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shankar0123
2cd2a5c52f
Audit 2026-05-10 MED-17 closure.
WHAT.
When the matched IdP's discovery doc advertises
authorization_response_iss_parameter_supported=true (RFC 9207 §3),
HandleCallback now REQUIRES a non-empty `iss` query parameter on
/auth/oidc/callback and enforces a constant-time compare against the
configured provider's IssuerURL. Mismatch maps to two new sentinel
errors (ErrIssParamMissing / ErrIssParamMismatch) that the handler's
classifyOIDCFailure dispatches via errors.Is BEFORE the substring
fall-through, so the audit failure_category remains distinguishable
between the RFC 9207 leg (iss_param_missing / iss_param_mismatch) and
the in-token iss claim leg (id_token_iss_mismatch).
WHY.
The RFC 9207 iss URL parameter is the load-bearing mix-up-attack
defense for multi-tenant IdPs (Keycloak realms, Authentik tenants,
Auth0 tenants, public-trust CAs). Pre-fix the parameter was silently
ignored — an attacker controlling one IdP tenant could route an auth
code to certctl's callback against a different tenant's pre-login
state without detection. Modern Keycloak / Authentik / public-trust
CAs ship the discovery flag by default; legacy IdPs that don't
advertise are unaffected (back-compat preserved).
HOW.
- internal/auth/oidc/service.go
- providerEntry gains issParamSupported bool.
- getOrLoad extends the discovery-claims read to include
authorization_response_iss_parameter_supported, alongside the
existing id_token_signing_alg_values_supported defense.
- HandleCallback's signature gains callbackIss string at position 5.
Step 2.5 runs after the state compare + provider load: when
issParamSupported is true, an empty callbackIss returns
ErrIssParamMissing; a present-but-mismatched value returns
ErrIssParamMismatch (constant-time compare).
- Two new sentinels: ErrIssParamMissing, ErrIssParamMismatch.
ErrIssuerMismatch's doc-string clarified to note it covers the
in-token leg only.
- internal/api/handler/auth_session_oidc.go
- OIDCAuthHandshaker.HandleCallback signature updated.
- LoginCallback reads r.URL.Query().Get("iss") (no TrimSpace —
byte-strict compare upstream) and threads it through.
- classifyOIDCFailure: typed errors.Is dispatch for the three
iss-family sentinels BEFORE the substring fall-through, so the
three cases stay distinguishable in the audit row.
- internal/api/handler/auth_session_oidc_test.go
- stubOIDCSvc.HandleCallback bumped to 7-arg signature.
- TestClassifyOIDCFailure extended with 5 new cases pinning the
iss-family dispatch + a wrapped-error round-trip.
- internal/auth/oidc/service_test.go
- mockIdP gains advertiseIssParameterSupported bool; the
/.well-known/openid-configuration handler emits the claim only
when set (so existing tests stay back-compat).
- 4 new regression tests:
* MED17_NoSupport_AnyIssAccepted — provider doesn't advertise;
arbitrary callbackIss is ignored (back-compat).
* MED17_SupportButMissing — provider advertises; missing iss →
ErrIssParamMissing.
* MED17_SupportButMismatch — provider advertises; wrong iss →
ErrIssParamMismatch (load-bearing mix-up defense).
* MED17_SupportAndCorrect — provider advertises; matching iss →
success path proves the gate isn't over-eager.
- internal/auth/oidc/bench_test.go,
internal/auth/oidc/logging_test.go,
internal/auth/oidc/integration_keycloak_test.go
- Mechanical: all existing HandleCallback call sites updated to
pass "" for callbackIss (matches pre-fix behavior for IdPs that
don't advertise support — the Keycloak integration suite tests
will be re-evaluated once the Keycloak fixture is run against a
realm with the discovery flag enabled).
VERIFY.
- go vet ./internal/auth/oidc/... ./internal/api/handler/... PASS
- go test -short -count=1 ./internal/auth/oidc/... PASS (3.4s)
- go test -short -count=1 ./internal/api/handler/... PASS (5.4s)
- 4 new MED-17 regression tests + extended TestClassifyOIDCFailure pass.
Refs: cowork/auth-bundles-audit-2026-05-10.md MED-17
cowork/auth-bundles-fixes-2026-05-10/HANDOFF.md item 7
RFC 9207 — OAuth 2.0 Authorization Server Issuer Identification