d-OAuth-Server/docs/SECURITY.md

Security

This guide states the threat model, the token and key design, the sender constraint model, the fail-closed policy, and the admin replacement procedure. The implementation is the authority; the citations point at it.

Purpose

For a security reviewer and an operator: what the server protects, how, and what to do when something goes wrong. The standards behind the design are RFC 9700 (BCP 240) and draft-ietf-oauth-v2-1-16 (published 3 September 2026; an Internet-Draft, not an RFC, at https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1).

Threat model

Assets

• Opaque access and refresh tokens, and the authorization codes and device codes that yield them.
• Client secrets, user passwords, TOTP secrets, and DCR initial-access tokens.
• The token pepper, the TOTP encryption key, and the PII field-encryption key.
• The directory itself: emails, client names, contacts, redirect URIs, JAR request URIs, JWKS URLs, software ids, and mTLS identities.
• The integrity of the authorization decision: which client may act for which user with which scopes.

Adversaries

• A network attacker who can read and modify traffic. TLS and sender constraints address this.
• A malicious or compromised client that tries to escalate scope, replay a code, or replay a token.
• A stolen token used from another key or certificate.
• An operator or job that could leak a secret through logs, argv, or an error body.

Trust boundaries

• The TLS public listener is the only route to OAuth handlers.
• The plaintext health listener carries no OAuth routes.
• Postgres holds durable domain state; Redis holds ephemeral state.
• Forwarded client-cert and client-IP headers are never trusted: the listener uses only the TLS peer address and the certificate chain it verified itself.

Attack surface

This section records the container and host attack surface: how an attacker could reach the container or the host through the image, the controls already in place, and the residual risk. It is durable threat-model material, not a backlog; update it whenever the deployment surface changes (a new listener, mount, dependency class, or runtime setting).

Vector	Existing controls	Residual risk
Application-level RCE in a parser (form bodies, DPoP/JAR/private_key_jwt JWTs, DCR JSON, jwks_uri/request_uri fetches)	#![forbid(unsafe_code)], no unwrap/panic in request paths, rustls/ring (no OpenSSL), fuzz targets for token-form/DCR-JSON/JAR-claims	Fuzzing is not exhaustive.
Dependency / supply-chain compromise	Committed Cargo.lock, cargo deny/cargo audit in CI, SBOM + provenance + keyless cosign, digest-pinned bases	A zero-day in a trusted crate.
Container escape via kernel or runtime bug	Non-root UID 65532, read-only root filesystem, all capabilities dropped, allowPrivilegeEscalation: false, seccompProfile: RuntimeDefault	A kernel LPE or runc bug, mitigated but not eliminated.
Secret exfiltration from the process environment	Secrets are injected at runtime, not baked into image layers; the pepper/TOTP/PII keys live outside the database; the Deployment mounts every secret — pepper, TOTP key, PII key, and the database/Redis passwords — as 0400 tmpfs files under /run/secrets (read through their *_FILE variables), and the passwordless DSNs come from the ConfigMap, so no secret enters the environment	An attacker with code execution can still read the mounted secret files under /run/secrets; that is the same capability the server has.
Exposed management surface	The plaintext health listener (8081) is unauthenticated but not published in the Service (only 8080 is)	A misconfigured Service/Ingress could expose it; it must never be internet-facing.
Host access via mounted volumes	/certs is mounted read-only at mode 0400; no Docker socket is mounted	Operator misconfiguration.
Unrestricted pod egress (SSRF to metadata, exfiltration)	networkpolicy.yaml default-denies egress except DNS (53 to kube-system) and Postgres/Redis (5432/6379) scoped to the namespace hosting them — no internet-wide 0.0.0.0/0 peer; any ipBlock peer excludes 169.254.0.0/16; ingress is limited to pod ports 8080/8081	Enforcement depends on a NetworkPolicy-capable CNI; a cluster that ignores NetworkPolicy gets no benefit. A managed database outside the cluster needs a specific CIDR or a CNI FQDN policy. The default-deny also blocks the JAR request_uri HTTPS fetch — remote request objects are unsupported unless the deployment adds a scoped allowance (see docs/DEPLOYMENT.md).
Operator exec into a running pod	No shell and a non-root, read-only-rootfs container limit what runs; every secret is a file the container user already reads, and no secret is in the environment	pods/exec (and docker exec) bypass the application's own authorization; restrict it with RBAC and audit it (docs/OPERATIONS.md).
Kubernetes API access from the pod	automountServiceAccountToken: false; the binary never calls the API	A future feature that needs the API would have to re-enable it deliberately.
Image/registry compromise	Digest pinning, cosign verification, and a committed digest reference with no shippable :latest; CI writes the signed build digest into kustomization.yaml	A registry compromise before the digest is recorded still ships; the pin step must run.

