@fastify/passport is a port of passport authentication library for the Fastify ecosystem. The CSRF (Cross-Site Request Forger) protection enforced by the @fastify/csrf-protection library, when combined with @fastify/passport in affected versions, can be bypassed by network and same-site attackers. fastify/csrf-protection implements the synchronizer token pattern (using plugins @fastify/session and @fastify/secure-session) by storing a random value used for CSRF token generation in the _csrf attribute of a user's session. The @fastify/passport library does not clear the session object upon authentication, preserving the _csrf attribute between pre-login and authenticated sessions.

Consequently, CSRF tokens generated before authentication are still valid. Network and same-site attackers can thus obtain a CSRF token for their pre-session, fixate that pre-session in the victim's browser via cookie tossing, and then perform a CSRF attack after the victim authenticates. As a solution, newer versions of @fastify/passport include the configuration options: clearSessionOnLogin (default: true) and clearSessionIgnoreFields (default: ['passport', 'session']) to clear all the session attributes by default, preserving those explicitly defined in clearSessionIgnoreFields.