ZITADEL: MFA bypass via session reuse in Login V2

### Summary A vulnerability in ZITADEL’s Login V2 UI allowed a password-verified browser session to be reused for a new authentication request without re-checking a user’s enrolled second factor (TOTP, OTP, or U2F). An attacker who already knows valid credentials can fully authenticate to an application without completing MFA. ### Impact ZITADEL Login V2 issues a session as soon as the user’s password is verified, before the MFA challenge is completed. If the MFA step is abandoned (for example by navigating back) and login is started again, Login V2 may reuse that existing session instead of requiring the second factor. Session validity checks only enforced MFA verification when the organization’s login policy had **Force MFA** (or **Force MFA for local users only**) enabled. They did not treat a voluntarily enrolled second factor as required. In the common case where MFA is available on the user but not organization-mandated, a password-only session was treated as fully authenticated and used to complete the OIDC or SAML callback, bypassing the user’s second factor. **Scope note:** This issue affects **customer applications** that authenticate users through the hosted **Login V2** UI (OIDC/SAML). It does **not** affect Login V1. It also does **not** affect authentication to ZITADEL itself — including the Console, the Management/Admin APIs, and user self-management — even when Login V2 is enabled. ### Affected Versions Systems running one of the following versions are affecte