Last reviewed: 2026-10-05. File-mounted DB/Redis passwords, digest pinning, NetworkPolicy, automountServiceAccountToken: false, and scheduled fuzzing are implemented in the shipped manifests and workflows.

Toplevel rules

• #![forbid(unsafe_code)] across the crate.
• unwrap, expect, and panic are denied by lint in request paths.
• A raw token, secret, password, OTP, or client secret never appears in Postgres, Redis, logs, an error body, or an evidence file.
• Secret comparisons are constant time.

Browser and intermediary caching

• Every response except /.well-known/… metadata carries Cache-Control: no-store (+ Pragma: no-cache for legacy clients). An authorization page, consent page, token body, request handle, or error is therefore never reused by a browser, the back/forward cache, or a shared intermediary.
• This covers the GET /login and GET /consent pages (which carry a session-bound CSRF token and the bound request_id), the /authorize redirects and error responses, the token endpoint (success and error), PAR, device authorization, DCR, introspection, and revocation.
• The plaintext health listener does not mount the security-header layer, so the health router inserts Cache-Control: no-store and X-Content-Type-Options: nosniff on each probe response itself. A regression test pins both headers so a route added to that listener cannot silently drop the policy.
• OAuth metadata discovery is cacheable by design and carries an explicit positive Cache-Control: public, max-age=3600 (not browser heuristic freshness). A metadata handler that fails closed returns a 503 error, which is itself no-store.

Opaque tokens, hashed at rest

• A token is at least 256 bits from a CSPRNG, base64url encoded.
• Only a keyed HMAC-SHA256 hash is persisted, under a versioned pepper key id.
• The newest pepper mints; verification retries across retained key ids, newest-first and bounded.
• A raw token is never a storage key or a stored value.
• A storage dump therefore yields no usable token.

Rotation, tombstones, and replay

• Refresh rotation is a Lua compare-and-set, keyed by a deterministic rotation_request_id derived from the old refresh token hash and the pepper.
• The old record becomes a tombstone with used and rotated_to, kept for the refresh lifetime.
• Reuse of a rotated token is detected, revokes the whole grant family, and emits token.refresh_reuse_detected.
• A lost post-write reply is resolved by re-reading the rotation mapping, so a retry returns the same tokens and never double-rotates.
• The write is followed by a topology-appropriate durability barrier before any token is returned. A barrier below the required level fails closed.

See REDIS.md for the barrier modes.

Sender constraints

Reference: RFC 9449 (DPoP), RFC 8705 (mTLS).

Every sender-constrained grant requires a freshly verified proof on every use.

Client class	Required binding
Public	DPoP only. A missing proof fails with invalid_grant.
Confidential	mTLS and/or DPoP. The confirmation records each proven component.

• cnf propagates to every newly minted token.
• A stored cnf alone never mints: the proof is re-verified per use.
• For token exchange, the proof must match the subject token's confirmation, not merely the caller's.
• private_key_jwt is client authentication only. It produces no confirmation and never satisfies sender constraint. This is deliberate. Its exp may lie at most 300 s past now (plus clock skew), and the jti replay marker is held for the assertion's full remaining validity, so a long-lived assertion cannot be replayed once its marker expires. RS256, PS256, ES256, and EdDSA are accepted; none and HMAC are rejected.
• mTLS identity is a client certificate subject DN (tls_client_auth) or a certificate thumbprint (self_signed_tls_client_auth), each unique per the partial indexes in the schema.
• A client certificate is matched only against the identity registered for the requesting client_id, and only while inside its validity window. A cross-wired or expired certificate is invalid_client and never binds a cnf.
• A DPoP proof is rejected if it is malformed, oversized, uses an unsupported algorithm, has the wrong typ, or has a bad or replayed jti, htm, htu, iat, or ath.

