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 affected:

• 4.x: 4.0.0 through 4.16.0 (including RC versions)

Patches

The vulnerability has been addressed in the latest releases. The patch ensures Login V2 validates that any second factor enrolled on the user has been verified before an existing session can be reused to complete authentication.

• 4.x: Upgrade to $\ge$ 4.16.1

Workarounds

If an immediate upgrade is not possible, enable Force MFA (or Force MFA for local users only, if you want to exempt IdP/federated logins) in the affected organization’s — or the instance default — login policy. This makes second-factor verification mandatory for password logins and closes this session-reuse bypass.

Note that this is a broader policy change (MFA becomes mandatory for the scoped local logins) rather than a narrow fix limited to users who already self-enrolled a second factor.

Questions

If you have any questions or comments about this advisory, please email us at security@zitadel.com

Credits

Thanks to Philippe Wechsler (@MadMonkey87) for finding and reporting the vulnerability.