testing-proxy/docs/draft-ietf-oauth-v2-1-15.txt

OAuth Working Group                                             D. Hardt
Internet-Draft                                                     Hellō
Intended status: Standards Track                              A. Parecki
Expires: 3 September 2026                                           Okta
                                                          T. Lodderstedt
                                                                  SPRIND
                                                            2 March 2026


                 The OAuth 2.1 Authorization Framework
                        draft-ietf-oauth-v2-1-15

Abstract

   The OAuth 2.1 authorization framework enables an application to
   obtain limited access to a protected resource, either on behalf of a
   resource owner by orchestrating an approval interaction between the
   resource owner and an authorization service, or by allowing the
   application to obtain access on its own behalf.  This specification
   replaces and obsoletes the OAuth 2.0 Authorization Framework
   described in RFC 6749 and the Bearer Token Usage in RFC 6750.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the OAuth Working Group
   mailing list (oauth@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/oauth/.

   Source for this draft and an issue tracker can be found at
   https://github.com/oauth-wg/oauth-v2-1.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."




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   This Internet-Draft will expire on 3 September 2026.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.1.  Roles . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     1.2.  Protocol Flow . . . . . . . . . . . . . . . . . . . . . .   8
     1.3.  Authorization Grant . . . . . . . . . . . . . . . . . . .  10
       1.3.1.  Authorization Code  . . . . . . . . . . . . . . . . .  10
       1.3.2.  Refresh Token . . . . . . . . . . . . . . . . . . . .  10
       1.3.3.  Client Credentials  . . . . . . . . . . . . . . . . .  12
     1.4.  Access Token  . . . . . . . . . . . . . . . . . . . . . .  13
       1.4.1.  Access Token Scope  . . . . . . . . . . . . . . . . .  14
       1.4.2.  Bearer Tokens . . . . . . . . . . . . . . . . . . . .  15
       1.4.3.  Sender-Constrained Access Tokens  . . . . . . . . . .  16
     1.5.  Communication security  . . . . . . . . . . . . . . . . .  16
     1.6.  HTTP Redirections . . . . . . . . . . . . . . . . . . . .  17
     1.7.  Interoperability  . . . . . . . . . . . . . . . . . . . .  17
     1.8.  Compatibility with OAuth 2.0  . . . . . . . . . . . . . .  17
     1.9.  Notational Conventions  . . . . . . . . . . . . . . . . .  18
   2.  Client Registration . . . . . . . . . . . . . . . . . . . . .  18
     2.1.  Client Types  . . . . . . . . . . . . . . . . . . . . . .  19
     2.2.  Client Identifier . . . . . . . . . . . . . . . . . . . .  21
     2.3.  Client Redirection Endpoint . . . . . . . . . . . . . . .  21
       2.3.1.  Registration Requirements . . . . . . . . . . . . . .  21
       2.3.2.  Multiple Redirect URIs  . . . . . . . . . . . . . . .  22
       2.3.3.  Preventing CSRF Attacks . . . . . . . . . . . . . . .  23
       2.3.4.  Preventing Mix-Up Attacks . . . . . . . . . . . . . .  23
       2.3.5.  Invalid Endpoint  . . . . . . . . . . . . . . . . . .  23
       2.3.6.  Endpoint Content  . . . . . . . . . . . . . . . . . .  23
     2.4.  Client Authentication . . . . . . . . . . . . . . . . . .  24
       2.4.1.  Client Secret . . . . . . . . . . . . . . . . . . . .  25
       2.4.2.  Other Authentication Methods  . . . . . . . . . . . .  26
     2.5.  Unregistered Clients  . . . . . . . . . . . . . . . . . .  26



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   3.  Protocol Endpoints  . . . . . . . . . . . . . . . . . . . . .  26
     3.1.  Authorization Endpoint  . . . . . . . . . . . . . . . . .  27
     3.2.  Token Endpoint  . . . . . . . . . . . . . . . . . . . . .  28
       3.2.1.  Client Authentication . . . . . . . . . . . . . . . .  28
       3.2.2.  Token Endpoint Request  . . . . . . . . . . . . . . .  29
       3.2.3.  Token Endpoint Response . . . . . . . . . . . . . . .  30
       3.2.4.  Token Endpoint Error Response . . . . . . . . . . . .  32
   4.  Grant Types . . . . . . . . . . . . . . . . . . . . . . . . .  33
     4.1.  Authorization Code Grant  . . . . . . . . . . . . . . . .  33
       4.1.1.  Authorization Request . . . . . . . . . . . . . . . .  35
       4.1.2.  Authorization Response  . . . . . . . . . . . . . . .  39
       4.1.3.  Token Endpoint Extension  . . . . . . . . . . . . . .  42
     4.2.  Client Credentials Grant  . . . . . . . . . . . . . . . .  43
       4.2.1.  Token Endpoint Extension  . . . . . . . . . . . . . .  44
     4.3.  Refresh Token Grant . . . . . . . . . . . . . . . . . . .  44
       4.3.1.  Token Endpoint Extension  . . . . . . . . . . . . . .  45
       4.3.2.  Refresh Token Response  . . . . . . . . . . . . . . .  46
       4.3.3.  Refresh Token Recommendations . . . . . . . . . . . .  47
     4.4.  Extension Grants  . . . . . . . . . . . . . . . . . . . .  47
   5.  Resource Requests . . . . . . . . . . . . . . . . . . . . . .  48
     5.1.  Bearer Token Requests . . . . . . . . . . . . . . . . . .  48
       5.1.1.  Authorization Request Header Field  . . . . . . . . .  48
       5.1.2.  Form-Encoded Content Parameter  . . . . . . . . . . .  49
     5.2.  Access Token Validation . . . . . . . . . . . . . . . . .  50
     5.3.  Error Response  . . . . . . . . . . . . . . . . . . . . .  50
       5.3.1.  The WWW-Authenticate Response Header Field  . . . . .  51
       5.3.2.  Error Codes . . . . . . . . . . . . . . . . . . . . .  52
   6.  Extensibility . . . . . . . . . . . . . . . . . . . . . . . .  53
     6.1.  Defining Access Token Types . . . . . . . . . . . . . . .  53
       6.1.1.  Registered Access Token Types . . . . . . . . . . . .  53
       6.1.2.  Vendor-Specific Access Token Types  . . . . . . . . .  54
     6.2.  Defining New Endpoint Parameters  . . . . . . . . . . . .  54
     6.3.  Defining New Authorization Grant Types  . . . . . . . . .  55
     6.4.  Defining New Authorization Endpoint Response Types  . . .  55
     6.5.  Defining Additional Error Codes . . . . . . . . . . . . .  55
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  56
     7.1.  Access Token Security Considerations  . . . . . . . . . .  56
       7.1.1.  Security Threats  . . . . . . . . . . . . . . . . . .  56
       7.1.2.  Threat Mitigation . . . . . . . . . . . . . . . . . .  57
       7.1.3.  Summary of Recommendations  . . . . . . . . . . . . .  57
       7.1.4.  Access Token Privilege Restriction  . . . . . . . . .  59
     7.2.  Client Authentication . . . . . . . . . . . . . . . . . .  59
     7.3.  Client Impersonation  . . . . . . . . . . . . . . . . . .  60
       7.3.1.  Impersonation of Native Apps  . . . . . . . . . . . .  60
       7.3.2.  Access Token Privilege Restriction  . . . . . . . . .  61
     7.4.  Client Impersonating Resource Owner . . . . . . . . . . .  61
     7.5.  Authorization Code Security Considerations  . . . . . . .  62
       7.5.1.  Authorization Code Injection  . . . . . . . . . . . .  62



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       7.5.2.  Reuse of Authorization Codes  . . . . . . . . . . . .  63
       7.5.3.  HTTP 307 Redirect . . . . . . . . . . . . . . . . . .  64
     7.6.  Ensuring Endpoint Authenticity  . . . . . . . . . . . . .  64
     7.7.  Credentials-Guessing Attacks  . . . . . . . . . . . . . .  64
     7.8.  Phishing Attacks  . . . . . . . . . . . . . . . . . . . .  65
     7.9.  Cross-Site Request Forgery  . . . . . . . . . . . . . . .  65
     7.10. Clickjacking  . . . . . . . . . . . . . . . . . . . . . .  66
     7.11. Injection and Input Validation  . . . . . . . . . . . . .  67
     7.12. Open Redirection  . . . . . . . . . . . . . . . . . . . .  68
       7.12.1.  Client as Open Redirector  . . . . . . . . . . . . .  68
       7.12.2.  Authorization Server as Open Redirector  . . . . . .  68
     7.13. Transport Security  . . . . . . . . . . . . . . . . . . .  69
     7.14. Authorization Server Mix-Up Mitigation  . . . . . . . . .  70
       7.14.1.  Mix-Up Defense via Issuer Identification . . . . . .  70
       7.14.2.  Mix-Up Defense via Distinct Redirect URIs  . . . . .  71
   8.  Native Applications . . . . . . . . . . . . . . . . . . . . .  71
     8.1.  Client Authentication of Native Apps  . . . . . . . . . .  73
       8.1.1.  Registration of Native App Clients  . . . . . . . . .  73
       8.1.2.  Native App Attestation  . . . . . . . . . . . . . . .  73
     8.2.  Using Inter-App URI Communication for OAuth in Native
           Apps  . . . . . . . . . . . . . . . . . . . . . . . . . .  73
     8.3.  Initiating the Authorization Request from a Native App  .  74
     8.4.  Receiving the Authorization Response in a Native App  . .  75
       8.4.1.  Claimed "https" Scheme URI Redirection  . . . . . . .  75
       8.4.2.  Loopback Interface Redirection  . . . . . . . . . . .  75
       8.4.3.  Private-Use URI Scheme Redirection  . . . . . . . . .  76
     8.5.  Security Considerations in Native Apps  . . . . . . . . .  77
       8.5.1.  Embedded User Agents in Native Apps . . . . . . . . .  77
       8.5.2.  Fake External User-Agents in Native Apps  . . . . . .  78
       8.5.3.  Malicious External User-Agents in Native Apps . . . .  79
       8.5.4.  Loopback Redirect Considerations in Native Apps . . .  79
   9.  Browser-Based Apps  . . . . . . . . . . . . . . . . . . . . .  79
   10. Differences from OAuth 2.0  . . . . . . . . . . . . . . . . .  80
     10.1.  Removal of the OAuth 2.0 Implicit grant  . . . . . . . .  80
     10.2.  Redirect URI Parameter in Token Request  . . . . . . . .  81
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  82
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  82
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  82
     12.2.  Informative References . . . . . . . . . . . . . . . . .  84
   Appendix A.  Augmented Backus-Naur Form (ABNF) Syntax . . . . . .  88
     A.1.  "client_id" Syntax  . . . . . . . . . . . . . . . . . . .  88
     A.2.  "client_secret" Syntax  . . . . . . . . . . . . . . . . .  88
     A.3.  "response_type" Syntax  . . . . . . . . . . . . . . . . .  88
     A.4.  "scope" Syntax  . . . . . . . . . . . . . . . . . . . . .  88
     A.5.  "state" Syntax  . . . . . . . . . . . . . . . . . . . . .  89
     A.6.  "redirect_uri" Syntax . . . . . . . . . . . . . . . . . .  89
     A.7.  "error" Syntax  . . . . . . . . . . . . . . . . . . . . .  89
     A.8.  "error_description" Syntax  . . . . . . . . . . . . . . .  89



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     A.9.  "error_uri" Syntax  . . . . . . . . . . . . . . . . . . .  89
     A.10. "grant_type" Syntax . . . . . . . . . . . . . . . . . . .  89
     A.11. "code" Syntax . . . . . . . . . . . . . . . . . . . . . .  90
     A.12. "access_token" Syntax . . . . . . . . . . . . . . . . . .  90
     A.13. "token_type" Syntax . . . . . . . . . . . . . . . . . . .  90
     A.14. "expires_in" Syntax . . . . . . . . . . . . . . . . . . .  90
     A.15. "refresh_token" Syntax  . . . . . . . . . . . . . . . . .  90
     A.16. Endpoint Parameter Syntax . . . . . . . . . . . . . . . .  90
     A.17. "code_verifier" Syntax  . . . . . . . . . . . . . . . . .  90
     A.18. "code_challenge" Syntax . . . . . . . . . . . . . . . . .  91
   Appendix B.  Use of application/x-www-form-urlencoded Media
           Type  . . . . . . . . . . . . . . . . . . . . . . . . . .  91
   Appendix C.  Serializations . . . . . . . . . . . . . . . . . . .  92
     C.1.  Query String Serialization  . . . . . . . . . . . . . . .  92
     C.2.  Form-Encoded Serialization  . . . . . . . . . . . . . . .  92
     C.3.  JSON Serialization  . . . . . . . . . . . . . . . . . . .  92
   Appendix D.  Extensions . . . . . . . . . . . . . . . . . . . . .  92
   Appendix E.  Acknowledgements . . . . . . . . . . . . . . . . . .  94
   Appendix F.  Document History . . . . . . . . . . . . . . . . . .  95
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 100

1.  Introduction

   OAuth introduces an authorization layer to the client-server
   authentication model by separating the role of the client from that
   of the resource owner.  In OAuth, the client requests access to
   resources controlled by the resource owner and hosted by the resource
   server.  Instead of using the resource owner's credentials to access
   protected resources, the client obtains an access token - a
   credential representing a specific set of access attributes such as
   scope and lifetime.  Access tokens are issued to clients by an
   authorization server with the approval of the resource owner.  The
   client uses the access token to access the protected resources hosted
   by the resource server.

   In the older, more limited client-server authentication model, the
   client requests an access-restricted resource (protected resource) on
   the server by authenticating to the server using the resource owner's
   credentials.  In order to provide applications access to restricted
   resources, the resource owner shares their credentials with the
   application.  This creates several problems and limitations:

   *  Applications are required to store the resource owner's
      credentials for future use, typically a password in clear-text.

   *  Servers are required to support password authentication, despite
      the security weaknesses inherent in passwords.




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   *  Applications gain overly broad access to the resource owner's
      protected resources, leaving resource owners without any ability
      to restrict duration or access to a limited subset of resources.

   *  Resource owners often reuse passwords with other unrelated
      services, despite best security practices.  This password reuse
      means a vulnerability or exposure in one service may have security
      implications in completely unrelated services.

   *  Resource owners cannot revoke access to an individual application
      without revoking access to all third parties, and must do so by
      changing their password.

   *  Compromise of any application results in compromise of the end-
      user's password and all of the data protected by that password.

   An example where OAuth is used is where an end user (resource owner)
   grants a financial management service (client) access to their
   sensitive transaction history stored at a banking service (resource
   server), without sharing their username and password with the
   financial management service.  Instead, they authenticate directly
   with their financial institution's server (authorization server),
   which issues the financial management service delegation-specific
   credentials (access token).

   This separation of concerns also provides the ability to use more
   advanced user authentication methods such as multi-factor
   authentication and even passwordless authentication, without any
   modification to the applications.  With all user authentication logic
   handled by the authorization server, applications don't need to be
   concerned with the specifics of implementing any particular
   authentication mechanism.  This provides the ability for the
   authorization server to manage the user authentication policies and
   even change them in the future without coordinating the changes with
   applications.
















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   The authorization layer can also simplify how a resource server
   determines if a request is authorized.  Traditionally, after
   authenticating the client, each resource server would evaluate
   policies to compute if the client is authorized on each API call.  In
   a distributed system, the policies need to be synchronized to all the
   resource servers, or the resource server must call a central policy
   server to process each request.  In OAuth, evaluation of the policies
   is performed only when a new access token is created by the
   authorization server.  If the authorized access is represented in the
   access token, the resource server no longer needs to evaluate the
   policies, and only needs to validate the access token.  This
   simplification applies when the application is acting on behalf of a
   resource owner, or on behalf of itself.

   OAuth is an authorization protocol, not an authentication protocol,
   as OAuth does not define the necessary components to achieve user
   authentication.  An authentication protocol is necessary if the goal
   is to authenticate users.  An example is OpenID Connect
   [OpenID.Connect], which builds on OAuth to provide the security
   characteristics and necessary components required of an
   authentication protocol.

   The access token represents the authorization granted to the client.
   It is a common practice for the client to present the access token to
   a proprietary API which returns a user identifier for the resource
   owner, and then using the result of the API as a proxy for
   authenticating the user.  This practice is not part of the OAuth
   standard or security considerations, and may not have been considered
   by the resource owner.  Implementors should carefully consult the
   documentation of the resource server before adopting this practice.

   This specification is designed for use with HTTP [RFC9110].  The use
   of OAuth over any protocol other than HTTP is out of scope.

   Since the publication of the OAuth 2.0 Authorization Framework
   [RFC6749] in October 2012, it has been updated by OAuth 2.0 for
   Native Apps [RFC8252], OAuth Security Best Current Practice
   [RFC9700], and OAuth 2.0 for Browser-Based Apps
   [I-D.ietf-oauth-browser-based-apps].  The OAuth 2.0 Authorization
   Framework: Bearer Token Usage [RFC6750] has also been updated with
   [RFC9700].  This Standards Track specification consolidates the
   information in all of these documents and removes features that have
   been found to be insecure in [RFC9700].

1.1.  Roles

   OAuth defines four roles:




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   "resource owner":  An entity capable of granting access to a
      protected resource.  When the resource owner is a person, it is
      referred to as an end user.  This is sometimes abbreviated as
      "RO".

   "resource server":  The server hosting the protected resources,
      capable of accepting and responding to protected resource requests
      using access tokens.  The resource server is often accessible via
      an API.  This is sometimes abbreviated as "RS".

   "client":  An application making protected resource requests on
      behalf of the resource owner and with its authorization.  The term
      "client" does not imply any particular implementation
      characteristics (e.g., whether the application executes on a
      server, a desktop, or other devices).

   "authorization server":  The server issuing access tokens to the
      client after successfully authenticating the resource owner and
      obtaining authorization.  This is sometimes abbreviated as "AS".

   Most of this specification defines the interaction between the client
   and the authorization server, as well as between the client and
   resource server.

   The interaction between the authorization server and resource server
   is beyond the scope of this specification, however several extensions
   have been defined to provide an option for interoperability between
   resource servers and authorization servers.  The authorization server
   may be the same server as the resource server or a separate entity.
   A single authorization server may issue access tokens accepted by
   multiple resource servers.

   The interaction between the resource owner and authorization server
   (e.g. how the end user authenticates themselves at the authorization
   server) is also out of scope of this specification, with some
   exceptions, such as security considerations around prompting the end
   user for consent.

   When the resource owner is the end user, the user will interact with
   the client.  When the client is a web-based application, the user
   will interact with the client through a user agent (as described in
   Section 3.5 of [RFC9110]).  When the client is a native application,
   the user will interact with the client directly through the operating
   system.  See Section 2.1 for further details.

1.2.  Protocol Flow





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        +--------+                               +---------------+
        |        |--(1)- Authorization Request ->|   Resource    |
        |        |                               |     Owner     |
        |        |<-(2)-- Authorization Grant ---|               |
        |        |                               +---------------+
        |        |
        |        |                               +---------------+
        |        |--(3)-- Authorization Grant -->| Authorization |
        | Client |                               |     Server    |
        |        |<-(4)----- Access Token -------|               |
        |        |                               +---------------+
        |        |
        |        |                               +---------------+
        |        |--(5)----- Access Token ------>|    Resource   |
        |        |                               |     Server    |
        |        |<-(6)--- Protected Resource ---|               |
        +--------+                               +---------------+

                      Figure 1: Abstract Protocol Flow

   The abstract OAuth 2.1 flow illustrated in Figure 1 describes the
   interaction between the four roles and includes the following steps:

   1.  The client requests authorization from the resource owner.  The
       authorization request can be made directly to the resource owner
       (as shown), or preferably indirectly via the authorization server
       as an intermediary.

   2.  The client receives an authorization grant, which is a credential
       representing the resource owner's authorization, expressed using
       one of the authorization grant types defined in this
       specification or using an extension grant type.  The
       authorization grant type depends on the method used by the client
       to request authorization and the types supported by the
       authorization server.

   3.  The client requests an access token by authenticating with the
       authorization server and presenting the authorization grant.

   4.  The authorization server authenticates the client and validates
       the authorization grant, and if valid, issues an access token.

   5.  The client requests the protected resource from the resource
       server and authenticates by presenting the access token.

   6.  The resource server validates the access token, and if valid,
       serves the request.




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   The preferred method for the client to obtain an authorization grant
   from the resource owner (depicted in steps (1) and (2)) is to use the
   authorization server as an intermediary, which is illustrated in
   Figure 3 in Section 4.1.

1.3.  Authorization Grant

   An authorization grant represents the resource owner's authorization
   (to access its protected resources) used by the client to obtain an
   access token.  This specification defines three grant types --
   authorization code, refresh token, and client credentials -- as well
   as an extensibility mechanism for defining additional types.

1.3.1.  Authorization Code

   An authorization code is a temporary credential used to obtain an
   access token.  Instead of the client requesting authorization
   directly from the resource owner, the client directs the resource
   owner to an authorization server (via its user agent) which in turn
   directs the resource owner back to the client with the authorization
   code.  The client can then exchange the authorization code for an
   access token.