PAR and JAR

Reference: RFC 9126, RFC 9101.

• A confidential client at /authorize must use a PAR request_uri or a signed request object. This is mandatory, not optional.
• A JAR request object must be signed; none and symmetric algorithms are rejected. iss must equal client_id, aud must be the issuer, and conflicts with the query string are rejected.
• A request_uri is one-time and client-bound, or exactly pre-registered, and is fetched only through the SSRF guard (https only, no redirects, global IPs only, size and time capped). The connection is pinned to the addresses validated before the fetch, so a rebinding hostname cannot redirect it to a blocked address.
• PAR credentials are read from the header and body only, never the query.

Key ring and rotation

• One versioned KeyRing holds three independent chains, newest first: the token-pepper chain, the TOTP-encryption chain, and the PII field-encryption chain. Each purpose also has an optional retained previous key.
• Opaque token hashes carry the pepper key id; the TOTP blob and every _enc PII blob carry their own key id, and the blind index carries its key id.
• Rotating the pepper requires retaining the old key for at least the refresh lifetime so live tokens still verify.
• Rotating the PII key re-encrypts every _enc value and recomputes every blind index (pii rotate); rotating the TOTP key re-wraps the per-row DEKs (totp rewrap). See OPERATIONS.md.
• Debug prints key ids only, never key material.
• There is no authorization-server JWT-signing key and no jwks_uri. The keygen command emits keys for test fixtures and client-side DPoP only.

PII field encryption

Reference: AES-256-GCM (ring), HKDF-SHA256, HMAC-SHA256.

• The directory columns listed under Assets are sealed in the application with a versioned AES-256-GCM field codec (src/crypto/field.rs). The key is derived from the PII master key with HKDF-SHA256 (info pii-field-gcm-v1), never from the TOTP key.
• A keyed-HMAC blind index ({kid}:{hex}, info pii-blind-index-v1) is stored for the columns that are looked up or must stay unique: email_index, the two mTLS indexes, and uri_index. Encrypted bytes cannot be compared or made unique directly, so the index is what enforces uniqueness and equality.
• A read accepts candidates across every retained key id, newest first, so a lookup keeps working while an old key is retained. A create or register path runs an explicit dual-predicate uniqueness pre-check because a partial unique index cannot span a sealed row and a not-yet-backfilled plaintext row.
• TOTP secrets use a per-row envelope (blob version 3): a random DEK wrapped by a key derived from the TOTP key (info totp-kek-v1). The data AAD excludes the key id, so a DEK can be re-wrapped under a new key without re-encrypting the secret. Versions 1 and 2 still decrypt.
• Decryption of a stored ciphertext is fail-closed: a tampered, truncated, unknown-key, or undecodable blob maps to AppError::PiiDecrypt (temporarily_unavailable, 503) and emits the pii.decrypt_failure audit event with the request id. No value or plaintext is written to the event.
• Out of scope. An attacker who already has database write access can move an entire encrypted body between rows (full-blob substitution) or replay a sealed value. Field encryption binds ciphertext to a key, not to a row identity, and defending against a write-capable adversary would require row binding or a MAC over the row that this release does not add. This is a documented threat-model boundary, not a defect.

