GHSA-xg6x-h9c9-2m83: 
Better Auth vulnerability analysis and mitigation

Summary

Under certain configurations, sessions may be considered valid before two-factor authentication (2FA) is fully completed. This can allow access to authenticated routes without verifying the second factor.

Description

When two-factor authentication is enabled, the authentication flow correctly identifies users who require additional verification and defers full authentication until the second factor is completed. However, when session.cookieCache is enabled, the session generated during the initial sign-in step may be cached as valid prior to 2FA verification. Subsequent session lookups may then return this cached session without re-evaluating the 2FA requirement. This results in a situation where session validity can be established before all authentication constraints are satisfied.

Impact

An attacker (or user) with valid primary credentials may gain access to protected application routes without completing the required second authentication factor. Any application using better-auth with both two-factor authentication and session cookie caching enabled may be affected.

Mitigation

• Upgrade to a version of better-auth that includes the fix for this issue.
• Ensure that session caching does not treat sessions as fully authenticated until all required authentication steps, including 2FA, are completed.
• As a temporary workaround, disable session.cookieCache when using two-factor authentication.