   Before directing the resource owner back to the client with the
   authorization code, the authorization server authenticates the
   resource owner, and may request the resource owner's consent or
   otherwise inform them of the client's request.  Because the resource
   owner only authenticates with the authorization server, the resource
   owner's credentials are never shared with the client, and the client
   does not need to have knowledge of any additional authentication
   steps such as multi-factor authentication or delegated accounts.

   The authorization code provides a few important security benefits,
   such as the ability to authenticate the client, as well as the
   transmission of the access token directly to the client without
   passing it through the resource owner's user agent and potentially
   exposing it to others, including the resource owner.

1.3.2.  Refresh Token

   Refresh tokens are credentials used to obtain access tokens.  Refresh
   tokens may be issued to the client by the authorization server and
   are used to obtain a new access token when the current access token
   becomes invalid or expires, or to obtain additional access tokens
   with identical or narrower scope (access tokens may have a shorter
   lifetime and fewer privileges than authorized by the resource owner).
   Issuing a refresh token is optional at the discretion of the
   authorization server, and may be issued based on properties of the



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   client, properties of the request, policies within the authorization
   server, or any other criteria.  If the authorization server issues a
   refresh token, it is included when issuing an access token (i.e.,
   step (2) in Figure 2).  The lifetime of the refresh token is also at
   the discretion of the authorization server.

   A refresh token is a string representing the authorization granted to
   the client by the resource owner.  The string is considered opaque to
   the client.  The refresh token may be an identifier used to retrieve
   the authorization information or may encode this information into the
   string itself.  Unlike access tokens, refresh tokens are intended for
   use only with authorization servers and are never sent to resource
   servers.

  +--------+                                           +---------------+
  |        |--(1)------- Authorization Grant --------->|               |
  |        |                                           |               |
  |        |<-(2)----------- Access Token -------------|               |
  |        |               & Refresh Token             |               |
  |        |                                           |               |
  |        |                            +----------+   |               |
  |        |--(3)---- Access Token ---->|          |   |               |
  |        |                            |          |   |               |
  |        |<-(4)- Protected Resource --| Resource |   | Authorization |
  | Client |                            |  Server  |   |     Server    |
  |        |--(5)---- Access Token ---->|          |   |               |
  |        |                            |          |   |               |
  |        |<-(6)- Invalid Token Error -|          |   |               |
  |        |                            +----------+   |               |
  |        |                                           |               |
  |        |--(7)----------- Refresh Token ----------->|               |
  |        |                                           |               |
  |        |<-(8)----------- Access Token -------------|               |
  +--------+           & Optional Refresh Token        +---------------+

               Figure 2: Refreshing an Expired Access Token

   The flow illustrated in Figure 2 includes the following steps:

   1.  The client requests an access token by authenticating with the
       authorization server and presenting an authorization grant.

   2.  The authorization server authenticates the client and validates
       the authorization grant, and if valid, issues an access token and
       optionally a refresh token.

   3.  The client makes a protected resource request to the resource
       server by presenting the access token.



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   4.  The resource server validates the access token, and if valid,
       serves the request.

   5.  Steps (3) and (4) repeat until the access token expires.  If the
       client knows the access token expired, it skips to step (7);
       otherwise, it makes another protected resource request.

   6.  Since the access token is invalid, the resource server returns an
       invalid token error.

   7.  The client requests a new access token by presenting the refresh
       token and providing client authentication if it has been issued
       credentials.  The client authentication requirements are based on
       the client type and on the authorization server policies.

   8.  The authorization server authenticates the client and validates
       the refresh token, and if valid, issues a new access token (and,
       optionally, a new refresh token).

   Note that there is no need to communicate the lifetime of the refresh
   token to the client, because the client can't do anything different
   with the knowledge of the lifetime.  Additionally, the authorization
   server might choose to use dynamic lifetimes (e.g. the refresh token
   expiry is extended as long as the refresh token is used at least once
   every 7 days), or the authorization server might revoke the refresh
   token before its scheduled expiration date for any reason, such as if
   the user revokes the application's access.  This means the client
   already has to handle the case of a refresh token expiring at an
   arbitrary time.

   Regardless of why or when the refresh token expires, the client has
   only one path to obtain new tokens, which is to start a new OAuth
   flow from the beginning.  For that reason, there is no property
   defined to communicate the expiration of a refresh token to the
   client.

1.3.3.  Client Credentials

   The client credentials or other forms of client authentication (e.g.,
   a private key used to sign a JWT, as described in [RFC7523] and its
   update [I-D.ietf-oauth-rfc7523bis]) can be used as an authorization
   grant when the authorization scope is limited to the protected
   resources under the control of the client, or to protected resources
   previously arranged with the authorization server.  Client
   credentials are used when the client is requesting access to
   protected resources based on an authorization previously arranged
   with the authorization server.




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1.4.  Access Token

   Access tokens are credentials used to access protected resources.  An
   access token is a string representing an authorization issued to the
   client.

   The string is considered opaque to the client, even if it has a
   structure.  The client MUST NOT expect to be able to parse the access
   token value.  The authorization server is not required to use a
   consistent access token encoding or format other than what is
   expected by the resource server.

   The access granted by the resource owner to the client is represented
   by the Access Token created by the authorization server.  Access
   Tokens are short lived to reduce the blast radius of a leaked Access
   Token.  The expiration of the Access Token is set by the
   authorization server.

   Depending on the authorization server implementation, the token
   string may be used by the resource server to retrieve the
   authorization information, or the token may self-contain the
   authorization information in a verifiable manner (i.e., a token
   string consisting of a signed data payload).  One example of a token
   retrieval mechanism is Token Introspection [RFC7662], in which the RS
   calls an endpoint on the AS to validate the token presented by the
   client.  One example of a structured token format is JWT Profile for
   Access Tokens [RFC9068], a method of encoding and signing access
   token data as a JSON Web Token [RFC7519].

   Additional authentication credentials, which are beyond the scope of
   this specification, may be required in order for the client to use an
   access token.  This is typically referred to as a sender-constrained
   access token, such as DPoP [RFC9449] and Mutual TLS Certificate-Bound
   Access Tokens [RFC8705].

   The access token provides an abstraction layer, replacing different
   authorization constructs (e.g., username and password) with a single
   token understood by the resource server.  This abstraction enables
   issuing access tokens more restrictive than the authorization grant
   used to obtain them, as well as removing the resource server's need
   to understand a wide range of authentication methods.

   Access tokens can have different formats, structures, and methods of
   utilization (e.g., cryptographic properties) based on the resource
   server security requirements.  Access token attributes and the
   methods used to access protected resources may be extended beyond
   what is described in this specification.




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   Access tokens (as well as any confidential access token attributes)
   MUST be kept confidential in transit and storage, and only shared
   among the authorization server, the resource servers the access token
   is valid for, and the client to which the access token is issued.

   The authorization server MUST ensure that access tokens cannot be
   generated, modified, or guessed to produce valid access tokens by
   unauthorized parties.

1.4.1.  Access Token Scope

   Access tokens are intended to be issued to clients with less
   privileges than the user granting the access has.  This is known as a
   limited "scope" access token.  The authorization server and resource
   server can use this scope mechanism to limit what types of resources
   or level of access a particular client can have.

   For example, a client may only need "read" access to a user's
   resources, but doesn't need to update resources, so the client can
   request the read-only scope defined by the authorization server, and
   obtain an access token that cannot be used to update resources.  This
   requires coordination between the authorization server, resource
   server, and client.  The authorization server provides the client the
   ability to request specific scopes, and associates those scopes with
   the access token issued to the client.  The resource server is then
   responsible for enforcing scopes when presented with a limited-scope
   access token.

   OAuth does not define any scope values, instead scopes are defined by
   the authorization server or by extensions or profiles of OAuth.  One
   such extension that defines scopes is [OpenID.Connect], which defines
   a set of scopes that provide granular access to a user's profile
   information.  It is recommended to avoid defining custom scopes that
   conflict with scopes from known extensions.

   To request a limited-scope access token, the client uses the scope
   request parameter at the authorization or token endpoints, depending
   on the grant type used.  In turn, the authorization server uses the
   scope response parameter to inform the client of the scope of the
   access token issued.

   The value of the scope parameter is expressed as a space- delimited
   list of case-sensitive strings.  The strings are defined by the
   authorization server.  If the value contains multiple space-delimited
   strings, their order does not matter, and each string adds an
   additional access range to the requested scope.





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       scope       = scope-token *( SP scope-token )
       scope-token = 1*( %x21 / %x23-5B / %x5D-7E )

   The authorization server MAY fully or partially ignore the scope
   requested by the client, based on the authorization server policy or
   the resource owner's instructions.  If the issued access token scope
   is different from the one requested by the client, the authorization
   server MUST include the scope response parameter in the token
   response (Section 3.2.3) to inform the client of the actual scope
   granted.

   If the client omits the scope parameter when requesting
   authorization, the authorization server MUST either process the
   request using a pre-defined default value or fail the request
   indicating an invalid scope.  The authorization server SHOULD
   document its scope requirements and default value (if defined).

1.4.2.  Bearer Tokens

   A Bearer Token is a security token with the property that any party
   in possession of the token (a "bearer") can use the token in any way
   that any other party in possession of it can.  Using a Bearer Token
   does not require a bearer to prove possession of cryptographic key
   material (proof-of-possession).

   Bearer Tokens may be enhanced with proof-of-possession specifications
   such as DPoP [RFC9449] and mTLS [RFC8705] to provide proof-of-
   possession characteristics.

   To protect against access token disclosure, the communication
   interaction between the client and the resource server MUST utilize
   confidentiality and integrity protection as described in Section 1.5.

   There is no requirement on the particular structure or format of a
   bearer token.  If a bearer token is a reference to authorization
   information, such references MUST be infeasible for an attacker to
   guess, such as using a sufficiently long cryptographically random
   string.  If a bearer token uses an encoding mechanism to contain the
   authorization information in the token itself, the access token MUST
   use integrity protection sufficient to prevent the token from being
   modified.  One example of an encoding and signing mechanism for
   access tokens is described in JSON Web Token Profile for Access
   Tokens [RFC9068].








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1.4.3.  Sender-Constrained Access Tokens

   A sender-constrained access token binds the use of an access token to
   a specific sender.  This sender is obliged to demonstrate knowledge
   of a certain secret as prerequisite for the acceptance of that access
   token at the recipient (e.g., a resource server).

   Authorization and resource servers SHOULD use mechanisms for sender-
   constraining access tokens, such as OAuth Demonstration of Proof of
   Possession (DPoP) [RFC9449] or Mutual TLS for OAuth 2.0 [RFC8705].
   See Section 4.10.1 of [RFC9700] to prevent misuse of stolen and
   leaked access tokens.

   It is RECOMMENDED to use end-to-end TLS between the client and the
   resource server.  If TLS traffic needs to be terminated at an
   intermediary, refer to Section 4.13 of [RFC9700] for further security
   advice.

1.5.  Communication security

   Implementations MUST use a mechanism to provide communication
   authentication, integrity and confidentiality such as Transport-Layer
   Security [RFC8446], to protect the exchange of clear-text credentials
   and tokens either in the content or in header fields from
   eavesdropping which enables replay (e.g., see Section 2.4.1,
   Section 7.5.1, Section 3.2, and Section 1.4.2).

   All the OAuth protocol URLs (URLs exposed by the AS, RS and Client)
   MUST use the https scheme except for loopback interface redirect
   URIs, which MAY use the http scheme.  When using https, TLS
   certificates MUST be checked according to Section 4.3.4 of [RFC9110].
   At the time of this writing, TLS version 1.3 [RFC8446] is the most
   recent version.

   Implementations MAY also support additional transport-layer security
   mechanisms that meet their security requirements.

   The identification of the TLS versions and algorithms is outside the
   scope of this specification.  Refer to [BCP195] for up to date
   recommendations on transport layer security, and to the relevant
   specifications for certificate validation and other security
   considerations.









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1.6.  HTTP Redirections

   This specification makes extensive use of HTTP redirections, in which
   the client or the authorization server directs the resource owner's
   user agent to another destination.  While the examples in this
   specification show the use of the HTTP 302 status code, any other
   method available via the user agent to accomplish this redirection,
   with the exception of HTTP 307, is allowed and is considered to be an
   implementation detail.  See Section 7.5.3 for details.

1.7.  Interoperability

   OAuth 2.1 provides a rich authorization framework with well-defined
   security properties.

   This specification leaves a few required components partially or
   fully undefined (e.g., client registration, authorization server
   capabilities, endpoint discovery).  Some of these behaviors are
   defined in optional extensions which implementations can choose to
   use, such as:

   *  [RFC8414]: Authorization Server Metadata, defining an endpoint
      clients can use to look up the information needed to interact with
      a particular OAuth server

   *  [RFC7591]: Dynamic Client Registration, providing a mechanism for
      programmatically registering clients with an authorization server

   *  [RFC7592]: Dynamic Client Management, providing a mechanism for
      updating dynamically registered client information

   *  [RFC7662]: Token Introspection, defining a mechanism for resource
      servers to obtain information about access tokens

   Please refer to Appendix D for a list of current known extensions at
   the time of this publication.

1.8.  Compatibility with OAuth 2.0

   OAuth 2.1 is compatible with OAuth 2.0 with the extensions and
   restrictions from known best current practices applied.
   Specifically, features not specified in OAuth 2.0 core, such as PKCE,
   are required in OAuth 2.1.  Additionally, some features available in
   OAuth 2.0, such as the Implicit or Resource Owner Credentials grant
   types, are not specified in OAuth 2.1.  Furthermore, some behaviors
   allowed in OAuth 2.0 are restricted in OAuth 2.1, such as the strict
   string matching of redirect URIs required by OAuth 2.1.




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   See Section 10 for more details on the differences from OAuth 2.0.

1.9.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234].  Additionally, the rule URI-reference is
   included from "Uniform Resource Identifier (URI): Generic Syntax"
   [RFC3986].

   Certain security-related terms are to be understood in the sense
   defined in [RFC4949].  These terms include, but are not limited to,
   "attack", "authentication", "authorization", "certificate",
   "confidentiality", "credential", "encryption", "identity", "sign",
   "signature", "trust", "validate", and "verify".

   The term "content" is to be interpreted as described in Section 6.4
   of [RFC9110].

   The term "user agent" is to be interpreted as described in
   Section 3.5 of [RFC9110].

   Unless otherwise noted, all the protocol parameter names and values
   are case sensitive.

2.  Client Registration

   Before initiating the protocol, the client must have established an
   identifier (Section 2.2) at the authorization server.  The means
   through which the client identifier is established with the
   authorization server are beyond the scope of this specification, but
   typically involve the client developer manually registering the
   client at the authorization server's website (after creating an
   account and agreeing to the service's Terms of Service), or by using
   Dynamic Client Registration [RFC7591].  Extensions may also define
   other programmatic methods of establishing client registration.










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   Client registration does not require a direct interaction between the
   client and the authorization server.  When supported by the
   authorization server, registration can rely on other means for
   establishing trust and obtaining the required client properties
   (e.g., redirect URI, client type).  For example, registration can be
   accomplished using a self-issued or third-party-issued assertion, or
   by the authorization server performing client discovery using a
   trusted channel.

   Client registration MUST include:

   *  the client type as described in Section 2.1,

   *  client details needed by the grant type in use, such as redirect
      URIs as described in Section 2.3, and

   *  any other information required by the authorization server (e.g.,
      application name, website, description, logo image, the acceptance
      of legal terms).

   Dynamic Client Registration [RFC7591] defines a common general data
   model for clients that may be used even with manual client
   registration.

2.1.  Client Types

   OAuth 2.1 defines two client types based on their ability to
   authenticate securely with the authorization server.

   "confidential":  Clients that have credentials with the AS are
      designated as "confidential clients"

   "public":  Clients without credentials are called "public clients"

   Any clients with credentials MUST take precautions to prevent leakage
   and abuse of their credentials.

   Client authentication allows an Authorization Server to ensure it is
   interacting with a certain client (identified by its client_id) in an
   OAuth flow.  The Authorization Server might make policy decisions
   about things such as whether to prompt the user for consent on every
   authorization or only the first based on the confidence that the
   Authorization Server is actually communicating with the legitimate
   client.

   Whether and how an Authorization Server validates the identity of a
   client or the party providing/operating this client is out of scope
   of this specification.  Authorization servers SHOULD consider the



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   level of confidence in a client's identity when deciding whether they
   allow a client access to more sensitive resources and operations such
   as the Client Credentials grant type and how often to prompt the user
   for consent.

   There is no requirement that an Authorization Server supports a
   particular client type.

   A single client_id SHOULD NOT be treated as more than one type of
   client.

   This specification has been designed around the following client
   profiles:

   "web application":  A web application is a client running on a web
      server.  Resource owners access the client via an HTML user
      interface rendered in a user agent on the device used by the
      resource owner.  The client credentials as well as any access
      tokens issued to the client are stored on the web server and are
      not exposed to or accessible by the resource owner.

   "browser-based application":  A browser-based application is a client
      in which the client code is downloaded from a web server and
      executes within a user agent (e.g., web browser) on the device
      used by the resource owner.  Protocol data and credentials are
      easily accessible (and often visible) to the resource owner.  If
      such applications wish to use client credentials, it is
      recommended to utilize the backend for frontend pattern.  Since
      such applications reside within the user agent, they can make
      seamless use of the user agent capabilities when requesting
      authorization.

   "native application":  A native application is a client installed and
      executed on the device used by the resource owner.  Protocol data
      and credentials are accessible to the resource owner.  It is
      assumed that any client authentication credentials included in the
      application can be extracted.  Dynamically issued access tokens
      and refresh tokens can receive an acceptable level of protection.
      On some platforms, these credentials are protected from other
      applications residing on the same device.  If such applications
      wish to use client credentials, it is recommended to utilize the
      backend for frontend pattern, or issue the credentials at runtime
      using Dynamic Client Registration [RFC7591].








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2.2.  Client Identifier

   Every client is identified in the context of an authorization server
   by a client identifier -- a unique string representing the
   registration information provided by the client.  While the
   Authorization Server typically issues the client identifier itself,
   it may also serve clients whose client identifier was created by a
   party other than the Authorization Server.  The client identifier is
   not a secret; it is exposed to the resource owner and MUST NOT be
   used alone for client authentication.  The client identifier is
   unique in the context of an authorization server.

   The client identifier is an opaque string whose size is left
   undefined by this specification.  The client should avoid making
   assumptions about the identifier size.  The authorization server
   SHOULD document the size of any identifier it issues.

   If the authorization server supports clients with client identifiers
   issued by parties other than the authorization server, the
   authorization server SHOULD take precautions to avoid clients
   impersonating resource owners as described in Section 7.4.

2.3.  Client Redirection Endpoint

   The client redirection endpoint (also referred to as "redirect
   endpoint") is the URI of the client that the authorization server
   redirects the user agent back to after completing its interaction
   with the resource owner.

   The authorization server redirects the user agent to one of the
   client's redirection endpoints previously established with the
   authorization server during the client registration process.

   The redirect URI MUST be an absolute URI as defined by Section 4.3 of
   [RFC3986].  The redirect URI MAY include an query string component
   (Appendix C.1), which MUST be retained when adding additional query
   parameters.  The redirect URI MUST NOT include a fragment component.

2.3.1.  Registration Requirements

   Authorization servers MUST require clients to register their complete
   redirect URI (including the path component).  Authorization servers
   MUST reject authorization requests that specify a redirect URI that
   doesn't exactly match one that was registered, with an exception for
   loopback redirects, where an exact match is required except for the
   port URI component, see Section 4.1.1 for details.





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   The authorization server MAY allow the client to register multiple
   redirect URIs.

   Registration may happen out of band, such as a manual step of
   configuring the client information at the authorization server, or
   may happen at runtime, such as in the initial POST in Pushed
   Authorization Requests [RFC9126].

   For private-use URI scheme-based redirect URIs, authorization servers
   SHOULD enforce the requirement in Section 8.4.3 that clients use
   schemes that are reverse domain name based.  At a minimum, any
   private-use URI scheme that doesn't contain a period character (.)
   SHOULD be rejected.

   In addition to the collision-resistant properties, this can help to
   prove ownership in the event of a dispute where two apps claim the
   same private-use URI scheme (where one app is acting maliciously).
   For example, if two apps claimed com.example.app, the owner of
   example.com could petition the app store operator to remove the
   counterfeit app.  Such a petition is harder to prove if a generic URI
   scheme was used.

   Clients MUST NOT expose URLs that forward the user's browser to
   arbitrary URIs obtained from a query parameter ("open redirector"),
   as described in Section 7.12.  Open redirectors can enable
   exfiltration of authorization codes and access tokens.

   The client MAY use the state request parameter to achieve per-request
   customization if needed rather than varying the redirect URI per
   request.

   Without requiring registration of redirect URIs, attackers can use
   the authorization endpoint as an open redirector as described in
   Section 7.12.

