Tinyauth vulnerable to TOTP/2FA bypass via OIDC authorize endpoint

QFinch assessment

The OIDC authorization endpoint allows users with a TOTP-pending session (password verified, TOTP not yet completed) to obtain authorization codes. An attacker who knows a user's password but not their TOTP secret can obtain valid OIDC tokens, completely bypassing the second factor.

Potential impact

Complete bypass of TOTP/MFA for any user account on any tinyauth instance that has OIDC clients configured. An attacker who has compromised a user's password (credential stuffing, phishing, database breach) can obtain SSO tokens for that user's identity without knowing the TOTP secret. This defeats the purpose of the second factor entirely. All downstream applications relying on tinyauth's OIDC provider for authentication are affected.

Technical context

When a user with TOTP enabled logs in at POST /api/user/login, the server creates a session with TotpPending: true and returns a session cookie. The context middleware (internal/middleware/contextmiddleware.go:56-66) correctly sets TotpPending: true and does not set IsLoggedIn for these sessions. However, the OIDC authorize handler (internal/controller/oidccontroller.go:105-116) only checks whether a user context exists via utils.GetContext(c). It does not check IsLoggedIn or TotpPending. Since the context middleware populates a context for TOTP-pending sessions (with the username filled in), GetContext succeeds, and the handler proceeds to issue an authorization code at line 156 using the username from the incomplete session. For comparison, the proxy controller (internal/controller/proxycontroller.go:176-179) correctly blocks TOTP-incomplete sessions by checking IsBasicAuth && TotpEnabled and setting IsLoggedIn = false. The OIDC authorize handler has no equivalent guard. StoreCode at internal/service/oidcservice.go:305 saves the code with the victim's sub claim. The attacker then exchanges this code at POST /api/oidc/token for a valid access token and ID token.