Master key custody

• The token pepper, the TOTP-encryption key, and the PII field-encryption key are the server's master keys. They live in the versioned KeyRing (src/keys.rs), loaded once at startup from configuration. Nothing re-reads or fetches them later.
• Hard requirement. A master key must never be stored in Postgres, captured in the same backup or restore as Postgres, baked into an image layer, placed in a ConfigMap, or injected into the process environment by the shipped manifests. A database dump plus its backup must not yield any of the three keys.
• Losing the token pepper invalidates every live opaque access and refresh token, because only keyed HMAC hashes are persisted and the pepper is the HMAC key. Losing the PII key makes every sealed directory value unreadable.
• The real config surface is small: inline token_pepper, totp_encryption_key, and pii_encryption_key, or the mounted-file forms token_pepper_file, totp_encryption_key_file, and pii_encryption_key_file. When both are set the inline value wins (resolve_secret in src/config/build.rs). The key ids default to v1 through token_pepper_id / totp_key_id / pii_key_id (src/config/raw.rs). Each purpose may retain one previous key (*_previous_key / *_previous_key_file) with an explicit, distinct *_previous_key_id; a previous key without its active key is a config error.
• The shipped cluster manifests never use the inline or environment forms: the Secret file-mounts the keys 0400 on a tmpfs volume at /run/secrets, and the loader reads them through D_OAUTH_TOKEN_PEPPER_FILE, D_OAUTH_TOTP_ENCRYPTION_KEY_FILE, and D_OAUTH_PII_ENCRYPTION_KEY_FILE (deploy/k8s/secret.yaml, deploy/k8s/configmap.yaml). The ConfigMap carries only non-secret settings; the previous keys stay optional and are never required in a workload.
• The server does not talk to a KMS or an HSM. It has no cloud SDK and no key-fetch code path. Sourcing a key from a KMS/HSM or a secret manager is the deployment's responsibility; this section is a documented contract, not an implemented feature.

Custody, in order of preference:

• Preferred. The key is held in a KMS/HSM or a secret manager that never co-locates the key with the database backup: cloud KMS, Vault Transit, or a PKCS#11 HSM.
• Acceptable. A secret manager writes a 0400 tmpfs file that the server reads through its *_FILE key. This is what the shipped manifests expect.
• Forbidden. The key is in the database, in a database backup, in an image layer, in the ConfigMap, or in the process environment of the shipped manifests.

Custody and rotation interact. Retain the old pepper key id for at least the refresh lifetime, or live tokens stop verifying; retain the old TOTP key id only long enough to re-enroll users or re-wrap blobs, then drop it; retain the old PII key id until pii rotate has re-encrypted every value and recomputed every blind index, then drop it. A rotation that removes an id early is a deliberate invalidation of what that key protects.

The operator procedure for custody and KMS/HSM sourcing lives in DEPLOYMENT.md.

Fail-closed policy

The seven security endpoints (authorize, token, introspect, revoke, par, device_authorization, register) are always fail-closed. An attempt to configure one as fail-open is a load-time error.

• A Postgres or Redis outage returns 503 temporarily_unavailable with Retry-After: 1, never a fail-open success.
• Only idempotent operations are retried, under a bounded budget. A non-idempotent token operation is never blindly replayed.
• Readiness reports 503 while a dependency is down; liveness stays 200 because it touches no dependency.
• A cold scope cache with Postgres down returns 503 rather than an empty list.

Replay and single-use

• An authorization code is consumed atomically, exactly once.
• A PAR request_uri is consumed atomically.
• A device code is consumed atomically on approval.
• DPoP and private_key_jwt jti values are recorded with SET NX, so a replayed proof or assertion is rejected. A private_key_jwt marker lives for the assertion's full remaining validity, bounded by the 300 s lifetime cap.
• A TOTP code is recorded in a replay cache for its time-step window.

Input and output safety

• Redirect URI matching is exact, with the RFC 8252 loopback exception only. There are no wildcards and no prefix matches.
• PKCE is S256 only. A verifier cannot be omitted to downgrade.
• Scope is intersected: requested ∩ client-registered ∩ user-entitled ∩ enabled. Refresh can only narrow and re-checks the current client-registered and user-entitled sets (a fresh Postgres read), so revoking a scope or an entitlement drops it on the next refresh.
• The login, consent, and device pages set a strict CSP with frame-ancestors 'none'. They use server-side sessions, CSRF protection, and no open redirect.
• GET /logout is a server-specific session endpoint (not OAuth 2.1/OIDC defined). It destroys the session and clears the cookie, then renders a neutral page or redirects to a post_logout URI. It accepts no query parameters: the target is read only from session meta, frozen when the authorization flow was bound, and used only if the owning client is still active and still registers that exact URI (exact-string match + RFC 8252 loopback). A forced navigation to /logout can therefore only end a session and, at most, send the browser to that client's own pre-registered URI — never to an attacker target. There is no post_logout_redirect_uri parameter and no id_token_hint.
• Failed-login and lockout counters live in Redis, not Postgres, so a login flood does not write rows.
• Rate limiting is per IP and per client. Counters live in a Redis keyspace separated from token state.
• CORS is empty by default and never a wildcard.
• DCR is protected by a one-time initial-access token; open registration is off by default.
• Secrets are read from stdin, a file, or the environment, never argv, and are printed only for an explicit one-time value.