2.3.2.  Multiple Redirect URIs

   If multiple redirect URIs have been registered to a client, the
   client MUST include a redirect URI with the authorization request
   using the redirect_uri request parameter (Section 4.1.1).  If only a
   single redirect URI has been registered to a client, the redirect_uri
   request parameter is optional.









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2.3.3.  Preventing CSRF Attacks

   Clients MUST prevent Cross-Site Request Forgery (CSRF) attacks.  In
   this context, CSRF refers to requests to the redirection endpoint
   that do not originate at the authorization server, but a malicious
   third party (see Section 4.4.1.8 of [RFC6819] for details).  Clients
   that have ensured that the authorization server supports the
   code_challenge parameter MAY rely on the CSRF protection provided by
   that mechanism.  In OpenID Connect flows, validating the nonce
   parameter provides CSRF protection.  Otherwise, one-time use CSRF
   tokens carried in the state parameter that are securely bound to the
   user agent MUST be used for CSRF protection (see Section 7.9).

2.3.4.  Preventing Mix-Up Attacks

   When an OAuth client can only interact with one authorization server,
   a mix-up defense is not required.  In scenarios where an OAuth client
   interacts with two or more authorization servers, however, clients
   MUST prevent mix-up attacks.  In order to prevent mix-up attacks,
   clients MUST only process redirect responses of the issuer they sent
   the respective request to and from the same user agent this
   authorization request was initiated with.

   See Section 7.14 for a detailed description of two different defenses
   against mix-up attacks.

2.3.5.  Invalid Endpoint

   If an authorization request fails validation due to a missing,
   invalid, or mismatching redirect URI, the authorization server SHOULD
   inform the resource owner of the error and MUST NOT automatically
   redirect the user agent to the invalid redirect URI.

2.3.6.  Endpoint Content

   The redirection request to the client's endpoint typically results in
   an HTML document response, processed by the user agent.  If the HTML
   response is served directly as the result of the redirection request,
   any script included in the HTML document will execute with full
   access to the redirect URI and the artifacts (e.g., authorization
   code) it contains.  Additionally, the request URL containing the
   authorization code may be sent in the HTTP Referer header to any
   embedded images, stylesheets and other elements loaded in the page.

   The client SHOULD NOT include any third-party scripts (e.g., third-
   party analytics, social plug-ins, ad networks) in the redirect URI
   endpoint response.  Instead, it SHOULD extract the artifacts from the
   URI and redirect the user agent again to another endpoint without



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   exposing the artifacts (in the URI or elsewhere).  If third-party
   scripts are included, the client MUST ensure that its own scripts
   (used to extract and remove the credentials from the URI) will
   execute first.

2.4.  Client Authentication

   The authorization server MUST only rely on client authentication if
   the process of issuance/registration and distribution of the
   underlying credentials ensures their confidentiality.

   For confidential clients, the authorization server MAY accept any
   form of client authentication meeting its security requirements
   (e.g., client secret, public/private key pair).

   It is RECOMMENDED to use asymmetric (public-key based) methods for
   client authentication such as mTLS [RFC8705] or using signed JWTs
   ("Private Key JWT") in accordance with [RFC7521], [RFC7523], and
   their update [I-D.ietf-oauth-rfc7523bis] (defined in [OpenID.Connect]
   as the client authentication method private_key_jwt).  When such
   methods for client authentication are used, authorization servers do
   not need to store sensitive symmetric keys, making these methods more
   robust against a number of attacks, and enables clients to manage
   their own keys and key rotation.

   When using JWT-based client authentication, clients and authorization
   servers MUST follow the updated guidance around aud values in
   [I-D.ietf-oauth-rfc7523bis].

   When client authentication is not possible, the authorization server
   SHOULD employ other means to validate the client's identity -- for
   example, by requiring the registration of the client redirect URI or
   enlisting the resource owner to confirm identity.  A valid redirect
   URI is not sufficient to verify the client's identity when asking for
   resource owner authorization but can be used to prevent delivering
   credentials to a counterfeit client after obtaining resource owner
   authorization.

   The client MUST NOT use more than one authentication method in each
   request to prevent a conflict of which authentication mechanism is
   authoritative for the request.

   The authorization server MUST consider the security implications of
   interacting with unauthenticated clients and take measures to limit
   the potential exposure of tokens issued to such clients, (e.g.,
   limiting the lifetime of refresh tokens).





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   The privileges an authorization server associates with a certain
   client identity MUST depend on the assessment of the overall process
   for client identification and client credential lifecycle management.
   See Section 7.2 for additional details.

2.4.1.  Client Secret

   To support confidential clients in possession of a client secret, the
   authorization server MUST support the client including the client
   credentials in the request body content using the following
   parameters:

   "client_id":  REQUIRED.  The client identifier issued to the client
      during the registration process described by Section 2.2.

   "client_secret":  REQUIRED.  The client secret.

   The parameters can only be transmitted in the request content and
   MUST NOT be included in the request URI.

   This is also known as client_secret_post as defined in Section 2 of
   [RFC7591].

   For example, a request to refresh an access token (Section 4.3) using
   the content parameters (with extra line breaks for display purposes
   only):

   POST /token HTTP/1.1
   Host: server.example.com
   Content-Type: application/x-www-form-urlencoded

   grant_type=refresh_token&refresh_token=tGzv3JOkF0XG5Qx2TlKWIA
   &client_id=s6BhdRkqt3&client_secret=7Fjfp0ZBr1KtDRbnfVdmIw

   The authorization server MAY support the HTTP Basic authentication
   scheme for authenticating clients that were issued a client secret.

   When using the HTTP Basic authentication scheme as defined in
   Section 11 of [RFC9110] to authenticate with the authorization
   server, the client identifier is encoded using the application/x-www-
   form-urlencoded encoding algorithm per Appendix B, and the encoded
   value is used as the username; the client secret is encoded using the
   same algorithm and used as the password.

   This is also known as client_secret_basic as defined in Section 2 of
   [RFC7591].

   For example (with extra line breaks for display purposes only):



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   Authorization: Basic czZCaGRSa3F0Mzo3RmpmcDBaQnIxS3REUmJuZlZkbUl3

   Note: This method of initially form-encoding the client identifier
   and secret, and then using the encoded values as the HTTP Basic
   authentication username and password, has led to many
   interoperability problems in the past.  Some implementations have
   missed the encoding step, or decided to only encode certain
   characters, or ignored the encoding requirement when validating the
   credentials, leading to clients having to special-case how they
   present the credentials to individual authorization servers.
   Including the credentials in the request body content avoids the
   encoding issues and leads to more interoperable implementations.

   Since the client secret authentication method involves a password,
   the authorization server MUST protect any endpoint utilizing it
   against brute force attacks.

2.4.2.  Other Authentication Methods

   The authorization server MAY support any suitable authentication
   scheme matching its security requirements.  When using other
   authentication methods, the authorization server MUST define a
   mapping between the client identifier (registration record) and
   authentication scheme.

   Some additional authentication methods such as mTLS [RFC8705] and
   Private Key JWT ([RFC7523], [I-D.ietf-oauth-rfc7523bis]) are defined
   in the "OAuth Token Endpoint Authentication Methods
   (https://www.iana.org/assignments/oauth-parameters/oauth-
   parameters.xhtml#token-endpoint-auth-method)" registry, and may be
   useful as generic client authentication methods beyond the specific
   use of protecting the token endpoint.

2.5.  Unregistered Clients

   This specification does not require that clients be registered with
   the authorization server.  However, the use of unregistered clients
   is beyond the scope of this specification and requires additional
   security analysis and review of its interoperability impact.

3.  Protocol Endpoints

   The authorization process utilizes two authorization server endpoints
   (HTTP resources):

   *  Authorization endpoint - used by the client to obtain
      authorization from the resource owner via user agent redirection.




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   *  Token endpoint - used by the client to exchange an authorization
      grant for an access token, typically with client authentication.

   As well as one client endpoint:

   *  Redirection endpoint - used by the authorization server to return
      responses containing authorization credentials to the client via
      the resource owner user agent.

   Not every authorization grant type utilizes both endpoints.
   Extension grant types MAY define additional endpoints as needed.

3.1.  Authorization Endpoint

   The authorization endpoint is used to interact with the resource
   owner and obtain an authorization grant.  The authorization server
   MUST first authenticate the resource owner.  The way in which the
   authorization server authenticates the resource owner (e.g., username
   and password login, passkey, federated login, or by using an
   established session) is beyond the scope of this specification.

   The means through which the client obtains the URL of the
   authorization endpoint are beyond the scope of this specification,
   but the URL is typically provided in the service documentation, or in
   the authorization server's metadata document [RFC8414].

   The authorization endpoint URL MUST NOT include a fragment component,
   and MAY include a query string component Appendix C.1, which MUST be
   retained when adding additional query parameters.

   The authorization server MUST support the use of the HTTP GET method
   Section 9.3.1 of [RFC9110] for the authorization endpoint and MAY
   support the POST method (Section 9.3.3 of [RFC9110]) as well.

   The authorization server MUST ignore unrecognized request parameters
   sent to the authorization endpoint.

   Request and response parameters defined by this specification MUST
   NOT be included more than once.  This requirement also applies to
   parameters defined by extensions unless the extension explicitly
   defines otherwise for a specific parameter.  Parameters sent without
   a value MUST be treated as if they were omitted from the request.

   An authorization server that redirects a request potentially
   containing user credentials MUST avoid forwarding these user
   credentials accidentally (see Section 7.5.3 for details).





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   Cross-Origin Resource Sharing [WHATWG.CORS] MUST NOT be supported at
   the Authorization Endpoint as the client does not access this
   endpoint directly, instead the client redirects the user agent to it.

3.2.  Token Endpoint

   The token endpoint is used by the client to obtain an access token
   using a grant such as those described in Section 4 and Section 4.3.

   The means through which the client obtains the URL of the token
   endpoint are beyond the scope of this specification, but the URL is
   typically provided in the service documentation and configured during
   development of the client, or provided in the authorization server's
   metadata document [RFC8414] and fetched programmatically at runtime.

   The token endpoint URL MUST NOT include a fragment component, and MAY
   include a query string component Appendix C.1.

   The client MUST use the HTTP POST method when making requests to the
   token endpoint.

   The authorization server MUST ignore unrecognized request parameters
   sent to the token endpoint.

   Parameters sent without a value MUST be treated as if they were
   omitted from the request.  Request and response parameters defined by
   this specification MUST NOT be included more than once.  This
   requirement also applies to parameters defined by extensions unless
   the extension explicitly defines otherwise for a specific parameter.

   Authorization servers that wish to support browser-based applications
   (for example, applications running exclusively in client-side
   JavaScript without access to a supporting backend server) will need
   to ensure the token endpoint supports the necessary CORS
   [WHATWG.CORS] headers to allow the responses to be visible to the
   application.  If the authorization server provides additional
   endpoints to the application, such as metadata URLs, dynamic client
   registration, revocation, introspection, discovery or user info
   endpoints, these endpoints may also be accessed by the browser-based
   application, and will also need to have the CORS headers defined to
   allow access.  See [I-D.ietf-oauth-browser-based-apps] for further
   details.

3.2.1.  Client Authentication

   Confidential clients MUST authenticate with the authorization server
   as described in Section 2.4 when making requests to the token
   endpoint.



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   Client authentication is used for:

   *  Enforcing the binding of refresh tokens and authorization codes to
      the client they were issued to.  Client authentication adds an
      additional layer of security when an authorization code is
      transmitted to the redirection endpoint over an insecure channel.

   *  Recovering from a compromised client by disabling the client or
      changing its credentials, thus preventing an attacker from abusing
      stolen refresh tokens.  Changing a single set of client
      credentials is significantly faster than revoking an entire set of
      refresh tokens.

   *  Implementing authentication management best practices, which
      require periodic credential rotation.  Rotation of an entire set
      of refresh tokens can be challenging, while rotation of a single
      set of client credentials is significantly easier.

3.2.2.  Token Endpoint Request

   The client makes a request to the token endpoint by sending the
   following parameters using the form-encoded serialization format per
   Appendix C.2 with a character encoding of UTF-8 in the HTTP request
   content:

   "grant_type":  REQUIRED.  Identifier of the grant type the client
      uses with the particular token request.  This specification
      defines the values authorization_code, refresh_token, and
      client_credentials.  The grant type determines the further
      parameters required or supported by the token request.  The
      details of those grant types are defined below.

   "client_id":  OPTIONAL.  The client identifier is needed when a form
      of client authentication that relies on the parameter is used, or
      the grant_type requires identification of public clients.

   Confidential clients MUST authenticate with the authorization server
   as described in Section 3.2.1.

   For example, the client makes the following HTTPS request (with extra
   line breaks for display purposes only):










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 POST /token HTTP/1.1
 Host: server.example.com
 Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
 Content-Type: application/x-www-form-urlencoded

 grant_type=authorization_code
 &code=SplxlOBeZQQYbYS6WxSbIA
 &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb
 &code_verifier=3641a2d12d66101249cdf7a79c000c1f8c05d2aafcf14bf146497bed

   The authorization server MUST:

   *  require client authentication for confidential clients (or clients
      with other authentication requirements),

   *  authenticate the client if client authentication is included

   Further grant type specific processing rules apply and are specified
   with the respective grant type.

3.2.3.  Token Endpoint Response

   If the access token request is valid and authorized, the
   authorization server issues an access token and optional refresh
   token.

   If the client authentication failed or is invalid, the authorization
   server returns an error response as described in Section 3.2.4.

   The authorization server issues an access token and optional refresh
   token by creating an HTTP response according to Appendix C.3, using
   the application/json media type as defined by [RFC8259], with the
   following parameters and an HTTP 200 (OK) status code:

   "access_token":  REQUIRED.  The access token issued by the
      authorization server.

   "token_type":  REQUIRED.  The type of the access token issued as
      described in Section 1.4.  Value is case insensitive.

   "expires_in":  RECOMMENDED.  A JSON number that represents the
      lifetime in seconds of the access token.  For example, the value
      3600 denotes that the access token will expire in one hour from
      the time the response was generated.  If omitted, the
      authorization server SHOULD provide the lifetime via other means
      or document the default value.  Note that the authorization server
      may prematurely expire an access token and clients MUST NOT expect
      an access token to be valid for the provided lifetime.



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   "scope":  RECOMMENDED, if identical to the scope requested by the
      client; otherwise, REQUIRED.  The scope of the access token as
      described by Section 1.4.1.

   "refresh_token":  OPTIONAL.  The refresh token, which can be used to
      obtain new access tokens based on the grant passed in the
      corresponding token request.

   Authorization servers SHOULD determine, based on a risk assessment
   and their own policies, whether to issue refresh tokens to a certain
   client.  If the authorization server decides not to issue refresh
   tokens, the client MAY obtain new access tokens by starting the OAuth
   flow over, for example initiating a new authorization code request.
   In such a case, the authorization server may utilize cookies and
   persistent grants to optimize the user experience.

   If refresh tokens are issued, those refresh tokens MUST be bound to
   the scope and resource servers as consented by the resource owner.
   This is to prevent privilege escalation by the legitimate client and
   reduce the impact of refresh token leakage.

   The parameters are serialized into a JavaScript Object Notation
   (JSON) structure as described in Appendix C.3.

   The authorization server MUST include the HTTP Cache-Control response
   header field (see Section 5.2 of [RFC9111]) with a value of no-store
   in any response containing tokens, credentials, or other sensitive
   information.

   For example:

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-store

   {
     "access_token": "2YotnFZFEjr1zCsicMWpAA",
     "token_type": "Bearer",
     "expires_in": 3600,
     "refresh_token": "tGzv3JOkF0XG5Qx2TlKWIA",
     "example_parameter": "example_value"
   }

   The client MUST ignore unrecognized value names in the response.  The
   sizes of tokens and other values received from the authorization
   server are left undefined.  The client should avoid making
   assumptions about value sizes.  The authorization server SHOULD
   document the size of any value it issues.



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3.2.4.  Token Endpoint Error Response

   The authorization server responds with an HTTP 400 (Bad Request)
   status code (unless specified otherwise) and includes the following
   parameters with the response:

   "error":  REQUIRED.  A single ASCII [USASCII] error code from the
      following:

      "invalid_request":  The request is missing a required parameter,
         includes an unsupported parameter value (other than grant
         type), repeats a parameter, includes multiple credentials,
         utilizes more than one mechanism for authenticating the client,
         contains a code_verifier although no code_challenge was sent in
         the authorization request, or is otherwise malformed.

      "invalid_client":  Client authentication failed (e.g., unknown
         client, no client authentication included, or unsupported
         authentication method).  The authorization server MAY return an
         HTTP 401 (Unauthorized) status code to indicate which HTTP
         authentication schemes are supported.  If the client attempted
         to authenticate via the Authorization request header field, the
         authorization server MUST respond with an HTTP 401
         (Unauthorized) status code and include the WWW-Authenticate
         response header field matching the authentication scheme used
         by the client.

      "invalid_grant":  The provided authorization grant (e.g.,
         authorization code, resource owner credentials) or refresh
         token is invalid, expired, revoked, does not match the redirect
         URI used in the authorization request, or was issued to another
         client.

      "unauthorized_client":  The authenticated client is not authorized
         to use this authorization grant type.

      "unsupported_grant_type":  The authorization grant type is not
         supported by the authorization server.

      "invalid_scope":  The requested scope is invalid, unknown,
         malformed, or exceeds the scope granted by the resource owner.

      Values for the error parameter MUST NOT include characters outside
      the set %x20-21 / %x23-5B / %x5D-7E.

   "error_description":  OPTIONAL.  Human-readable ASCII [USASCII] text





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      providing additional information, used to assist the client
      developer in understanding the error that occurred.  Values for
      the error_description parameter MUST NOT include characters
      outside the set %x20-21 / %x23-5B / %x5D-7E.

   "error_uri":  OPTIONAL.  A URI identifying a human-readable web page
      with information about the error, used to provide the client
      developer with additional information about the error.  Values for
      the error_uri parameter MUST conform to the URI-reference syntax
      and thus MUST NOT include characters outside the set %x21 /
      %x23-5B / %x5D-7E.

   The parameters are included in the content of the HTTP response using
   the application/json media type as defined in Appendix C.3.

   For example:

   HTTP/1.1 400 Bad Request
   Content-Type: application/json
   Cache-Control: no-store

   {
    "error": "invalid_request"
   }

4.  Grant Types

   To request an access token, the client obtains authorization from the
   resource owner.  This specification defines the following
   authorization grant types:

   *  authorization code

   *  client credentials, and

   *  refresh token

   It also provides an extension mechanism for defining additional grant
   types.

4.1.  Authorization Code Grant

   The authorization code grant type is used to obtain both access
   tokens and refresh tokens.

   The grant type uses the additional authorization endpoint to let the
   authorization server interact with the resource owner in order to get
   consent for resource access.



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   Since this is a redirect-based flow, the client must be capable of
   initiating the flow with the resource owner's user agent (typically a
   web browser) and capable of being redirected back to from the
   authorization server.

    +----------+
    | Resource |
    |   Owner  |
    +----------+
          ^
          |
          |
    +-----|----+          Client Identifier      +---------------+
    | .---+---------(1)-- & Redirect URI ------->|               |
    | |   |    |                                 |               |
    | |   '---------(2)-- User authenticates --->|               |
    | | User-  |                                 | Authorization |
    | | Agent  |                                 |     Server    |
    | |        |                                 |               |
    | |    .--------(3)-- Authorization Code ---<|               |
    +-|----|---+                                 +---------------+
      |    |                                         ^      v
      |    |                                         |      |
      ^    v                                         |      |
    +---------+                                      |      |
    |         |>---(4)-- Authorization Code ---------'      |
    |  Client |          & Redirect URI                     |
    |         |                                             |
    |         |<---(5)----- Access Token -------------------'
    +---------+       (w/ Optional Refresh Token)

                     Figure 3: Authorization Code Flow

   The flow illustrated in Figure 3 includes the following steps:

   (1) The client initiates the flow by directing the resource owner's
   user agent to the authorization endpoint.  The client includes its
   client identifier, code challenge (derived from a generated code
   verifier), optional requested scope, optional local state, and a
   redirect URI to which the authorization server will send the user
   agent back once access is granted (or denied).

   (2) The authorization server authenticates the resource owner (via
   the user agent) and establishes whether the resource owner grants or
   denies the client's access request.






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   (3) Assuming the resource owner grants access, the authorization
   server redirects the user agent back to the client using the redirect
   URI provided earlier (in the request or during client registration).
   The redirect URI includes an authorization code and any local state
   provided by the client earlier.

   (4) The client requests an access token from the authorization
   server's token endpoint by including the authorization code received
   in the previous step, and including its code verifier.  When making
   the request, the client authenticates with the authorization server
   if it can.  The client includes the redirect URI used to obtain the
   authorization code for verification.

   (5) The authorization server authenticates the client when possible,
   validates the authorization code, validates the code verifier, and
   ensures that the redirect URI received matches the URI used to
   redirect the user agent to the client in step (3).  If valid, the
   authorization server responds back with an access token and,
   optionally, a refresh token.

4.1.1.  Authorization Request

   To begin the authorization request, the client builds the
   authorization request URI by adding parameters to the authorization
   server's authorization endpoint URI.  The client will eventually
   redirect the user agent to this URI to initiate the request.

   Clients use a unique secret, called the "code verifier", per
   authorization request to protect against authorization code injection
   and CSRF attacks.  The client first generates the code verifier, then
   derives the "code challenge" to include in the authorization request.
   The client uses the code verifier when exchanging the authorization
   code at the token endpoint to prove that the client using the
   authorization code is the same client that requested it.

   The client constructs the request URI by adding the following
   parameters to the query component of the authorization endpoint URI
   as described by Appendix C.1:

   "response_type":  REQUIRED.  The authorization endpoint supports
      different sets of request and response parameters.  The client
      determines the type of flow by using a certain response_type
      value.  This specification defines the value code, which must be
      used to signal that the client wants to use the authorization code
      flow.






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   Extension response types MAY contain a space-delimited (%x20) list of
   values, where the order of values does not matter (e.g., response
   type a b is the same as b a).  The meaning of such composite response
   types is defined by their respective specifications.

   Some extension response types are defined by [OpenID.Connect].

   If an authorization request is missing the response_type parameter,
   or if the response type is not understood, the authorization server
   MUST return an error response as described in Section 4.1.2.1.

   "client_id":  REQUIRED.  The client identifier as described in
      Section 2.2.

   "code_challenge":  REQUIRED unless the specific requirements of
      Section 7.5.1 are met.  Code challenge derived from the code
      verifier.

   "code_challenge_method":  OPTIONAL, defaults to plain if not present
      in the request.  Code verifier transformation method is S256 or
      plain.

   "redirect_uri":  OPTIONAL if only one redirect URI is registered for
      this client.  REQUIRED if multiple redirect URIs are registered
      for this client.  See Section 2.3.2.

   "scope":  OPTIONAL.  The scope of the access request as described by
      Section 1.4.1.

   "state":  OPTIONAL.  An opaque value used by the client to maintain
      state between the request and callback.  The authorization server
      includes this value when redirecting the user agent back to the
      client.

   The code_verifier is a unique high-entropy cryptographically random
   string generated for each authorization request, using the unreserved
   characters [A-Z] / [a-z] / [0-9] / "-" / "." / "_" / "~", with a
   minimum length of 43 characters and a maximum length of 128
   characters.

   The client stores the code_verifier temporarily, and calculates the
   code_challenge which it uses in the authorization request.

   ABNF for code_verifier is as follows.







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   code-verifier = 43*128unreserved
   unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
   ALPHA = %x41-5A / %x61-7A
   DIGIT = %x30-39

   Clients SHOULD use code challenge methods that do not expose the
   code_verifier in the authorization request.  Otherwise, attackers
   that can read the authorization request (cf. Attacker A4 in
   [RFC9700]) can break the security provided by this mechanism.
   Currently, S256 is the only such method.

   NOTE: The code verifier SHOULD have enough entropy to make it
   impractical to guess the value.  It is RECOMMENDED that the output of
   a suitable random number generator be used to create a 32-octet
   sequence.  The octet sequence is then base64url-encoded to produce a
   43-octet URL-safe string to use as the code verifier.

   The client then creates a code_challenge derived from the code
   verifier by using one of the following transformations on the code
   verifier:

   S256
     code_challenge = BASE64URL-ENCODE(SHA256(ASCII(code_verifier)))

   plain
     code_challenge = code_verifier

   If the client is capable of using S256, it MUST use S256, as S256 is
   Mandatory To Implement (MTI) on the server.  Clients are permitted to
   use plain only if they cannot support S256 for some technical reason,
   for example constrained environments that do not have a hashing
   function available, and know via out-of-band configuration or via
   Authorization Server Metadata [RFC8414] that the server supports
   plain.

   ABNF for code_challenge is as follows.

   code-challenge = 43*128unreserved
   unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
   ALPHA = %x41-5A / %x61-7A
   DIGIT = %x30-39

   The properties code_challenge and code_verifier are adopted from the
   OAuth 2.0 extension known as "Proof-Key for Code Exchange", or PKCE
   [RFC7636] where this technique was originally developed.

   Authorization servers MUST support the code_challenge and
   code_verifier parameters.



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   Clients MUST use code_challenge and code_verifier and authorization
   servers MUST enforce their use except under the conditions described
   in Section 7.5.1.  Even in this case, using and enforcing
   code_challenge and code_verifier as described above is still
   RECOMMENDED.

   The state and scope parameters SHOULD NOT include sensitive client or
   resource owner information in plain text, as they can be transmitted
   over insecure channels or stored insecurely.

   The client directs the resource owner to the constructed URI using an
   HTTP redirection, or by other means available to it via the user
   agent.

   For example, the client directs the user agent to make the following
   HTTPS request (with extra line breaks for display purposes only):

   GET /authorize?response_type=code&client_id=s6BhdRkqt3&state=xyz
       &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb
       &code_challenge=6fdkQaPm51l13DSukcAH3Mdx7_ntecHYd1vi3n0hMZY
       &code_challenge_method=S256 HTTP/1.1
   Host: server.example.com

   The authorization server validates the request to ensure that all
   required parameters are present and valid.

   In particular, the authorization server MUST validate the
   redirect_uri in the request if present, ensuring that it matches one
   of the registered redirect URIs previously established during client
   registration (Section 2).  When comparing the two URIs the
   authorization server MUST ensure that the two URIs are equal, see
   Section 6.2.1 of [RFC3986], Simple String Comparison, for details.
   The only exception is native apps using a localhost URI: In this
   case, the authorization server MUST allow variable port numbers as
   described in Section 7.3 of [RFC8252].

   If the request is valid, the authorization server authenticates the
   resource owner and obtains an authorization decision (by asking the
   resource owner or by establishing approval via other means).

   When a decision is established, the authorization server directs the
   user agent to the provided client redirect URI using an HTTP
   redirection response, or by other means available to it via the user
   agent.







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4.1.2.  Authorization Response

   If the resource owner grants the access request, the authorization
   server issues an authorization code and delivers it to the client by
   adding the following parameters to the query component of the
   redirect URI using the query string serialization described by
   Appendix C.1, unless specified otherwise by an extension:

   "code":  REQUIRED.  The authorization code is generated by the
      authorization server and opaque to the client.  The authorization
      code MUST expire shortly after it is issued to mitigate the risk
      of leaks.  A maximum authorization code lifetime of 10 minutes is
      RECOMMENDED.  The authorization code is bound to the client
      identifier, code challenge and redirect URI.

   "state":  REQUIRED if the state parameter was present in the client
      authorization request.  The exact value received from the client.

   "iss":  OPTIONAL.  The identifier of the authorization server which
      the client can use to prevent mix-up attacks, if the client
      interacts with more than one authorization server.  See
      Section 7.14 and [RFC9207] for additional details on when this
      parameter is necessary, and how the client can use it to prevent
      mix-up attacks.

   For example, the authorization server redirects the user agent by
   sending the following HTTP response:

 HTTP/1.1 302 Found
 Location: https://client.example.com/cb?code=SplxlOBeZQQYbYS6WxSbIA
           &state=xyz&iss=https%3A%2F%2Fauthorization-server.example.com

   The client MUST ignore unrecognized response parameters.  The
   authorization code string size is left undefined by this
   specification.  The client should avoid making assumptions about code
   value sizes.  The authorization server SHOULD document the size of
   any value it issues.

   The authorization server MUST associate the code_challenge and
   code_challenge_method values with the issued authorization code so
   the code challenge can be verified later.










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   The exact method that the server uses to associate the code_challenge
   with the issued code is out of scope for this specification.  The
   code challenge could be stored on the server and associated with the
   code there.  The code_challenge and code_challenge_method values may
   be stored in encrypted form in the code itself, but the server MUST
   NOT include the code_challenge value in a response parameter in a
   form that entities other than the AS can extract.

   Clients MUST prevent injection (replay) of authorization codes into
   the authorization response by attackers.  Using code_challenge and
   code_verifier prevents injection of authorization codes since the
   authorization server will reject a token request with a mismatched
   code_verifier.  See Section 7.5.1 for more details.

4.1.2.1.  Authorization Error Response

   If the request fails due to a missing, invalid, or mismatching
   redirect URI, or if the client identifier is missing or invalid, the
   authorization server MUST NOT redirect the user agent to the invalid
   redirect URI and SHOULD inform the resource owner of the error, for
   example by displaying a message to the user in their browser.

   An authorization server MUST reject requests without a code_challenge
   from public clients, and MUST reject such requests from other clients
   unless there is reasonable assurance that the client mitigates
   authorization code injection in other ways.  See Section 7.5.1 for
   details.

   If the server does not support the requested code_challenge_method
   transformation, the authorization endpoint MUST return the
   authorization error response with error value set to invalid_request.
   The error_description or the response of error_uri SHOULD explain the
   nature of error, e.g., transform algorithm not supported.

   If the resource owner denies the access request or if the request
   fails for reasons other than a missing or invalid redirect URI, the
   authorization server informs the client by redirecting the user agent
   to the redirect URI and adding the following parameters to the query
   component of the redirect URI as described by Appendix C.1:

   "error":  REQUIRED.  A single ASCII [USASCII] error code from the
      following:

      "invalid_request":  The request is missing a required parameter,
         includes an invalid parameter value, includes a parameter more
         than once, or is otherwise malformed.

      "unauthorized_client":  The client is not authorized to request an



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         authorization code using this method.

      "access_denied":  The resource owner or authorization server
         denied the request.

      "unsupported_response_type":  The authorization server does not
         support obtaining an authorization code using this method.

      "invalid_scope":  The requested scope is invalid, unknown, or
         malformed.

      "server_error":  The authorization server encountered an
         unexpected condition that prevented it from fulfilling the
         request.  (This error code is needed because a 500 Internal
         Server Error HTTP status code cannot be returned to the client
         via an HTTP redirect.)

      "temporarily_unavailable":  The authorization server is currently
         unable to handle the request due to a temporary overloading or
         maintenance of the server.  (This error code is needed because
         a 503 Service Unavailable HTTP status code cannot be returned
         to the client via an HTTP redirect.)

      Values for the error parameter MUST NOT include characters outside
      the set %x20-21 / %x23-5B / %x5D-7E.

   "error_description":  OPTIONAL.  Human-readable ASCII [USASCII] text
      providing additional information, used to assist the client
      developer in understanding the error that occurred.  Values for
      the error_description parameter MUST NOT include characters
      outside the set %x20-21 / %x23-5B / %x5D-7E.

   "error_uri":  OPTIONAL.  A URI identifying a human-readable web page
      with information about the error, used to provide the client
      developer with additional information about the error.  Values for
      the error_uri parameter MUST conform to the URI-reference syntax
      and thus MUST NOT include characters outside the set %x21 /
      %x23-5B / %x5D-7E.

   "state":  REQUIRED if a state parameter was present in the client
      authorization request.  The exact value received from the client.

   "iss":  OPTIONAL.  The identifier of the authorization server.  See
      Section 4.1.2 above for details.

   For example, the authorization server indicates the request was
   denied by redirecting the user agent with the following HTTP
   response:



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 HTTP/1.1 302 Found
 Location: https://client.example.com/cb?error=access_denied
           &state=xyz&iss=https%3A%2F%2Fauthorization-server.example.com

4.1.3.  Token Endpoint Extension

   The authorization grant type is identified at the token endpoint with
   the grant_type value of authorization_code.

   If this value is set, the following additional token request
   parameters beyond Section 3.2.2 are supported:

   "code":  REQUIRED.  The authorization code received from the
      authorization server.

   "code_verifier":  REQUIRED, if the code_challenge parameter was
      included in the authorization request.  MUST NOT be used
      otherwise.  The original code verifier string.

   "client_id":  REQUIRED, if the client is not authenticating with the
      authorization server as described in Section 3.2.1.

   The authorization server MUST return an access token only once for a
   given authorization code.

   If a second valid token request is made with the same authorization
   code as a previously successful token request, the authorization
   server MUST deny the request and SHOULD revoke (when possible) all
   access tokens and refresh tokens previously issued based on that
   authorization code.  See Section 7.5.2 for further details.

   For example, the client makes the following HTTPS request (with extra
   line breaks for display purposes only):

 POST /token HTTP/1.1
 Host: server.example.com
 Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
 Content-Type: application/x-www-form-urlencoded

 grant_type=authorization_code
 &code=SplxlOBeZQQYbYS6WxSbIA
 &code_verifier=3641a2d12d66101249cdf7a79c000c1f8c05d2aafcf14bf146497bed

   In addition to the processing rules in Section 3.2.2, the
   authorization server MUST:






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   *  ensure that the authorization code was issued to the authenticated
      confidential client, or if the client is public, ensure that the
      code was issued to client_id in the request,

   *  verify that the authorization code is valid,

   *  verify that the code_verifier parameter is present if and only if
      a code_challenge parameter was present in the authorization
      request,

   *  if a code_verifier is present, verify the code_verifier by
      calculating the code challenge from the received code_verifier and
      comparing it with the previously associated code_challenge, after
      first transforming it according to the code_challenge_method
      method specified by the client, and

   *  If there was no code_challenge in the authorization request
      associated with the authorization code in the token request, the
      authorization server MUST reject the token request.

   See Section 10.2 for details on backwards compatibility with OAuth
   2.0 clients regarding the redirect_uri parameter in the token
   request.

4.2.  Client Credentials Grant

   The client can request an access token using only its client
   credentials (or other supported means of authentication) when the
   client is requesting access to the protected resources under its
   control, or those of another resource owner that have been previously
   arranged with the authorization server (the method of which is beyond
   the scope of this specification).

   The client credentials grant type MUST only be used by confidential
   clients.

        +---------+                                  +---------------+
        |         |                                  |               |
        |         |>--(1)- Client Authentication --->| Authorization |
        | Client  |                                  |     Server    |
        |         |<--(2)---- Access Token ---------<|               |
        |         |                                  |               |
        +---------+                                  +---------------+

                     Figure 4: Client Credentials Grant

   The use of the client credentials grant illustrated in Figure 4
   includes the following steps:



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   (1) The client authenticates with the authorization server and
   requests an access token from the token endpoint.

   (2) The authorization server authenticates the client, and if valid,
   issues an access token.

4.2.1.  Token Endpoint Extension

   The client credentials grant type is identified at the token endpoint
   with the grant_type value of client_credentials.

   If this value is set, the following additional token request
   parameters beyond Section 3.2.2 are supported:

   "scope":  OPTIONAL.  The scope of the access request as described by
      Section 1.4.1.

   For example, the client makes the following HTTP request using
   transport-layer security (with extra line breaks for display purposes
   only):

   POST /token HTTP/1.1
   Host: server.example.com
   Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
   Content-Type: application/x-www-form-urlencoded

   grant_type=client_credentials

   The authorization server MUST authenticate the client.

4.3.  Refresh Token Grant

   The refresh token is a credential issued by the authorization server
   to a client, which can be used to obtain new (fresh) access tokens
   based on an existing grant.  The client uses this option either
   because the previous access token has expired or the client
   previously obtained an access token with a scope more narrow than
   approved by the respective grant and later requires an access token
   with a different scope under the same grant.

   Refresh tokens MUST be kept confidential in transit and storage, and
   shared only among the authorization server and the client to whom the
   refresh tokens were issued.  The authorization server MUST maintain
   the binding between a refresh token and the client to whom it was
   issued.






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   The authorization server MUST verify the binding between the refresh
   token and client identity whenever the client identity can be
   authenticated.  When client authentication is not possible, the
   authorization server SHOULD issue sender-constrained refresh tokens
   or use refresh token rotation as described in Section 4.3.1.

   The authorization server MUST ensure that refresh tokens cannot be
   generated, modified, or guessed to produce valid refresh tokens by
   unauthorized parties.

4.3.1.  Token Endpoint Extension

   The refresh token grant type is identified at the token endpoint with
   the grant_type value of refresh_token.

   If this value is set, the following additional parameters beyond
   Section 3.2.2 are supported:

   "refresh_token":  REQUIRED.  The refresh token issued to the client.

   "scope":  OPTIONAL.  The scope of the access request as described by
      Section 1.4.1.  The requested scope MUST NOT include any scope not
      originally granted by the resource owner, and if omitted is
      treated as equal to the scope originally granted by the resource
      owner.

   Because refresh tokens are typically long-lasting credentials used to
   request additional access tokens, the refresh token is bound to the
   client to which it was issued.  Confidential clients MUST
   authenticate with the authorization server as described in
   Section 3.2.1.

   For example, the client makes the following HTTP request using
   transport-layer security (with extra line breaks for display purposes
   only):

   POST /token HTTP/1.1
   Host: server.example.com
   Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
   Content-Type: application/x-www-form-urlencoded

   grant_type=refresh_token&refresh_token=tGzv3JOkF0XG5Qx2TlKWIA

   In addition to the processing rules in Section 3.2.2, the
   authorization server MUST:






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   *  if client authentication is included in the request, ensure that
      the refresh token was issued to the authenticated client, OR if a
      client_id is included in the request, ensure the refresh token was
      issued to the matching client

   *  validate that the grant corresponding to this refresh token is
      still active

   *  validate the refresh token

   Authorization servers MUST utilize one of these methods to detect
   refresh token replay by malicious actors for public clients:

   *  _Sender-constrained refresh tokens:_ the authorization server
      cryptographically binds the refresh token to a certain client
      instance, e.g., by utilizing DPoP [RFC9449] or mTLS [RFC8705].

   *  _Refresh token rotation:_ the authorization server issues a new
      refresh token with every access token refresh response.  The
      previous refresh token is invalidated but information about the
      relationship is retained by the authorization server.  If a
      refresh token is compromised and subsequently used by both the
      attacker and the legitimate client, one of them will present an
      invalidated refresh token, which will inform the authorization
      server of the breach.  The authorization server cannot determine
      which party submitted the invalid refresh token, but it will
      revoke the active refresh token as well as the access
      authorization grant associated with it.  This stops the attack at
      the cost of forcing the legitimate client to obtain a fresh
      authorization grant.

   Implementation note: the grant to which a refresh token belongs may
   be encoded into the refresh token itself.  This can enable an
   authorization server to efficiently determine the grant to which a
   refresh token belongs, and by extension, all refresh tokens that need
   to be revoked.  Authorization servers MUST ensure the integrity of
   the refresh token value in this case, for example, using signatures.

4.3.2.  Refresh Token Response

   If valid and authorized, the authorization server issues an access
   token as described in Section 3.2.3.

   The authorization server MAY issue a new refresh token, in which case
   the client MUST discard the old refresh token and replace it with the
   new refresh token.





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4.3.3.  Refresh Token Recommendations

   The authorization server MAY revoke the old refresh token after
   issuing a new refresh token to the client.  If a new refresh token is
   issued, the refresh token scope MUST be identical to that of the
   refresh token included by the client in the request.

   Authorization servers MAY revoke refresh tokens automatically in case
   of a security event, such as:

   *  password change

   *  logout at the authorization server

   Refresh tokens SHOULD expire if the client has been inactive for some
   time, i.e., the refresh token has not been used to obtain new access
   tokens for some time.  The expiration time is at the discretion of
   the authorization server.  It might be a global value or determined
   based on the client policy or the grant associated with the refresh
   token (and its sensitivity).

4.4.  Extension Grants

   The client uses an extension grant type by specifying the grant type
   using an absolute URI (defined by the authorization server) as the
   value of the grant_type parameter of the token endpoint, and by
   adding any additional parameters necessary.

   For example, to request an access token using the Device
   Authorization Grant as defined by [RFC8628] after the user has
   authorized the client on a separate device, the client makes the
   following HTTPS request (with extra line breaks for display purposes
   only):

     POST /token HTTP/1.1
     Host: server.example.com
     Content-Type: application/x-www-form-urlencoded

     grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Adevice_code
     &device_code=GmRhmhcxhwEzkoEqiMEg_DnyEysNkuNhszIySk9eS
     &client_id=C409020731

   If the access token request is valid and authorized, the
   authorization server issues an access token and optional refresh
   token as described in Section 3.2.3.  If the request failed client
   authentication or is invalid, the authorization server returns an
   error response as described in Section 3.2.4.




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5.  Resource Requests

   The client accesses protected resources by presenting an access token
   to the resource server.  The resource server MUST validate the access
   token and ensure that it has not expired and that its scope covers
   the requested resource.  The methods used by the resource server to
   validate the access token are beyond the scope of this specification,
   but generally involve an interaction or coordination between the
   resource server and the authorization server.  For example, when the
   resource server and authorization server are colocated or are part of
   the same system, they may share a database or other storage; when the
   two components are operated independently, they may use Token
   Introspection [RFC7662] or a structured access token format such as a
   JWT [RFC9068].

5.1.  Bearer Token Requests

   This section defines two methods of sending Bearer tokens in resource
   requests to resource servers.  Clients MUST use one of the two
   methods defined below, and MUST NOT use more than one method to
   transmit the token in each request.

   In particular, clients MUST NOT send the access token in a URI query
   parameter, and resource servers MUST ignore access tokens in a URI
   query parameter.