Admin bootstrap and replace immediately

No default admin credential ships. migrate seeds zero users and serve never creates one. The first admin is created by admin bootstrap, which runs inside a single transaction under an advisory lock, so two concurrent bootstraps cannot both create an admin. Once any enabled admin exists, a second bootstrap refuses unless --force is given, which prints a loud warning and emits admin.bootstrap.force.

The bootstrap admin starts with must_change_password = true and totp_enabled = false. Its first login yields a restricted session that can only change the password and enroll TOTP; /authorize, the OAuth endpoints, and admin-gated actions deny it until both complete.

Replace it immediately:

• Log in as the bootstrap admin and change the password (minimum 12 characters). This clears must_change_password and stamps password_changed_at.
• Enroll TOTP and confirm with an authenticator code; the session is re-issued with full authority only now.
• Capture the recovery codes shown once and store them offline.
• Create a named second admin: d-oauth-server user create --admin --email <name>@example.com.
• Verify the named admin has full access.
• Disable the bootstrap account: d-oauth-server user disable --email <bootstrap>@example.com.
• Rotate or expire the bootstrap secret: delete the mounted password file and revoke any generated one-time password.

The step-by-step command runbook lives in RUNBOOK.md. The split is deliberate: RUNBOOK.md is the task-47 operator procedure, and this section is the task-48 security rationale and checklist.

Detecting an incident

• token.refresh_reuse_detected means a rotated refresh token was replayed. Treat the grant family as compromised.
• Repeated login.failure for one subject means a credential-stuffing attempt.
• admin.bootstrap.force means an admin was created despite an existing one.
• pii.decrypt_failure means a stored PII or TOTP ciphertext failed to open (tamper, truncation, or a missing key id). Treat it as possible data corruption or key-store drift; it carries only the request id, never a value.
• A spike in 503 on security endpoints means a dependency is down; the server is failing closed, not open.

Operational runbook

1. Rotate the suspect credential: disable the user, revoke the client, or rotate the client secret.
2. Revoke the affected tokens. Reuse detection already cascades the family.
3. If the token pepper may be compromised, rotate it and accept that tokens minted under the old pepper stop verifying once it is removed.
4. If a TLS key is compromised, replace the mounted material and roll the replicas (there is no hot reload).
5. Preserve logs. The X-Request-Id header correlates a request to its logs; audit events never contain a secret.

Full-database-disclosure blast radius

A full dump is worse than one leaked token: it exposes the whole directory and every hash or ciphertext at once. The token pepper and the TOTP encryption key are supplied outside the database (environment variable or mounted file), so a pure dump does not carry them; treat them as compromised anyway unless the key store can be shown untouched.