5.1.1.  Authorization Request Header Field

   When sending the access token in the Authorization request header
   field defined by HTTP/1.1 [RFC7235], the client uses the Bearer
   scheme to transmit the access token.

   For example:

    GET /resource HTTP/1.1
    Host: server.example.com
    Authorization: Bearer mF_9.B5f-4.1JqM

   The syntax of the Authorization header field for this scheme follows
   the usage of the Basic scheme defined in Section 2 of [RFC2617].
   Note that, as with Basic, it does not conform to the generic syntax
   defined in Section 1.2 of [RFC2617] but is compatible with the
   general authentication framework in HTTP 1.1 Authentication
   [RFC7235], although it does not follow the preferred practice
   outlined therein in order to reflect existing deployments.  The
   syntax for Bearer credentials is as follows:





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   token68    = 1*( ALPHA / DIGIT /
                    "-" / "." / "_" / "~" / "+" / "/" ) *"="
   credentials = "bearer" 1*SP token68

   Clients SHOULD make authenticated requests with a bearer token using
   the Authorization request header field with the Bearer HTTP
   authorization scheme.  Resource servers MUST support this method.

   As described in Section 11.1 of [RFC9110], the string bearer is case-
   insensitive.  This means all of the following are valid uses of the
   Authorization header:

   *  Authorization: Bearer mF_9.B5f-4.1JqM

   *  Authorization: bearer mF_9.B5f-4.1JqM

   *  Authorization: BEARER mF_9.B5f-4.1JqM

   *  Authorization: bEaReR mF_9.B5f-4.1JqM

5.1.2.  Form-Encoded Content Parameter

   When sending the access token in the HTTP request content, the client
   adds the access token to the request content using the access_token
   parameter.  The client MUST NOT use this method unless all of the
   following conditions are met:

   *  The HTTP request includes the Content-Type header field set to
      application/x-www-form-urlencoded.

   *  The content follows the encoding requirements of the application/
      x-www-form-urlencoded content-type as defined by the URL Living
      Standard [WHATWG.URL].

   *  The HTTP request content is single-part.

   *  The content to be encoded in the request MUST consist entirely of
      ASCII [USASCII] characters.

   *  The HTTP request method is one for which the content has defined
      semantics.  In particular, this means that the GET method MUST NOT
      be used.

   The content MAY include other request-specific parameters, in which
   case the access_token parameter MUST be properly separated from the
   request-specific parameters using & character(s) (ASCII code 38).





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   For example, the client makes the following HTTP request using
   transport-layer security:

   POST /resource HTTP/1.1
   Host: server.example.com
   Content-Type: application/x-www-form-urlencoded

   access_token=mF_9.B5f-4.1JqM

   The application/x-www-form-urlencoded method SHOULD NOT be used
   except in application contexts where participating clients do not
   have access to the Authorization request header field.  Resource
   servers MAY support this method.

5.2.  Access Token Validation

   After receiving the access token, the resource server MUST check that
   the access token is not yet expired, is authorized to access the
   requested resource, was issued with the appropriate scope, and meets
   other policy requirements of the resource server to access the
   protected resource.

   Access tokens generally fall into two categories: reference tokens or
   self-encoded tokens.  Reference tokens can be validated by querying
   the authorization server or looking up the token in a token database,
   whereas self-encoded tokens contain the authorization information in
   an encrypted and/or signed string which can be extracted by the
   resource server.

   A standardized method to query the authorization server to check the
   validity of an access token is defined in Token Introspection
   [RFC7662].

   A standardized method of encoding information in a token string is
   defined in JWT Profile for Access Tokens [RFC9068].

   See Section 7.1 for additional considerations around creating and
   validating access tokens.

5.3.  Error Response

   If a resource access request fails, the resource server SHOULD inform
   the client of the error.  The details of the error response is
   determined by the particular token type, such as the description of
   Bearer tokens in Section 5.3.2.






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5.3.1.  The WWW-Authenticate Response Header Field

   If the protected resource request does not include authentication
   credentials or does not contain an access token that enables access
   to the protected resource, the resource server MUST include the HTTP
   WWW-Authenticate response header field; it MAY include it in response
   to other conditions as well.  The WWW-Authenticate header field uses
   the framework defined by HTTP/1.1 [RFC7235].

   All challenges for this token type MUST use the auth-scheme value
   Bearer.  This scheme MUST be followed by one or more auth-param
   values.  The auth-param attributes used or defined by this
   specification for this token type are as follows.  Other auth-param
   attributes MAY be used as well.

   "realm":  A realm attribute MAY be included to indicate the scope of
      protection in the manner described in HTTP/1.1 [RFC7235].  The
      realm attribute MUST NOT appear more than once.

   "scope":  The scope attribute is defined in Section 1.4.1.  The scope
      attribute is a space-delimited list of case-sensitive scope values
      indicating the required scope of the access token for accessing
      the requested resource. scope values are implementation defined;
      there is no centralized registry for them; allowed values are
      defined by the authorization server.  The order of scope values is
      not significant.  In some cases, the scope value will be used when
      requesting a new access token with sufficient scope of access to
      utilize the protected resource.  Use of the scope attribute is
      OPTIONAL.  The scope attribute MUST NOT appear more than once.
      The scope value is intended for programmatic use and is not meant
      to be displayed to end users.

      Two example scope values follow; these are taken from the OpenID
      Connect [OpenID.Messages] and the Open Authentication Technology
      Committee (OATC) Online Multimedia Authorization Protocol [OMAP]
      OAuth 2.0 use cases, respectively:

      scope="openid profile email"
      scope="urn:example:channel=HBO&urn:example:rating=G,PG-13"

   "error":  If the protected resource request included an access token
      and failed authentication, the resource server SHOULD include the
      error attribute to provide the client with the reason why the
      access request was declined.  The parameter value is described in
      Section 5.3.2.

   "error_description":  The resource server MAY include the




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      error_description attribute to provide developers a human-readable
      explanation that is not meant to be displayed to end users.

   "error_uri":  The resource server MAY include the error_uri attribute
      with an absolute URI identifying a human-readable web page
      explaining the error.

   The error, error_description, and error_uri attributes MUST NOT
   appear more than once.

   Values for the scope attribute (specified in Appendix A.4) MUST NOT
   include characters outside the set %x21 / %x23-5B / %x5D-7E for
   representing scope values and %x20 for delimiters between scope
   values.  Values for the error and error_description attributes
   (specified in Appendix A.7 and Appendix A.8) MUST NOT include
   characters outside the set %x20-21 / %x23-5B / %x5D-7E.  Values for
   the error_uri attribute (specified in Appendix A.9 of) MUST conform
   to the URI-reference syntax and thus MUST NOT include characters
   outside the set %x21 / %x23-5B / %x5D-7E.

5.3.2.  Error Codes

   When a request fails, the resource server responds using the
   appropriate HTTP status code (typically, 400, 401, 403, or 405) and
   includes one of the following error codes in the response:

   "invalid_request":  The request is missing a required parameter,
      includes an unsupported parameter or parameter value, repeats the
      same parameter, uses more than one method for including an access
      token, or is otherwise malformed.  The resource server SHOULD
      respond with the HTTP 400 (Bad Request) status code.

   "invalid_token":  The access token provided is expired, revoked,
      malformed, or invalid for other reasons.  The resource server
      SHOULD respond with the HTTP 401 (Unauthorized) status code.  The
      client MAY request a new access token and retry the protected
      resource request.

   "insufficient_scope":  The request requires higher privileges
      (scopes) than provided by the scopes granted to the client and
      represented by the access token.  The resource server SHOULD
      respond with the HTTP 403 (Forbidden) status code and MAY include
      the scope attribute with the scope necessary to access the
      protected resource.

   Extensions may define additional error codes or specify additional
   circumstances in which the above error codes are returned.




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   If the request lacks any authentication information (e.g., the client
   was unaware that authentication is necessary or attempted using an
   unsupported authentication method), the resource server SHOULD NOT
   include an error code or other error information.

   For example:

   HTTP/1.1 401 Unauthorized
   WWW-Authenticate: Bearer realm="example"

   And in response to a protected resource request with an
   authentication attempt using an expired access token:

   HTTP/1.1 401 Unauthorized
   WWW-Authenticate: Bearer realm="example",
                     error="invalid_token",
                     error_description="The access token expired"

6.  Extensibility

6.1.  Defining Access Token Types

   Access token types can be defined in one of two ways: registered in
   the Access Token Types registry (following the procedures in
   Section 11.1 of [RFC6749]), or by using a unique absolute URI as its
   name.

6.1.1.  Registered Access Token Types

   [RFC6750] establishes a common registry in Section 11.4 of [RFC6749]
   for error values to be shared among OAuth token authentication
   schemes.

   New authentication schemes designed primarily for OAuth token
   authentication SHOULD define a mechanism for providing an error
   status code to the client, in which the error values allowed are
   registered in the error registry established by this specification.

   Such schemes MAY limit the set of valid error codes to a subset of
   the registered values.  If the error code is returned using a named
   parameter, the parameter name SHOULD be error.

   Other schemes capable of being used for OAuth token authentication,
   but not primarily designed for that purpose, MAY bind their error
   values to the registry in the same manner.






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   New authentication schemes MAY choose to also specify the use of the
   error_description and error_uri parameters to return error
   information in a manner parallel to their usage in this
   specification.

   Type names MUST conform to the type-name ABNF.  If the type
   definition includes a new HTTP authentication scheme, the type name
   SHOULD be identical to the HTTP authentication scheme name (as
   defined by [RFC2617]).  The token type example is reserved for use in
   examples.

   type-name  = 1*name-char
   name-char  = "-" / "." / "_" / DIGIT / ALPHA

6.1.2.  Vendor-Specific Access Token Types

   Types utilizing a URI name SHOULD be limited to vendor-specific
   implementations that are not commonly applicable, and are specific to
   the implementation details of the resource server where they are
   used.

   All other types MUST be registered.

6.2.  Defining New Endpoint Parameters

   New request or response parameters for use with the authorization
   endpoint or the token endpoint are defined and registered in the
   OAuth Parameters registry following the procedure in Section 11.2 of
   [RFC6749].

   Parameter names MUST conform to the param-name ABNF, and parameter
   values syntax MUST be well-defined (e.g., using ABNF, or a reference
   to the syntax of an existing parameter).

   param-name  = 1*name-char
   name-char   = "-" / "." / "_" / DIGIT / ALPHA

   Unregistered vendor-specific parameter extensions that are not
   commonly applicable and that are specific to the implementation
   details of the authorization server where they are used SHOULD
   utilize a vendor-specific prefix that is not likely to conflict with
   other registered values (e.g., begin with 'companyname_').









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6.3.  Defining New Authorization Grant Types

   New authorization grant types can be defined by assigning them a
   unique absolute URI for use with the grant_type parameter.  If the
   extension grant type requires additional token endpoint parameters,
   they MUST be registered in the OAuth Parameters registry as described
   by Section 11.2 of [RFC6749].

6.4.  Defining New Authorization Endpoint Response Types

   New response types for use with the authorization endpoint are
   defined and registered in the Authorization Endpoint Response Types
   registry following the procedure in Section 11.3 of [RFC6749].
   Response type names MUST conform to the response-type ABNF.

   response-type  = response-name *( SP response-name )
   response-name  = 1*response-char
   response-char  = "_" / DIGIT / ALPHA

   If a response type contains one or more space characters (%x20), it
   is compared as a space-delimited list of values in which the order of
   values does not matter.  Only one order of values can be registered,
   which covers all other arrangements of the same set of values.

   For example, an extension can define and register the code
   other_token response type.  Once registered, the same combination
   cannot be registered as other_token code, but both values can be used
   to denote the same response type.

6.5.  Defining Additional Error Codes

   In cases where protocol extensions (i.e., access token types,
   extension parameters, or extension grant types) require additional
   error codes to be used with the authorization code grant error
   response (Section 4.1.2.1), the token error response (Section 3.2.4),
   or the resource access error response (Section 5.3), such error codes
   MAY be defined.

   Extension error codes MUST be registered (following the procedures in
   Section 11.4 of [RFC6749]) if the extension they are used in
   conjunction with is a registered access token type, a registered
   endpoint parameter, or an extension grant type.  Error codes used
   with unregistered extensions MAY be registered.

   Error codes MUST conform to the error ABNF and SHOULD be prefixed by
   an identifying name when possible.  For example, an error identifying
   an invalid value set to the extension parameter example SHOULD be
   named example_invalid.



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   error      = 1*error-char
   error-char = %x20-21 / %x23-5B / %x5D-7E

7.  Security Considerations

   As a flexible and extensible framework, OAuth's security
   considerations depend on many factors.  The following sections
   provide implementers with security guidelines focused on the three
   client profiles described in Section 2.1: web application, browser-
   based application, and native application.

   A comprehensive OAuth security model and analysis, as well as
   background for the protocol design, is provided by [RFC6819] and
   [RFC9700].

7.1.  Access Token Security Considerations

7.1.1.  Security Threats

   The following list presents several common threats against protocols
   utilizing some form of tokens.  This list of threats is based on NIST
   Special Publication 800-63 [NIST800-63].

7.1.1.1.  Access token manufacture/modification

   An attacker may generate a bogus access token or modify the token
   contents (such as the authentication or attribute statements) of an
   existing token, causing the resource server to grant inappropriate
   access to the client.  For example, an attacker may modify the token
   to extend the validity period; a malicious client may modify the
   assertion to gain access to information that they should not be able
   to view.

7.1.1.2.  Access token information disclosure

   Access tokens may contain authentication and attribute statements
   that include sensitive information.

   If the client should be prevented from observing the contents of the
   access token, content encryption MUST be applied.

   Since cookies are by default transmitted in cleartext, any
   information contained in them is at risk of disclosure: Bearer tokens
   MUST NOT be stored in cookies that can be sent in the clear.  See
   Section 7 and 8 of [RFC6265] for security considerations about
   cookies.





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7.1.1.3.  Access token redirect

   An attacker uses an access token generated for consumption by one
   resource server to gain access to a different resource server that
   mistakenly believes the token to be for it.

7.1.1.4.  Access token replay

   An attacker attempts to use an access token that has already been
   used with that resource server in the past.

7.1.2.  Threat Mitigation

   A large range of threats can be mitigated by protecting the contents
   of the access token by using a digital signature, and by following
   best practices for signing key management such as periodic key
   rotation.

   Alternatively, a bearer token can contain a reference to
   authorization information, rather than encoding the information
   directly.  Using a reference may require an extra interaction between
   a resource server and authorization server to resolve the reference
   to the authorization information.  The mechanics of such an
   interaction are not defined by this specification, but one such
   mechanism is defined in Token Introspection [RFC7662].

   This document does not specify the encoding or the contents of the
   access token; hence, detailed recommendations about the means of
   guaranteeing access token integrity protection are outside the scope
   of this specification.  One example of an encoding and signing
   mechanism for access tokens is described in JSON Web Token Profile
   for Access Tokens [RFC9068].

   To deal with access token redirects, it is important for the
   authorization server to include the identity of the intended
   recipients (the audience), typically a single resource server (or a
   list of resource servers), in the token.  Restricting the use of the
   token to a specific scope is also RECOMMENDED.

   Section 1.5 provides information to protect against access token
   disclosure and providing confidentiality and integrity for the
   communications between client, resource server and authorization
   server.

7.1.3.  Summary of Recommendations






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7.1.3.1.  Safeguard bearer tokens

   Client implementations MUST ensure that bearer tokens are not leaked
   to unintended parties, as they will be able to use them to gain
   access to protected resources.  This is the primary security
   consideration when using bearer tokens and underlies all the more
   specific recommendations that follow.

7.1.3.2.  Validate TLS certificate chains

   The client MUST validate the TLS certificate chain when making
   requests to protected resources.  Failing to do so may enable DNS
   hijacking attacks to steal the token and gain unintended access.

7.1.3.3.  Always use TLS (https)

   Clients MUST always use TLS (https) or equivalent transport security
   when making requests with bearer tokens.  Failing to do so exposes
   the token to numerous attacks that could give attackers unintended
   access.

7.1.3.4.  Don't store bearer tokens in HTTP cookies

   Implementations MUST NOT store bearer tokens within cookies that can
   be sent in the clear (which is the default transmission mode for
   cookies).  Implementations that do store bearer tokens in cookies
   MUST take precautions against cross-site request forgery.

7.1.3.5.  Issue short-lived bearer tokens

   Authorization servers SHOULD issue short-lived bearer tokens,
   particularly when issuing tokens to clients that run within a web
   browser or other environments where information leakage may occur.
   Using short-lived bearer tokens can reduce the impact of them being
   leaked.

7.1.3.6.  Issue scoped bearer tokens

   Authorization servers SHOULD issue bearer tokens that contain an
   audience restriction, scoping their use to the intended resource
   server or set of resource servers.










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7.1.3.7.  Don't pass bearer tokens in page URLs

   Bearer tokens MUST NOT be passed in page URLs (for example, as query
   string parameters).  Instead, bearer tokens SHOULD be passed in HTTP
   message headers or message bodies for which confidentiality measures
   are taken.  Browsers, web servers, and other software may not
   adequately secure URLs in the browser history, web server logs, and
   other data structures.  If bearer tokens are passed in page URLs,
   attackers might be able to steal them from the history data, logs, or
   other unsecured locations.

7.1.4.  Access Token Privilege Restriction

   The privileges associated with an access token SHOULD be restricted
   to the minimum required for the particular application or use case.
   This prevents clients from exceeding the privileges authorized by the
   resource owner.  It also prevents users from exceeding their
   privileges authorized by the respective security policy.  Privilege
   restrictions also help to reduce the impact of access token leakage.

   In particular, access tokens SHOULD be restricted to certain resource
   servers (audience restriction), preferably to a single resource
   server.  To put this into effect, the authorization server associates
   the access token with certain resource servers and every resource
   server is obliged to verify, for every request, whether the access
   token sent with that request was meant to be used for that particular
   resource server.  If not, the resource server MUST refuse to serve
   the respective request.  Clients and authorization servers MAY
   utilize the parameters scope or resource as specified in this
   document and [RFC8707], respectively, to determine the resource
   server they want to access.

   Additionally, access tokens SHOULD be restricted to certain resources
   and actions on resource servers or resources.  To put this into
   effect, the authorization server associates the access token with the
   respective resource and actions and every resource server is obliged
   to verify, for every request, whether the access token sent with that
   request was meant to be used for that particular action on the
   particular resource.  If not, the resource server must refuse to
   serve the respective request.  Clients and authorization servers MAY
   utilize the parameter scope and authorization_details as specified in
   [RFC9396] to determine those resources and/or actions.

7.2.  Client Authentication

   Depending on the overall process of client registration and
   credential lifecycle management, this may affect the confidence an
   authorization server has in a particular client.



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   For example, authentication of a dynamically registered client does
   not prove the identity of the client, it only ensures that repeated
   requests to the authorization server were made from the same client
   instance.  Such clients may be limited in terms of which scopes they
   are allowed to request, or may have other limitations such as shorter
   token lifetimes.

   In contrast, if there is a registered application whose developer's
   identity was verified, who signed a contract and is issued a client
   secret that is only used in a secure backend service, the
   authorization server might allow this client to request more
   sensitive scopes or to be issued longer-lasting tokens.

7.3.  Client Impersonation

   If a confidential client has its credentials stolen, a malicious
   client can impersonate the client and obtain access to protected
   resources.

   The authorization server SHOULD enforce explicit resource owner
   authentication and provide the resource owner with information about
   the client and the requested authorization scope and lifetime.  It is
   up to the resource owner to review the information in the context of
   the current client and to authorize or deny the request.

   The authorization server SHOULD NOT process repeated authorization
   requests automatically (without active resource owner interaction)
   without authenticating the client or relying on other measures to
   ensure that the repeated request comes from the original client and
   not an impersonator.

7.3.1.  Impersonation of Native Apps

   As stated above, the authorization server SHOULD NOT process
   authorization requests automatically without user consent or
   interaction, except when the identity of the client can be assured.
   This includes the case where the user has previously approved an
   authorization request for a given client ID -- unless the identity of
   the client can be proven, the request SHOULD be processed as if no
   previous request had been approved.

   Measures such as claimed https scheme redirects MAY be accepted by
   authorization servers as identity proof.  Some operating systems may
   offer alternative platform-specific identity features that MAY be
   accepted, as appropriate.






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7.3.2.  Access Token Privilege Restriction

   The client SHOULD request access tokens with the minimal scope
   necessary.  The authorization server SHOULD take the client identity
   into account when choosing how to honor the requested scope and MAY
   issue an access token with fewer scopes than requested.

   The privileges associated with an access token SHOULD be restricted
   to the minimum required for the particular application or use case.
   This prevents clients from exceeding the privileges authorized by the
   resource owner.  It also prevents users from exceeding their
   privileges authorized by the respective security policy.  Privilege
   restrictions also help to reduce the impact of access token leakage.

   In particular, access tokens SHOULD be restricted to certain resource
   servers (audience restriction), preferably to a single resource
   server.  To put this into effect, the authorization server associates
   the access token with certain resource servers and every resource
   server is obliged to verify, for every request, whether the access
   token sent with that request was meant to be used for that particular
   resource server.  If not, the resource server MUST refuse to serve
   the respective request.  Clients and authorization servers MAY
   utilize the parameters scope or resource as specified in [RFC8707],
   respectively, to determine the resource server they want to access.

7.4.  Client Impersonating Resource Owner

   Resource servers may make access control decisions based on the
   identity of a resource owner for which an access token was issued, or
   based on the identity of a client in the client credentials grant.
   If both options are possible, depending on the details of the
   implementation, a client's identity may be mistaken for the identity
   of a resource owner.  For example, if a client is able to choose its
   own client_id during registration with the authorization server, a
   malicious client may set it to a value identifying an end user (e.g.,
   a sub value if OpenID Connect is used).  If the resource server
   cannot properly distinguish between access tokens issued to clients
   and access tokens issued to end users, the client may then be able to
   access resource of the end user.












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   If the authorization server has a common namespace for client IDs and
   user identifiers, causing the resource server to be unable to
   distinguish an access token authorized by a resource owner from an
   access token authorized by a client itself, authorization servers
   SHOULD NOT allow clients to influence their client_id or any other
   Claim if that can cause confusion with a genuine resource owner.
   Where this cannot be avoided, authorization servers MUST provide
   other means for the resource server to distinguish between the two
   types of access tokens.

7.5.  Authorization Code Security Considerations

7.5.1.  Authorization Code Injection

   Authorization code injection is an attack where the client receives
   an authorization code from the attacker in its redirect URI instead
   of the authorization code from the legitimate authorization server.
   Without protections in place, there is no mechanism by which the
   client can know that the attack has taken place.  Authorization code
   injection can lead to both the attacker obtaining access to a
   victim's account, as well as a victim accidentally gaining access to
   the attacker's account.

7.5.1.1.  Countermeasures

   To prevent injection of authorization codes into the client, using
   code_challenge and code_verifier is REQUIRED for clients, and
   authorization servers MUST enforce their use, unless both of the
   following criteria are met:

   *  The client is a confidential client.

   *  In the specific deployment and the specific request, there is
      reasonable assurance by the authorization server that the client
      implements the OpenID Connect nonce mechanism properly.

   In this case, using and enforcing code_challenge and code_verifier is
   still RECOMMENDED.

   The code_challenge or OpenID Connect nonce value MUST be transaction-
   specific and securely bound to the client and the user agent in which
   the transaction was started.  If a transaction leads to an error,
   fresh values for code_challenge or nonce MUST be chosen.

   Relying on the client to validate the OpenID Connect nonce parameter
   means the authorization server has no way to confirm that the client
   has actually protected itself against authorization code injection
   attacks.  If an attacker is able to inject an authorization code into



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   a client, the client would still exchange the injected authorization
   code and obtain tokens, and would only later reject the ID token
   after validating the nonce and seeing that it doesn't match.  In
   contrast, the authorization server enforcing the code_challenge and
   code_verifier parameters provides a higher security outcome, since
   the authorization server is able to recognize the authorization code
   injection attack pre-emptively and avoid issuing any tokens in the
   first place.

   Historic note: Although PKCE [RFC7636] (where the code_challenge and
   code_verifier parameters were created) was originally designed as a
   mechanism to protect native apps from authorization code exfiltration
   attacks, all kinds of OAuth clients, including web applications and
   other confidential clients, are susceptible to authorization code
   injection attacks, which are solved by the code_challenge and
   code_verifier mechanism.

7.5.2.  Reuse of Authorization Codes

   Several types of attacks are possible if authorization codes are able
   to be used more than once.

   As described in Section 4.1.3, the authorization server must reject a
   token request and revoke any issued tokens when receiving a second
   valid request with an authorization code that has already been used
   to issue an access token.  If an attacker is able to exfiltrate an
   authorization code and use it before the legitimate client, the
   attacker will obtain the access token and the legitimate client will
   not.  Revoking any issued tokens means the attacker's tokens will
   then be revoked, stopping the attack from proceeding any further.

   However, the authorization server should only revoke issued tokens if
   the request containing the authorization code is also valid,
   including any other parameters such as the code_verifier and client
   authentication.  The authorization server SHOULD NOT revoke any
   issued tokens when receiving a replayed authorization code that
   contains invalid parameters.  If it were to do so, this would create
   a denial of service opportunity for an attacker who is able to obtain
   an authorization code but unable to obtain the client authentication
   or code_verifier by sending an invalid authorization code request
   before the legitimate client and thereby revoking the legitimate
   client's tokens once it makes the valid request.









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7.5.3.  HTTP 307 Redirect

   An authorization server which redirects a request that potentially
   contains user credentials MUST NOT use the 307 status code
   (Section 15.4.8 of [RFC9110]) for redirection.  If an HTTP
   redirection (and not, for example, JavaScript) is used for such a
   request, AS SHOULD use the status code 303 ("See Other").

   At the authorization endpoint, a typical protocol flow is that the AS
   prompts the user to enter their credentials in a form that is then
   submitted (using the POST method) back to the authorization server.
   The AS checks the credentials and, if successful, redirects the user
   agent to the client's redirect URI.

   If the status code 307 were used for redirection, the user agent
   would send the user credentials via a POST request to the client.

   This discloses the sensitive credentials to the client.  If the
   client is malicious, it can use the credentials to impersonate the
   user at the AS.

   The behavior might be unexpected for developers, but is defined in
   Section 15.4.8 of [RFC9110].  This status code does not require the
   user agent to rewrite the POST request to a GET request and thereby
   drop the form data in the POST request content.

   In HTTP [RFC9110], only the status code 303 unambiguously enforces
   rewriting the HTTP POST request to an HTTP GET request.  For all
   other status codes, including the popular 302, user agents can opt
   not to rewrite POST to GET requests and therefore reveal the user
   credentials to the client.  (In practice, however, most user agents
   will only show this behaviour for 307 redirects.)

7.6.  Ensuring Endpoint Authenticity

   The risk related to man-in-the-middle attacks is mitigated by the
   mandatory use of channel security mechanisms such as [RFC8446] for
   communicating with the Authorization and Token Endpoints.  See
   Section 1.5 for further details.

7.7.  Credentials-Guessing Attacks

   The authorization server MUST prevent attackers from guessing access
   tokens, authorization codes, refresh tokens, resource owner
   passwords, and client credentials.






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   The probability of an attacker guessing generated tokens (and other
   credentials not intended for handling by end users) MUST be less than
   or equal to 2^(-128) and SHOULD be less than or equal to 2^(-160).

   The authorization server MUST utilize other means to protect
   credentials intended for end-user usage.

7.8.  Phishing Attacks

   Wide deployment of this and similar protocols may cause end users to
   become inured to the practice of being redirected to websites where
   they are asked to enter their passwords.  If end users are not
   careful to verify the authenticity of these websites before entering
   their credentials, it will be possible for attackers to exploit this
   practice to steal resource owners' passwords, and other phishable
   credentials such as OTPs.

   Service providers should attempt to educate end users about the risks
   phishing attacks pose and should provide mechanisms that make it easy
   for end users to confirm the authenticity of their sites, such as
   using phishing-resistant authenticators, as phishing resistant
   authenticators will offer a credential to log in to a certain site to
   the user only if the platform has successfully verified the site's
   origin.  Client developers should consider the security implications
   of how they interact with the user agent (e.g., external, embedded),
   and the ability of the end user to verify the authenticity of the
   authorization server.

   See Section 1.5 for further details on mitigating the risk of
   phishing attacks.

7.9.  Cross-Site Request Forgery

   An attacker might attempt to inject a request to the redirect URI of
   the legitimate client on the victim's device, e.g., to cause the
   client to access resources under the attacker's control.  This is a
   variant of an attack known as Cross-Site Request Forgery (CSRF).

   The traditional countermeasure is that clients pass a random value,
   also known as a CSRF Token, in the state parameter that links the
   request to the redirect URI to the user agent session as described.
   This countermeasure is described in detail in Section 5.3.5 of
   [RFC6819].  The same protection is provided by the code_verifier
   parameter or the OpenID Connect nonce value.

   When using code_verifier instead of state or nonce for CSRF
   protection, it is important to note that:




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   *  Clients MUST ensure that the AS supports the code_challenge_method
      intended to be used by the client.  If an authorization server
      does not support the requested method, state or nonce MUST be used
      for CSRF protection instead.

   *  If state is used for carrying application state, and integrity of
      its contents is a concern, clients MUST protect state against
      tampering and swapping.  This can be achieved by binding the
      contents of state to the browser session and/or signed/encrypted
      state values [I-D.bradley-oauth-jwt-encoded-state].

   AS therefore MUST provide a way to detect their supported code
   challenge methods either via AS metadata according to [RFC8414] or
   provide a deployment-specific way to ensure or determine support.

7.10.  Clickjacking

   As described in Section 4.4.1.9 of [RFC6819], the authorization
   request is susceptible to clickjacking attacks, also called user
   interface redressing.  In such an attack, an attacker embeds the
   authorization endpoint user interface in an innocuous context.  A
   user believing to interact with that context, for example, clicking
   on buttons, inadvertently interacts with the authorization endpoint
   user interface instead.  The opposite can be achieved as well: A user
   believing to interact with the authorization endpoint might
   inadvertently type a password into an attacker-provided input field
   overlaid over the original user interface.  Clickjacking attacks can
   be designed such that users can hardly notice the attack, for example
   using almost invisible iframes overlaid on top of other elements.

   An attacker can use this vector to obtain the user's authentication
   credentials, change the scope of access granted to the client, and
   potentially access the user's resources.

   Authorization servers MUST prevent clickjacking attacks.  Multiple
   countermeasures are described in [RFC6819], including the use of the
   X-Frame-Options HTTP response header field and frame-busting
   JavaScript.  In addition to those, authorization servers SHOULD also
   use Content Security Policy (CSP) level 2 [CSP-2] or greater.












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   To be effective, CSP must be used on the authorization endpoint and,
   if applicable, other endpoints used to authenticate the user and
   authorize the client (e.g., the device authorization endpoint, login
   pages, error pages, etc.).  This prevents framing by unauthorized
   origins in user agents that support CSP.  The client MAY permit being
   framed by some other origin than the one used in its redirection
   endpoint.  For this reason, authorization servers SHOULD allow
   administrators to configure allowed origins for particular clients
   and/or for clients to register these dynamically.

   Using CSP allows authorization servers to specify multiple origins in
   a single response header field and to constrain these using flexible
   patterns (see [CSP-2] for details).  Level 2 of this standard
   provides a robust mechanism for protecting against clickjacking by
   using policies that restrict the origin of frames (using frame-
   ancestors) together with those that restrict the sources of scripts
   allowed to execute on an HTML page (by using script-src).  A non-
   normative example of such a policy is shown in the following listing:

   HTTP/1.1 200 OK
   Content-Security-Policy: frame-ancestors https://ext.example.org:8000
   Content-Security-Policy: script-src 'self'
   X-Frame-Options: ALLOW-FROM https://ext.example.org:8000
   ...

   Because some user agents do not support [CSP-2], this technique
   SHOULD be combined with others, including those described in
   [RFC6819], unless such legacy user agents are explicitly unsupported
   by the authorization server.  Even in such cases, additional
   countermeasures SHOULD still be employed.

7.11.  Injection and Input Validation

   An injection attack occurs when an input or otherwise external
   variable is used by an application unsanitized and causes
   modification to the application logic.  This may allow an attacker to
   gain access to the application device or its data, cause denial of
   service, or introduce a wide range of malicious side-effects.

   The authorization server and client MUST treat parameters received as
   potentially malicious external input and apply appropriate
   protections, in particular, the values of the state and redirect_uri
   parameters.








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7.12.  Open Redirection

   An open redirector is an endpoint that forwards a user's browser to
   an arbitrary URI obtained from a query parameter.  Such endpoints are
   sometimes implemented, for example, to show a message before a user
   is then redirected to an external website, or to redirect users back
   to a URL they were intending to visit before being interrupted, e.g.,
   by a login prompt.

   The following attacks can occur when an AS or client has an open
   redirector.

7.12.1.  Client as Open Redirector

   Clients MUST NOT expose open redirectors.  Attackers may use open
   redirectors to produce URLs pointing to the client and utilize them
   to exfiltrate authorization codes, as described in Section 4.1.1 of
   [RFC9700].  Another abuse case is to produce URLs that appear to
   point to the client.  This might trick users into trusting the URL
   and follow it in their browser.  This can be abused for phishing.

   In order to prevent open redirection, clients should only redirect if
   the target URLs are whitelisted or if the origin and integrity of a
   request can be authenticated.  Countermeasures against open
   redirection are described by OWASP [owasp_redir].

7.12.2.  Authorization Server as Open Redirector

   Just as with clients, attackers could try to utilize a user's trust
   in the authorization server (and its URL in particular) for
   performing phishing attacks.  OAuth authorization servers regularly
   redirect users to other web sites (the clients), but must do so
   safely.

   Section 4.1.2.1 already prevents open redirects by stating that the
   authorization server MUST NOT automatically redirect the user agent
   in case of an invalid combination of client_id and redirect_uri.

   However, an attacker could also utilize a correctly registered
   redirect URI to perform phishing attacks.  The attacker could, for
   example, register a client via dynamic client registration [RFC7591]
   and execute one of the following attacks:

   1.  Intentionally send an erroneous authorization request, e.g., by
       using an invalid scope value, thus instructing the authorization
       server to redirect the user agent to its phishing site.





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   2.  Intentionally send a valid authorization request with client_id
       and redirect_uri controlled by the attacker.  After the user
       authenticates, the authorization server prompts the user to
       provide consent to the request.  If the user notices an issue
       with the request and declines the request, the authorization
       server still redirects the user agent to the phishing site.  In
       this case, the user agent will be redirected to the phishing site
       regardless of the action taken by the user.

   3.  Intentionally send a valid silent authentication request
       (prompt=none) with client_id and redirect_uri controlled by the
       attacker.  In this case, the authorization server will
       automatically redirect the user agent to the phishing site.

   The authorization server MUST take precautions to prevent these
   threats.  The authorization server MUST always authenticate the user
   first and, with the exception of the silent authentication use case,
   prompt the user for credentials when needed, before redirecting the
   user.  Based on its risk assessment, the authorization server needs
   to decide whether it can trust the redirect URI or not.  It could
   take into account URI analytics done internally or through some
   external service to evaluate the credibility and trustworthiness
   content behind the URI, and the source of the redirect URI and other
   client data.

   The authorization server SHOULD only automatically redirect the user
   agent if it trusts the redirect URI.  If the URI is not trusted, the
   authorization server MAY inform the user and rely on the user to make
   the correct decision.

7.13.  Transport Security

   In some deployments, including those utilizing load balancers, the
   TLS connection to the resource server terminates prior to the actual
   server that provides the resource.  This could leave the token
   unprotected between the front-end server where the TLS connection
   terminates and the back-end server that provides the resource.  In
   such deployments, sufficient measures MUST be employed to ensure
   confidentiality of the access token between the front-end and back-
   end servers; encryption of the token is one such possible measure.

   See Section 17.2 of [RFC9110] for further information.









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7.14.  Authorization Server Mix-Up Mitigation

   Mix-up is an attack on scenarios where an OAuth client interacts with
   two or more authorization servers and at least one authorization
   server is under the control of the attacker.  This can be the case,
   for example, if the attacker uses dynamic registration to register
   the client at his own authorization server or if an authorization
   server becomes compromised.

   When an OAuth client can only interact with one authorization server,
   a mix-up defense is not required.  In scenarios where an OAuth client
   interacts with two or more authorization servers, however, clients
   MUST prevent mix-up attacks.  Two different methods are discussed in
   the following.

   For both defenses, clients MUST store, for each authorization
   request, the issuer they sent the authorization request to, bind this
   information to the user agent, and check that the authorization
   response was received from the correct issuer.  Clients MUST ensure
   that the subsequent access token request, if applicable, is sent to
   the same issuer.  The issuer serves, via the associated metadata, as
   an abstract identifier for the combination of the authorization
   endpoint and token endpoint that are to be used in the flow.  If an
   issuer identifier is not available, for example, if neither OAuth 2.0
   Authorization Server Metadata [RFC8414] nor OpenID Connect Discovery
   [OpenID.Discovery] are used, a different unique identifier for this
   tuple or the tuple itself can be used instead.  For brevity of
   presentation, such a deployment-specific identifier will be subsumed
   under the issuer (or issuer identifier) in the following.

   Note: Just storing the authorization server URL is not sufficient to
   identify mix-up attacks.  An attacker might declare an uncompromised
   AS's authorization endpoint URL as "their" AS URL, but declare a
   token endpoint under their own control.

   See Section 4.4 of [RFC9700] for a detailed description of several
   types of mix-up attacks.

7.14.1.  Mix-Up Defense via Issuer Identification

   This defense requires that the authorization server sends his issuer
   identifier in the authorization response to the client.  When
   receiving the authorization response, the client MUST compare the
   received issuer identifier to the stored issuer identifier.  If there
   is a mismatch, the client MUST abort the interaction.

   There are different ways this issuer identifier can be transported to
   the client:



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   *  The issuer information can be transported, for example, via an
      optional response parameter iss (see Section 4.1.2).

   *  When OpenID Connect is used and an ID Token is returned in the
      authorization response, the client can evaluate the iss claim in
      the ID Token.

   In both cases, the iss value MUST be evaluated according to
   [RFC9207].

   While this defense may require using an additional parameter to
   transport the issuer information, it is a robust and relatively
   simple defense against mix-up.

7.14.2.  Mix-Up Defense via Distinct Redirect URIs

   For this defense, clients MUST use a distinct redirect URI for each
   issuer they interact with.

   Clients MUST check that the authorization response was received from
   the correct issuer by comparing the distinct redirect URI for the
   issuer to the URI where the authorization response was received on.
   If there is a mismatch, the client MUST abort the flow.

   While this defense builds upon existing OAuth functionality, it
   cannot be used in scenarios where clients only register once for the
   use of many different issuers (as in some open banking schemes) and
   due to the tight integration with the client registration, it is
   harder to deploy automatically.

   Furthermore, an attacker might be able to circumvent the protection
   offered by this defense by registering a new client with the "honest"
   AS using the redirect URI that the client assigned to the attacker's
   AS.  The attacker could then run the attack as described above,
   replacing the client ID with the client ID of his newly created
   client.

   This defense SHOULD therefore only be used if other options are not
   available.

8.  Native Applications

   Native applications are clients installed and executed on the device
   used by the resource owner (i.e., desktop applications or native
   mobile applications).  Native applications require special
   consideration related to security, platform capabilities, and overall
   end-user experience.




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   The guidance in this section is primarily in the context of native
   mobile apps as opposed to desktop apps.  The native mobile platforms
   have matured more than the desktop platforms in terms of the
   capabilities provided to app developers relevant to the OAuth flows
   described here.  While the guidance is primarily focused on mobile
   apps, much of it generally can apply to desktop apps as well.

   The authorization endpoint requires interaction between the client
   and the resource owner's user agent.  The best current practice is to
   perform the OAuth authorization request in an external user agent
   (typically the browser) rather than an embedded user agent (such as
   one implemented with web-views).

   The native application can capture the response from the
   authorization server in several different ways with differing
   security properties of each.  For example, using a redirect URI with
   an "app-claimed URL" or custom URL scheme registered with the
   operating system to invoke the client as the handler, manual copy-
   and-paste of the credentials, running a local web server, installing
   a user agent extension, or by providing a redirect URI identifying a
   server-hosted resource under the client's control, which in turn
   makes the response available to the native application.

   Previously, it was common for native apps to use embedded user agents
   (commonly implemented with web-views) for OAuth authorization
   requests.  That approach has many drawbacks, including the host app
   being able to copy user credentials and cookies as well as the user
   needing to authenticate from scratch in each app.  See Section 8.5.1
   for a deeper analysis of the drawbacks of using embedded user agents
   for OAuth.

   Native app authorization requests that use the system browser are
   more secure and can take advantage of the user's authentication state
   on the device.  Being able to use the existing authentication session
   in the browser enables single sign-on, as users don't need to
   authenticate to the authorization server each time they use a new app
   (unless required by the authorization server policy).

   Supporting authorization flows between a native app and the browser
   is possible without changing the OAuth protocol itself, as the OAuth
   authorization request and response are already defined in terms of
   URIs.  This encompasses URIs that can be used for inter-app
   communication.  Some OAuth server implementations that assume all
   clients are confidential web clients will need to add an
   understanding of public native app clients and the types of redirect
   URIs they use to support this best practice.





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8.1.  Client Authentication of Native Apps

   Secrets that are statically included as part of an app distributed to
   multiple users are not confidential secrets, as one user may inspect
   their copy and learn the shared secret.  For this reason,
   authorization servers MUST NOT require client authentication of
   native app clients using a shared secret, as this serves no value
   beyond client identification which is already provided by the
   client_id request parameter.

   Authorization servers that still require a statically included shared
   secret for native app clients MUST treat the client as a public
   client (as defined in Section 2.1), and not accept the secret as
   proof of the client's identity.  Without additional measures, such
   clients are subject to client impersonation (see Section 7.3.1).