Dumped class	Protection at rest	What the dump yields
User passwords	Argon2id PHC, salted	Offline cracking only.
Client secrets	Argon2id PHC	Offline cracking only.
TOTP secrets	Per-row DEK wrapped by AES-256-GCM under HKDF-SHA256 (info totp-kek-v1; a re-wrap keeps the data ciphertext unchanged); v1/v2 blobs still decrypt	Readable only if the TOTP key also leaks.
TOTP recovery codes	Argon2id PHC	Offline cracking only.
DCR registration access tokens	Keyed HMAC (registration_access_token_hash)	Opaque while the pepper stays outside the dump.
Emails, client names, contacts, redirect URIs, JAR request_uris, jwks_uri, software ids, mTLS subject DNs and certificate thumbprints	Sealed AES-256-GCM under HKDF-SHA256 (info pii-field-gcm-v1), with keyed-HMAC blind indexes in _index	Opaque without the PII key. The blind indexes are deterministic tags, not plaintext, but they do reveal equality within one key generation.
Scopes, entitlements, timestamps, actor ids	Plaintext	Metadata disclosure.
Client public keys (jwks)	Plaintext	Public by design.
is_admin, disabled, must_change_password, totp_enabled, timestamps	Plaintext	Metadata disclosure.
Client registration metadata (grant_types, response_types, token_endpoint_auth_method, software_id, active, dpop_bound_access_tokens, client_id_issued_at, client_secret_expires_at) and catalog metadata (scope/redirect description, enabled, kind, actor ids granted_by/created_by/attached_by)	Plaintext	Metadata disclosure.

If the key store leaked too, the dump is a full compromise: TOTP secrets and the sealed directory decrypt, the Argon2id hashes are the only barrier left on passwords and client secrets, and registration-access-token hashes can be recomputed. Respond in the order below.

Full-database-disclosure rotation order

Rotate in this order. Each step retires a class of credential the dump already exposed; do not skip ahead because a later step looks cheaper.

1. Token pepper first. Make a new pepper active and remove the old key id. Removing the old id kills every live opaque access and refresh token, so every grant is forced back through authentication. This is the only step that retires tokens the dump cannot reconstruct. Retain the old id only to keep live tokens verifying, which is the opposite of what a breach response wants; a full disclosure drops it.
2. TOTP encryption key second. Make a new key active with the old key retained. Existing TOTP blobs stay readable while the old key is retained; run totp rewrap to re-wrap every v3 per-row DEK under the new key (the data ciphertext is untouched), or re-enroll with user enroll-totp before the old key is dropped. Once the old key is removed, a blob still naming it cannot decrypt.
3. PII field-encryption key third. Make a new PII key active with the old key retained, then run pii rotate under a write freeze (the app tier scaled to zero). Rotation re-encrypts every _enc value and recomputes every blind index under the new key, in bounded transactions under an advisory lock. The blind index is key-dependent, so uniqueness is only enforced within one key generation during the window; the freeze is what keeps a duplicate from slipping between generations. Run pii verify and only then retire the old key. See OPERATIONS.md.
4. Client secrets fourth. Rotate every confidential client secret through RFC 7592 PUT /register/{client_id}, which mints a fresh secret when the method is secret-based and returns it once.
5. DCR registration tokens last. The same PUT /register/{client_id} call rotates the registration access token on every update, and the response carries the only copy. Do not re-issue DCR tokens by replaying an initial-access token; it is single-use and consumed at registration.

The seven security endpoints stay fail-closed throughout. A Postgres or Redis outage while responding returns 503 temporarily_unavailable with Retry-After: 1, never a fail-open success; do not relax the policy to keep service up mid-rotation. The operator checklist with concrete commands is in RUNBOOK.md.

Edge cases

• A Redis flush loses ephemeral tokens but not clients, users, scopes, or redirect URIs. DCR client management survives because its token hash is in Postgres.
• An L7 ingress that terminates TLS strips the client certificate; in-process mTLS then fails. Use L4 passthrough.
• A public client that loses its DPoP key cannot use its tokens; there is no bearer fallback for a sender-constrained grant.

File and directory references

• src/crypto/ for token minting, hashing, TOTP, and constant-time compare.
• src/crypto/field.rs for the PII field codec and keyed blind index.
• src/keys.rs for the key ring.
• src/protocol/token/types.rs for the sender-constraint matrix.
• src/jose/dpop.rs, src/jose/jar.rs, src/protocol/client_auth/assertion.rs for proof verification.
• src/protocol/mtls.rs for mTLS matching.
• src/store/mod.rs, src/store/tokens.rs for rotation and tombstones.
• src/config/policy.rs for the fail-closed policy.
• src/middleware/ for redaction, rate limiting, and security headers.
• src/cli/admin.rs for bootstrap.
• RUNBOOK.md for the operator runbook (admin bootstrap, full-database-disclosure response).