8.1.1.  Registration of Native App Clients

   Except when using a mechanism like Dynamic Client Registration
   [RFC7591] to provision per-instance credentials, native apps are
   classified as public clients, as defined in Section 2.1, and MUST be
   registered with the authorization server as such.  Authorization
   servers MUST record the client type in the client registration
   details in order to identify and process requests accordingly.

8.1.2.  Native App Attestation

   The draft specification
   [I-D.ietf-oauth-attestation-based-client-auth] defines a mechanism
   that can be used by a native app to obtain a key-bound attestation to
   authenticate to an authorization server or resource server.  This can
   provide a higher level of assurance of a mobile app's identity.

8.2.  Using Inter-App URI Communication for OAuth in Native Apps

   Just as URIs are used for OAuth on the web to initiate the
   authorization request and return the authorization response to the
   requesting website, URIs can be used by native apps to initiate the
   authorization request in the device's browser and return the response
   to the requesting native app.

   By adopting the same methods used on the web for OAuth, benefits seen
   in the web context like the usability of a single sign-on session and
   the security of a separate authentication context are likewise gained
   in the native app context.  Reusing the same approach also reduces
   the implementation complexity and increases interoperability by
   relying on standards-based web flows that are not specific to a
   particular platform.



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   Native apps MUST use an external user agent to perform OAuth
   authorization requests.  This is achieved by opening the
   authorization request in the browser (detailed in Section 8.3) and
   using a redirect URI that will return the authorization response back
   to the native app (defined in Section 8.4).

8.3.  Initiating the Authorization Request from a Native App

   Native apps needing user authorization create an authorization
   request URI with the authorization code grant type per Section 4.1
   using a redirect URI capable of being received by the native app.

   The function of the redirect URI for a native app authorization
   request is similar to that of a web-based authorization request.
   Rather than returning the authorization response to the OAuth
   client's server, the redirect URI used by a native app returns the
   response to the app.  Several options for a redirect URI that will
   return the authorization response to the native app in different
   platforms are documented in Section 8.4.  Any redirect URI that
   allows the app to receive the URI and inspect its parameters is
   viable.

   After constructing the authorization request URI, the app uses
   platform-specific APIs to open the URI in an external user agent.
   Typically, the external user agent used is the default browser, that
   is, the application configured for handling http and https scheme
   URIs on the system; however, different browser selection criteria and
   other categories of external user agents MAY be used.

   This best practice focuses on the browser as the RECOMMENDED external
   user agent for native apps.  An external user agent designed
   specifically for user authorization and capable of processing
   authorization requests and responses like a browser MAY also be used.
   Other external user agents, such as a native app provided by the
   authorization server may meet the criteria set out in this best
   practice, including using the same redirect URI properties, but their
   use is out of scope for this specification.

   Some platforms support a browser feature known as "in-app browser
   tabs", where an app can present a tab of the browser within the app
   context without switching apps, but still retain key benefits of the
   browser such as a shared authentication state and security context.
   On platforms where they are supported, it is RECOMMENDED, for
   usability reasons, that apps use in-app browser tabs for the
   authorization request.






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8.4.  Receiving the Authorization Response in a Native App

   There are several redirect URI options available to native apps for
   receiving the authorization response from the browser, the
   availability and user experience of which varies by platform.

8.4.1.  Claimed "https" Scheme URI Redirection

   Some operating systems, in particular mobile operating systems, allow
   apps to claim https URIs (see Section 4.2.2 of [RFC9110]) in the
   domains they control.  When the browser encounters a claimed URI,
   instead of the page being loaded in the browser, the native app is
   launched with the URI supplied as a launch parameter.

   Such URIs can be used as redirect URIs by native apps.  They are
   indistinguishable to the authorization server from a regular web-
   based client redirect URI.  An example is:

   https://app.example.com/oauth2redirect/example-provider

   As the redirect URI alone is not enough to distinguish public native
   app clients from confidential web clients, it is REQUIRED in
   Section 8.1.1 that the client type be recorded during client
   registration to enable the server to determine the client type and
   act accordingly.

   App-claimed https scheme redirect URIs have some advantages compared
   to other native app redirect options in that the identity of the
   destination app is guaranteed to the authorization server by the
   operating system.  For this reason, native apps SHOULD use them over
   the other options where possible.

8.4.2.  Loopback Interface Redirection

   Native apps that are able to open a port on the loopback network
   interface without needing special permissions (typically, those on
   desktop operating systems) can use the loopback interface to receive
   the OAuth redirect.

   Loopback redirect URIs use the http scheme and are constructed with
   the loopback IP literal and whatever port the client is listening on.

   That is, http://127.0.0.1:{port}/{path} for IPv4, and
   http://[::1]:{port}/{path} for IPv6.  An example redirect using the
   IPv4 loopback interface with a randomly assigned port:

   http://127.0.0.1:51004/oauth2redirect/example-provider




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   An example redirect using the IPv6 loopback interface with a randomly
   assigned port:

   http://[::1]:61023/oauth2redirect/example-provider

   While redirect URIs using the name localhost (i.e.,
   http://localhost:{port}/{path}) function similarly to loopback IP
   redirects, the use of localhost is NOT RECOMMENDED.  Specifying a
   redirect URI with the loopback IP literal rather than localhost
   avoids inadvertently listening on network interfaces other than the
   loopback interface.  It is also less susceptible to client-side
   firewalls and misconfigured host name resolution on the user's
   device.

   The authorization server MUST allow any port to be specified at the
   time of the request for loopback IP redirect URIs, to accommodate
   clients that obtain an available ephemeral port from the operating
   system at the time of the request.

   Clients SHOULD NOT assume that the device supports a particular
   version of the Internet Protocol.  It is RECOMMENDED that clients
   attempt to bind to the loopback interface using both IPv4 and IPv6
   and use whichever is available.

8.4.3.  Private-Use URI Scheme Redirection

   Many mobile and desktop computing platforms support inter-app
   communication via URIs by allowing apps to register private-use URI
   schemes (sometimes colloquially referred to as "custom URL schemes")
   like com.example.app.  When the browser or another app attempts to
   load a URI with a private-use URI scheme, the app that registered it
   is launched to handle the request.

   Many environments that support private-use URI schemes do not provide
   a mechanism to claim a scheme and prevent other parties from using
   another application's scheme.  As such, clients using private-use URI
   schemes are vulnerable to potential attacks on their redirect URIs,
   so this option should only be used if the previously mentioned more
   secure options are not available.

   To perform an authorization request with a private-use URI scheme
   redirect, the native app launches the browser with a standard
   authorization request, but one where the redirect URI utilizes a
   private-use URI scheme it registered with the operating system.







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   When choosing a URI scheme to associate with the app, apps MUST use a
   URI scheme based on a domain name under their control, expressed in
   reverse order, as recommended by Section 3.8 of [RFC7595] for
   private-use URI schemes.

   For example, an app that controls the domain name app.example.com can
   use com.example.app as their scheme.  Some authorization servers
   assign client identifiers based on domain names, for example,
   client1234.usercontent.example.net, which can also be used as the
   domain name for the scheme when reversed in the same manner.  A
   scheme such as myapp, however, would not meet this requirement, as it
   is not based on a domain name.

   When there are multiple apps by the same publisher, care must be
   taken so that each scheme is unique within that group.  On platforms
   that use app identifiers based on reverse-order domain names, those
   identifiers can be reused as the private-use URI scheme for the OAuth
   redirect to help avoid this problem.

   Following the requirements of Section 3.2 of [RFC3986], as there is
   no naming authority for private-use URI scheme redirects, only a
   single slash (/) appears after the scheme component.  A complete
   example of a redirect URI utilizing a private-use URI scheme is:

   com.example.app:/oauth2redirect/example-provider

   When the authorization server completes the request, it redirects to
   the client's redirect URI as it would normally.  As the redirect URI
   uses a private-use URI scheme, it results in the operating system
   launching the native app, passing in the URI as a launch parameter.
   Then, the native app uses normal processing for the authorization
   response.

8.5.  Security Considerations in Native Apps

8.5.1.  Embedded User Agents in Native Apps

   Embedded user agents are a technically possible method for
   authorizing native apps.  These embedded user agents are unsafe for
   use by third parties to the authorization server by definition, as
   the app that hosts the embedded user agent can access the user's full
   authentication credentials, not just the OAuth authorization grant
   that was intended for the app.  They are also typically sandboxed by
   the operating system and mechanisms such as WebAuthn that rely on the
   web origin are disabled.






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   In typical web-view-based implementations of embedded user agents,
   the host application can record every keystroke entered in the login
   form to capture usernames and passwords, automatically submit forms
   to bypass user consent, and copy session cookies and use them to
   perform authenticated actions as the user.

   Even when used by trusted apps belonging to the same party as the
   authorization server, embedded user agents violate the principle of
   least privilege by having access to more powerful credentials than
   they need, potentially increasing the attack surface.

   Encouraging users to enter credentials in an embedded user agent
   without the usual address bar and visible certificate validation
   features that browsers have makes it impossible for the user to know
   if they are signing in to the legitimate site; even when they are, it
   trains them that it's OK to enter credentials without validating the
   site first.

   Aside from the security concerns, embedded user agents do not share
   the authentication state with other apps or the browser, requiring
   the user to log in for every authorization request, which is often
   considered an inferior user experience.

8.5.2.  Fake External User-Agents in Native Apps

   The native app that is initiating the authorization request has a
   large degree of control over the user interface and can potentially
   present a fake external user agent, that is, an embedded user agent
   made to appear as an external user agent.

   When all good actors are using external user agents, the advantage is
   that it is possible for security experts to detect bad actors, as
   anyone faking an external user agent is provably bad.  On the other
   hand, if good and bad actors alike are using embedded user agents,
   bad actors don't need to fake anything, making them harder to detect.
   Once a malicious app is detected, it may be possible to use this
   knowledge to blacklist the app's signature in malware scanning
   software, take removal action (in the case of apps distributed by app
   stores) and other steps to reduce the impact and spread of the
   malicious app.

   Authorization servers can also directly protect against fake external
   user agents by requiring an authentication factor only available to
   true external user agents.







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   Users who are particularly concerned about their security when using
   in-app browser tabs may also take the additional step of opening the
   request in the full browser from the in-app browser tab and complete
   the authorization there, as most implementations of the in-app
   browser tab pattern offer such functionality.

8.5.3.  Malicious External User-Agents in Native Apps

   If a malicious app is able to configure itself as the default handler
   for https scheme URIs in the operating system, it will be able to
   intercept authorization requests that use the default browser and
   abuse this position of trust for malicious ends such as phishing the
   user.

   This attack is not confined to OAuth; a malicious app configured in
   this way would present a general and ongoing risk to the user beyond
   OAuth usage by native apps.  Many operating systems mitigate this
   issue by requiring an explicit user action to change the default
   handler for http and https scheme URIs.

8.5.4.  Loopback Redirect Considerations in Native Apps

   Loopback interface redirect URIs MAY use the http scheme (i.e.,
   without TLS).  This is acceptable for loopback interface redirect
   URIs as the HTTP request never leaves the device.

   Clients SHOULD open the network port only when starting the
   authorization request and close it once the response is returned.

   Clients SHOULD listen on the loopback network interface only, in
   order to avoid interference by other network actors.

   Clients SHOULD use loopback IP literals rather than the string
   localhost as described in Section 8.4.2.

9.  Browser-Based Apps

   Browser-based apps are clients that run in a web browser, typically
   written in JavaScript, also known as "single-page apps".  These types
   of apps have particular security considerations similar to native
   apps.

   TODO: Bring in the normative text of the browser-based apps BCP when
   it is finalized.







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10.  Differences from OAuth 2.0

   This draft consolidates the functionality in OAuth 2.0 [RFC6749],
   OAuth 2.0 for Native Apps [RFC8252], Proof Key for Code Exchange
   [RFC7636], OAuth 2.0 for Browser-Based Apps
   [I-D.ietf-oauth-browser-based-apps], OAuth Security Best Current
   Practice [RFC9700], and Bearer Token Usage [RFC6750].

   Where a later draft updates or obsoletes functionality found in the
   original [RFC6749], that functionality in this draft is updated with
   the normative changes described in a later draft, or removed
   entirely.

   A non-normative list of changes from OAuth 2.0 is listed below:

   *  The authorization code grant is extended with the functionality
      from PKCE [RFC7636] such that the default method of using the
      authorization code grant according to this specification requires
      the addition of the PKCE parameters

   *  Redirect URIs must be compared using exact string matching as per
      Section 4.1.3 of [RFC9700]

   *  The Implicit grant (response_type=token) is omitted from this
      specification as per Section 2.1.2 of [RFC9700]

   *  The Resource Owner Password Credentials grant is omitted from this
      specification as per Section 2.4 of [RFC9700]

   *  Bearer token usage omits the use of bearer tokens in the query
      string of URIs as per Section 4.3.2 of [RFC9700]

   *  Refresh tokens for public clients must either be sender-
      constrained or one-time use as per Section 4.14.2 of [RFC9700]

   *  The token endpoint request containing an authorization code no
      longer contains the redirect_uri parameter

   *  Authorization servers must support client credentials in the
      request body

10.1.  Removal of the OAuth 2.0 Implicit grant

   The OAuth 2.0 Implicit grant is omitted from OAuth 2.1 as it was
   deprecated in [RFC9700].






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   The intent of removing the Implicit grant is to no longer issue
   access tokens in the authorization response, as such tokens are
   vulnerable to leakage and injection, and are unable to be sender-
   constrained to a client.  This behavior was indicated by clients
   using the response_type=token parameter.  This value for the
   response_type parameter is no longer defined in OAuth 2.1.

   Removal of response_type=token does not have an effect on other
   extension response types returning other artifacts from the
   authorization endpoint, for example, response_type=id_token defined
   by [OpenID.Connect].

10.2.  Redirect URI Parameter in Token Request

   In OAuth 2.0, the request to the token endpoint in the authorization
   code flow (Section 4.1.3 of [RFC6749]) contains an optional
   redirect_uri parameter.  The parameter was intended to prevent an
   authorization code injection attack, and was required if the
   redirect_uri parameter was sent in the original authorization
   request.  The authorization request only required the redirect_uri
   parameter if multiple redirect URIs were registered to the specific
   client.  However, in practice, many authorization server
   implementations required the redirect_uri parameter in the
   authorization request even if only one was registered, leading the
   redirect_uri parameter to be required at the token endpoint as well.

   In OAuth 2.1, authorization code injection is prevented by the
   code_challenge and code_verifier parameters, making the inclusion of
   the redirect_uri parameter serve no purpose in the token request.  As
   such, it has been removed.

   For backwards compatibility of an authorization server wishing to
   support both OAuth 2.0 and OAuth 2.1 clients, the authorization
   server MUST allow clients to send the redirect_uri parameter in the
   token request (Section 4.1.3), and MUST enforce the parameter as
   described in [RFC6749].  The authorization server can use the
   client_id in the request to determine whether to enforce this
   behavior for the specific client that it knows will be using the
   older OAuth 2.0 behavior.

   A client following only the OAuth 2.1 recommendations will not send
   the redirect_uri in the token request, and therefore will not be
   compatible with an authorization server that expects the parameter in
   the token request.







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11.  IANA Considerations

   This document does not require any IANA actions.

   All referenced registries are defined by [RFC6749] and related
   documents that this work is based upon.  No changes to those
   registries are required by this specification.

12.  References

12.1.  Normative References

   [BCP195]   Saint-Andre, P., "Recommendations for Secure Use of
              Transport Layer Security (TLS)", 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, DOI 10.17487/RFC2617, June 1999,
              <https://www.rfc-editor.org/info/rfc2617>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/info/rfc4949>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/info/rfc6749>.




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   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750,
              DOI 10.17487/RFC6750, October 2012,
              <https://www.rfc-editor.org/info/rfc6750>.

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,
              <https://www.rfc-editor.org/info/rfc7235>.

   [RFC7521]  Campbell, B., Mortimore, C., Jones, M., and Y. Goland,
              "Assertion Framework for OAuth 2.0 Client Authentication
              and Authorization Grants", RFC 7521, DOI 10.17487/RFC7521,
              May 2015, <https://www.rfc-editor.org/info/rfc7521>.

   [RFC7523]  Jones, M., Campbell, B., and C. Mortimore, "JSON Web Token
              (JWT) Profile for OAuth 2.0 Client Authentication and
              Authorization Grants", RFC 7523, DOI 10.17487/RFC7523, May
              2015, <https://www.rfc-editor.org/info/rfc7523>.

   [RFC7595]  Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
              and Registration Procedures for URI Schemes", BCP 35,
              RFC 7595, DOI 10.17487/RFC7595, June 2015,
              <https://www.rfc-editor.org/info/rfc7595>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8252]  Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps",
              BCP 212, RFC 8252, DOI 10.17487/RFC8252, October 2017,
              <https://www.rfc-editor.org/info/rfc8252>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/info/rfc9110>.





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   [RFC9111]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Caching", STD 98, RFC 9111,
              DOI 10.17487/RFC9111, June 2022,
              <https://www.rfc-editor.org/info/rfc9111>.

   [RFC9207]  Meyer zu Selhausen, K. and D. Fett, "OAuth 2.0
              Authorization Server Issuer Identification", RFC 9207,
              DOI 10.17487/RFC9207, March 2022,
              <https://www.rfc-editor.org/info/rfc9207>.

   [RFC9700]  Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett,
              "Best Current Practice for OAuth 2.0 Security", BCP 240,
              RFC 9700, DOI 10.17487/RFC9700, January 2025,
              <https://www.rfc-editor.org/info/rfc9700>.

   [USASCII]  Institute, A. N. S., "Coded Character Set -- 7-bit
              American Standard Code for Information Interchange, ANSI
              X3.4", 1986.

   [W3C.REC-xml-20081126]
              Bray, T., Paoli, J., Sperberg-McQueen, C. M., Maler, E.,
              and F. Yergeau, "Extensible Markup Language", November
              2008,
              <https://www.w3.org/TR/REC-xml/REC-xml-20081126.xml>.

   [WHATWG.CORS]
              WHATWG, "Fetch Standard: CORS protocol", June 2023,
              <https://fetch.spec.whatwg.org/#http-cors-protocol>.

   [WHATWG.URL]
              WHATWG, "URL", May 2022, <https://url.spec.whatwg.org/>.

12.2.  Informative References

   [CSP-2]    "Content Security Policy Level 2", December 2016,
              <https://www.w3.org/TR/CSP2>.

   [I-D.bradley-oauth-jwt-encoded-state]
              Bradley, J., Lodderstedt, T., and H. Zandbelt, "Encoding
              claims in the OAuth 2 state parameter using a JWT", Work
              in Progress, Internet-Draft, draft-bradley-oauth-jwt-
              encoded-state-09, 4 November 2018,
              <https://datatracker.ietf.org/doc/html/draft-bradley-
              oauth-jwt-encoded-state-09>.

   [I-D.ietf-oauth-attestation-based-client-auth]
              Looker, T., Bastian, P., and C. Bormann, "OAuth 2.0
              Attestation-Based Client Authentication", Work in



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              Progress, Internet-Draft, draft-ietf-oauth-attestation-
              based-client-auth-07, 15 September 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
              attestation-based-client-auth-07>.

   [I-D.ietf-oauth-browser-based-apps]
              Parecki, A., De Ryck, P., and D. Waite, "OAuth 2.0 for
              Browser-Based Applications", Work in Progress, Internet-
              Draft, draft-ietf-oauth-browser-based-apps-26, 3 December
              2025, <https://datatracker.ietf.org/doc/html/draft-ietf-
              oauth-browser-based-apps-26>.

   [I-D.ietf-oauth-rfc7523bis]
              Jones, M. B., Campbell, B., Mortimore, C., and F. Skokan,
              "Updates to OAuth 2.0 JSON Web Token (JWT) Client
              Authentication and Assertion-Based Authorization Grants",
              Work in Progress, Internet-Draft, draft-ietf-oauth-
              rfc7523bis-05, 12 January 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
              rfc7523bis-05>.

   [NIST800-63]
              Burr, W., Dodson, D., Newton, E., Perlner, R., Polk, T.,
              Gupta, S., and E. Nabbus, "NIST Special Publication
              800-63-1, INFORMATION SECURITY", December 2011,
              <http://csrc.nist.gov/publications/>.

   [OMAP]     Huff, J., Schlacht, D., Nadalin, A., Simmons, J.,
              Rosenberg, P., Madsen, P., Ace, T., Rickelton-Abdi, C.,
              and B. Boyer, "Online Multimedia Authorization Protocol:
              An Industry Standard for Authorized Access to Internet
              Multimedia Resources", August 2012,
              <https://www.svta.org/product/online-multimedia-
              authorization-protocol/>.

   [OpenID.Connect]
              Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
              C. Mortimore, "OpenID Connect Core 1.0 incorporating
              errata set 2", December 2023,
              <https://openid.net/specs/openid-connect-core-1_0.html>.

   [OpenID.Discovery]
              Sakimura, N., Bradley, J., Jones, M., and E. Jay, "OpenID
              Connect Discovery 1.0 incorporating errata set 2",
              December 2023, <https://openid.net/specs/openid-connect-
              discovery-1_0.html>.





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   [OpenID.Messages]
              Sakimura, N., Bradley, J., Jones, M., de Medeiros, B.,
              Mortimore, C., and E. Jay, "OpenID Connect Messages 1.0",
              June 2012, <http://openid.net/specs/openid-connect-
              messages-1_0.html>.

   [owasp_redir]
              "OWASP Cheat Sheet Series - Unvalidated Redirects and
              Forwards", 2020,
              <https://cheatsheetseries.owasp.org/cheatsheets/
              Unvalidated_Redirects_and_Forwards_Cheat_Sheet.html>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/info/rfc6265>.

   [RFC6819]  Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
              Threat Model and Security Considerations", RFC 6819,
              DOI 10.17487/RFC6819, January 2013,
              <https://www.rfc-editor.org/info/rfc6819>.

   [RFC7009]  Lodderstedt, T., Ed., Dronia, S., and M. Scurtescu, "OAuth
              2.0 Token Revocation", RFC 7009, DOI 10.17487/RFC7009,
              August 2013, <https://www.rfc-editor.org/info/rfc7009>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [RFC7591]  Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
              P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
              RFC 7591, DOI 10.17487/RFC7591, July 2015,
              <https://www.rfc-editor.org/info/rfc7591>.

   [RFC7592]  Richer, J., Ed., Jones, M., Bradley, J., and M. Machulak,
              "OAuth 2.0 Dynamic Client Registration Management
              Protocol", RFC 7592, DOI 10.17487/RFC7592, July 2015,
              <https://www.rfc-editor.org/info/rfc7592>.

   [RFC7636]  Sakimura, N., Ed., Bradley, J., and N. Agarwal, "Proof Key
              for Code Exchange by OAuth Public Clients", RFC 7636,
              DOI 10.17487/RFC7636, September 2015,
              <https://www.rfc-editor.org/info/rfc7636>.

   [RFC7662]  Richer, J., Ed., "OAuth 2.0 Token Introspection",
              RFC 7662, DOI 10.17487/RFC7662, October 2015,
              <https://www.rfc-editor.org/info/rfc7662>.




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   [RFC8414]  Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", RFC 8414,
              DOI 10.17487/RFC8414, June 2018,
              <https://www.rfc-editor.org/info/rfc8414>.

   [RFC8628]  Denniss, W., Bradley, J., Jones, M., and H. Tschofenig,
              "OAuth 2.0 Device Authorization Grant", RFC 8628,
              DOI 10.17487/RFC8628, August 2019,
              <https://www.rfc-editor.org/info/rfc8628>.

   [RFC8705]  Campbell, B., Bradley, J., Sakimura, N., and T.
              Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
              and Certificate-Bound Access Tokens", RFC 8705,
              DOI 10.17487/RFC8705, February 2020,
              <https://www.rfc-editor.org/info/rfc8705>.

   [RFC8707]  Campbell, B., Bradley, J., and H. Tschofenig, "Resource
              Indicators for OAuth 2.0", RFC 8707, DOI 10.17487/RFC8707,
              February 2020, <https://www.rfc-editor.org/info/rfc8707>.

   [RFC9068]  Bertocci, V., "JSON Web Token (JWT) Profile for OAuth 2.0
              Access Tokens", RFC 9068, DOI 10.17487/RFC9068, October
              2021, <https://www.rfc-editor.org/info/rfc9068>.

   [RFC9126]  Lodderstedt, T., Campbell, B., Sakimura, N., Tonge, D.,
              and F. Skokan, "OAuth 2.0 Pushed Authorization Requests",
              RFC 9126, DOI 10.17487/RFC9126, September 2021,
              <https://www.rfc-editor.org/info/rfc9126>.

   [RFC9396]  Lodderstedt, T., Richer, J., and B. Campbell, "OAuth 2.0
              Rich Authorization Requests", RFC 9396,
              DOI 10.17487/RFC9396, May 2023,
              <https://www.rfc-editor.org/info/rfc9396>.

   [RFC9449]  Fett, D., Campbell, B., Bradley, J., Lodderstedt, T.,
              Jones, M., and D. Waite, "OAuth 2.0 Demonstrating Proof of
              Possession (DPoP)", RFC 9449, DOI 10.17487/RFC9449,
              September 2023, <https://www.rfc-editor.org/info/rfc9449>.

   [RFC9470]  Bertocci, V. and B. Campbell, "OAuth 2.0 Step Up
              Authentication Challenge Protocol", RFC 9470,
              DOI 10.17487/RFC9470, September 2023,
              <https://www.rfc-editor.org/info/rfc9470>.








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   [W3C.REC-html401-19991224]
              Hors, A. L., Ed., Raggett, D., Ed., and I. Jacobs, Ed.,
              "HTML 4.01 Specification", W3C REC REC-html401-19991224,
              W3C REC-html401-19991224, 24 December 1999,
              <https://www.w3.org/TR/1999/REC-html401-19991224/>.

Appendix A.  Augmented Backus-Naur Form (ABNF) Syntax

   This section provides Augmented Backus-Naur Form (ABNF) syntax
   descriptions for the elements defined in this specification using the
   notation of [RFC5234].  The ABNF below is defined in terms of Unicode
   code points [W3C.REC-xml-20081126]; these characters are typically
   encoded in UTF-8.  Elements are presented in the order first defined.

   Some of the definitions that follow use the "URI-reference"
   definition from [RFC3986].

   Some of the definitions that follow use these common definitions:

   VSCHAR     = %x20-7E
   NQCHAR     = %x21 / %x23-5B / %x5D-7E
   NQSCHAR    = %x20-21 / %x23-5B / %x5D-7E

A.1.  "client_id" Syntax

   The client_id element is defined in Section 2.4.1:

   client-id     = *VSCHAR

A.2.  "client_secret" Syntax

   The client_secret element is defined in Section 2.4.1:

   client-secret = *VSCHAR

A.3.  "response_type" Syntax

   The response_type element is defined in Section 4.1.1 and
   Section 6.4:

   response-type = response-name *( SP response-name )
   response-name = 1*response-char
   response-char = "_" / DIGIT / ALPHA

A.4.  "scope" Syntax

   The scope element is defined in Section 1.4.1:




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    scope       = scope-token *( SP scope-token )
    scope-token = 1*NQCHAR

A.5.  "state" Syntax

   The state element is defined in Section 4.1.1, Section 4.1.2, and
   Section 4.1.2.1:

    state      = 1*VSCHAR

A.6.  "redirect_uri" Syntax

   The redirect_uri element is defined in Section 4.1.1, and
   Section 4.1.3:

    redirect-uri      = URI-reference

A.7.  "error" Syntax

   The error element is defined in Sections Section 4.1.2.1,
   Section 3.2.4, and Section 5.3:

    error             = 1*NQSCHAR

A.8.  "error_description" Syntax

   The error_description element is defined in Sections Section 4.1.2.1,
   Section 3.2.4, and Section 5.3:

    error-description = 1*NQSCHAR

A.9.  "error_uri" Syntax

   The error_uri element is defined in Sections Section 4.1.2.1,
   Section 3.2.4, and Section 5.3:

    error-uri         = URI-reference

A.10.  "grant_type" Syntax

   The grant_type element is defined in Section Section 3.2.2:

    grant-type = grant-name / URI-reference
    grant-name = 1*name-char
    name-char  = "-" / "." / "_" / DIGIT / ALPHA






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A.11.  "code" Syntax

   The code element is defined in Section 4.1.3:

    code       = 1*VSCHAR

A.12.  "access_token" Syntax

   The access_token element is defined in Section 3.2.3:

    access-token = 1*VSCHAR

A.13.  "token_type" Syntax

   The token_type element is defined in Section 3.2.3, and Section 6.1:

    token-type = type-name / URI-reference
    type-name  = 1*name-char
    name-char  = "-" / "." / "_" / DIGIT / ALPHA

A.14.  "expires_in" Syntax

   The expires_in element is defined in Section 3.2.3:

    expires-in = 1*DIGIT

A.15.  "refresh_token" Syntax

   The refresh_token element is defined in Section 3.2.3 and
   Section 4.3:

    refresh-token = 1*VSCHAR

A.16.  Endpoint Parameter Syntax

   The syntax for new endpoint parameters is defined in Section 6.2:

    param-name = 1*name-char
    name-char  = "-" / "." / "_" / DIGIT / ALPHA

A.17.  "code_verifier" Syntax

   ABNF for code_verifier is as follows.

   code-verifier = 43*128unreserved
   unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
   ALPHA = %x41-5A / %x61-7A
   DIGIT = %x30-39



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A.18.  "code_challenge" Syntax

   ABNF for code_challenge is as follows.

   code-challenge = 43*128unreserved
   unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
   ALPHA = %x41-5A / %x61-7A
   DIGIT = %x30-39

Appendix B.  Use of application/x-www-form-urlencoded Media Type

   At the time of publication of [RFC6749], the application/x-www-form-
   urlencoded media type was defined in Section 17.13.4 of
   [W3C.REC-html401-19991224] but not registered in the IANA MIME Media
   Types registry (http://www.iana.org/assignments/media-types
   (http://www.iana.org/assignments/media-types)).  Furthermore, that
   definition is incomplete, as it does not consider non-US-ASCII
   characters.

   To address this shortcoming when generating contents using this media
   type, names and values MUST be encoded using the UTF-8 character
   encoding scheme [RFC3629] first; the resulting octet sequence then
   needs to be further encoded using the escaping rules defined in
   [W3C.REC-html401-19991224].

   When parsing data from a content using this media type, the names and
   values resulting from reversing the name/value encoding consequently
   need to be treated as octet sequences, to be decoded using the UTF-8
   character encoding scheme.

   For example, the value consisting of the six Unicode code points (1)
   U+0020 (SPACE), (2) U+0025 (PERCENT SIGN), (3) U+0026 (AMPERSAND),
   (4) U+002B (PLUS SIGN), (5) U+00A3 (POUND SIGN), and (6) U+20AC (EURO
   SIGN) would be encoded into the octet sequence below (using
   hexadecimal notation):

   20 25 26 2B C2 A3 E2 82 AC

   and then represented in the content as:

   +%25%26%2B%C2%A3%E2%82%AC










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Appendix C.  Serializations

   Various messages in this specification are serialized using one of
   the methods described below.  This section describes the syntax of
   these serialization methods; other sections describe when they can
   and must be used.  Note that not all methods can be used for all
   messages.

C.1.  Query String Serialization

   In order to serialize the parameters using the Query String
   Serialization, the Client constructs the string by adding the
   parameters and values to the query component of a URL using the
   application/x-www-form-urlencoded format as defined by [WHATWG.URL].
   Query String Serialization is typically used in HTTP GET requests.

C.2.  Form-Encoded Serialization

   Parameters and their values are Form Serialized by adding the
   parameter names and values to the entity body of the HTTP request
   using the application/x-www-form-urlencoded format as defined by
   Appendix B.  Form Serialization is typically used in HTTP POST
   requests.

C.3.  JSON Serialization

   The parameters are serialized into a JSON [RFC8259] object structure
   by adding each parameter at the highest structure level.  Parameter
   names and string values are represented as JSON strings.  Numerical
   values are represented as JSON numbers.  Boolean values are
   represented as JSON booleans.  Omitted parameters and parameters with
   no value SHOULD be omitted from the object and not represented by a
   JSON null value, unless otherwise specified.  A parameter MAY have a
   JSON object or a JSON array as its value.  The order of parameters
   does not matter and can vary.

Appendix D.  Extensions

   Below is a list of well-established extensions at the time of
   publication:

   *  [RFC7009]: Token Revocation

      -  The Token Revocation extension defines a mechanism for clients
         to indicate to the authorization server that an access token is
         no longer needed.

   *  [RFC7591]: Dynamic Client Registration



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      -  Dynamic Client Registration provides a mechanism for
         programmatically registering clients with an authorization
         server.

   *  [RFC7662]: Token Introspection

      -  The Token Introspection extension defines a mechanism for
         resource servers to obtain information about access tokens.

   *  [RFC8414]: Authorization Server Metadata

      -  Authorization Server Metadata (also known as OAuth Discovery)
         defines an endpoint clients can use to look up the information
         needed to interact with a particular OAuth server, such as the
         location of the authorization and token endpoints and the
         supported grant types.

   *  [RFC8628]: OAuth 2.0 Device Authorization Grant

      -  The Device Authorization Grant (formerly known as the Device
         Flow) is an extension that enables devices with no browser or
         limited input capability to obtain an access token.  This is
         commonly used by smart TV apps, or devices like hardware video
         encoders that can stream video to a streaming video service.

   *  [RFC8705]: Mutual TLS

      -  Mutual TLS describes a mechanism of binding tokens to the
         clients they were issued to, as well as a client authentication
         mechanism, via TLS certificate authentication.

   *  [RFC8707]: Resource Indicators

      -  Provides a way for the client to explicitly signal to the
         authorization server where it intends to use the access token
         it is requesting.

   *  [RFC9068]: JSON Web Token (JWT) Profile for OAuth 2.0 Access
      Tokens

      -  This specification defines a profile for issuing OAuth access
         tokens in JSON Web Token (JWT) format.

   *  [RFC9126]: Pushed Authorization Requests







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      -  The Pushed Authorization Requests extension describes a
         technique of initiating an OAuth flow from the back channel,
         providing better security and more flexibility for building
         complex authorization requests.

   *  [RFC9207]: Authorization Server Issuer Identification

      -  The iss parameter in the authorization response indicates the
         identity of the authorization server to prevent mix-up attacks
         in the client.

   *  [RFC9396]: Rich Authorization Requests

      -  Rich Authorization Requests specifies a new parameter
         authorization_details that is used to carry fine-grained
         authorization data in the OAuth authorization request.

   *  [RFC9449]: Demonstrating Proof of Possession (DPoP)

      -  DPoP describes a mechanism for sender-constraining OAuth 2.0
         tokens via a proof-of-possession mechanism on the application
         level.

   *  [RFC9470]: Step-Up Authentication Challenge Protocol

      -  Step-Up Auth describes a mechanism that resource servers can
         use to signal to a client that the authentication event
         associated with the access token of the current request does
         not meet its authentication requirements.

Appendix E.  Acknowledgements

   This specification is the work of the OAuth Working Group, and its
   starting point was based on the contents of the following
   specifications: OAuth 2.0 Authorization Framework (RFC 6749), OAuth
   2.0 for Native Apps (RFC 8252), OAuth Security Best Current Practice,
   and OAuth 2.0 for Browser-Based Apps.  The editors would like to
   thank everyone involved in the creation of those specifications upon
   which this is built.

   The editors would also like to thank the following individuals for
   their ideas, feedback, corrections, and wording that helped shape
   this version of the specification: Andrii Deinega, Bob Hamburg, Brian
   Campbell, Daniel Fett, Deng Chao, Emelia Smith, Falko, Filip Skokan,
   Joseph Heenan, Justin Richer, Karsten Meyer zu Selhausen, Michael
   Jones, Michael Peck, Roberto Polli, Tim Würtele and Vittorio
   Bertocci.




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   Discussions around this specification have also occurred at the OAuth
   Security Workshop in 2021 and 2022.  The authors thank the organizers
   of the workshop (Guido Schmitz, Steinar Noem, and Daniel Fett) for
   hosting an event that's conducive to collaboration and community
   input.

Appendix F.  Document History

   [[ To be removed from the final specification ]]

   -15

   *  add additional context for JWT client authentication and
      specifically recommend RFC7523bis

   *  editorial clarifications and updates

   *  clarify error responses in authorization endpoint and token
      endpoint

   *  synced language from RFC9700 for AS open redirect considerations

   *  applied RFC6750 erratas 3500 and 6613

   *  resolved ambiguity around repeated parameters

   -14

   *  Editorial clarifications

   *  Corrected an instance of "relying party" vs "resource server"

   *  Add references to client_secret_post and client_secret_basic terms
      from RFC7591

   *  Replaced "sanitize" language with treating as untrusted input

   *  Clarified that native apps guidance applies primarily to mobile
      app platforms

   *  Clarify that there is no requirement that an AS supports public or
      confidential clients in particular

   -13

   *  Updated references to RFC 9700

   *  Updated and sorted list of OAuth extensions



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   *  Updated references to link to section numbers

   -12

   *  Updated language around client registration to better reflect
      alternative registration methods such as those in use by OpenID
      Federation and open ecosystems

   *  Added DPoP and Step-Up Auth to appendix of extensions

   *  Updated reference for case insensitivity of auth scheme to HTTP
      instead of ABNF

   *  Corrected an instance of "relying party" vs "client"

   *  Moved client_id requirement to the individual grant types

   *  Consolidated the descriptions of serialization methods to the
      appendix

   -11

   *  Explicitly mention that Bearer is case insensitive

   *  Recommend against defining custom scopes that conflict with known
      scopes

   *  Change client credentials to be required to be supported in the
      request body to avoid HTTP Basic authentication encoding interop
      issues

   -10

   *  Clarify that the client id is an opaque string

   *  Extensions may define additional error codes on a resource request

   *  Improved formatting for error field definitions

   *  Moved and expanded "scope" definition to introduction section

   *  Split access token section into structure and request

   *  Renamed b64token to token68 for consistency with RFC7235

   *  Restored content from old appendix B about application/x-www-form-
      urlencoded




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   *  Clarified that clients must not parse access tokens

   *  Expanded text around when redirect_uri parameter is required in
      the authorization request

   *  Changed "permissions" to "privileges" in refresh token section for
      consistency

   *  Consolidated authorization code flow security considerations

   *  Clarified authorization code reuse - an authorization code can
      only obtain an access token once

   -09

   *  AS MUST NOT support CORS requests at authorization endpoint

   *  more detail on asymmetric client authentication

   *  sync CSRF description from security BCP

   *  update and move sender-constrained access tokens section

   *  sync client impersonating resource owner with security BCP

   *  add reference to authorization request from redirect URI
      registration section

   *  sync refresh rotation section from security BCP

   *  sync redirect URI matching text from security BCP

   *  updated references to RAR (RFC9396)

   *  clarifications on URIs

   *  removed redirect_uri from the token request

   *  expanded security considerations around code_verifier

   *  revised introduction section

   -08

   *  Updated acknowledgments

   *  Swap "by a trusted party" with "by an outside party" in client ID
      definition



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   *  Replaced "verify the identity of the resource owner" with
      "authenticate"

   *  Clarified refresh token rotation to match RFC6819

   *  Added appendix to hold application/x-www-form-urlencoded examples

   *  Fixed references to entries in appendix

   *  Incorporated new "Phishing via AS" section from Security BCP

   *  Rephrase description of the motivation for client authentication

   *  Moved "scope" parameter in token request into specific grant types
      to match OAuth 2.0

   *  Updated Clickjacking and Open Redirection description from the
      latest version of the Security BCP

   *  Moved normative requirements out of authorization code security
      considerations section

   *  Security considerations clarifications, and removed a duplicate
      section

   -07

   *  Removed "third party" from abstract

   *  Added MFA and passwordless as additional motiviations in
      introduction

   *  Mention PAR as one way redirect URI registration can happen

   *  Added a reference to requiring CORS headers on the token endpoint

   *  Updated reference to OMAP extension

   *  Fixed numbering in sequence diagram

   -06

   *  Removed "credentialed client" term

   *  Simplified definition of "confidential" and "public" clients

   *  Incorporated the iss response parameter referencing RFC9207




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   *  Added section on access token validation by the RS

   *  Removed requirement for authorization servers to support all 3
      redirect methods for native apps

   *  Fixes for some references

   *  Updates HTTP references to RFC 9110

   *  Clarifies "authorization grant" term

   *  Clarifies client credential grant usage

   *  Clean up authorization code diagram

   *  Updated reference for application/x-www-form-urlencoded and
      removed outdated note about it not being in the IANA registry

   -05

   *  Added a section about the removal of the implicit flow

   *  Moved many normative requirements from security considerations
      into the appropriate inline sections

   *  Reorganized and consolidated TLS language

   *  Require TLS on redirect URIs except for localhost/custom URL
      scheme

   *  Updated refresh token guidance to match security BCP

   -04

   *  Added explicit mention of not sending access tokens in URI query
      strings

   *  Clarifications on definition of client types

   *  Consolidated text around loopback vs localhost

   *  Editorial clarifications throughout the document

   -03

   *  refactoring to collect all the grant types under the same top-
      level header in section 4




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   *  Better split normative and security consideration text into the
      appropriate places, both moving text that was really security
      considerations out of the main part of the document, as well as
      pulling normative requirements from the security considerations
      sections into the appropriate part of the main document

   *  Incorporated many of the published errata on RFC6749

   *  Updated references to various RFCs

   *  Editorial clarifications throughout the document

   -02

   -01

   -00

   *  initial revision

Authors' Addresses

   Dick Hardt
   Hellō
   Email: dick.hardt@gmail.com


   Aaron Parecki
   Okta
   Email: aaron@parecki.com
   URI:   https://aaronparecki.com


   Torsten Lodderstedt
   SPRIND
   Email: torsten@lodderstedt.net















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