CSRF token fixation in fastify-passport

The CSRF protection enforced by the @fastify/csrf-protection library, when combined with @fastify/passport, can be bypassed by network and same-site attackers.

Details

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.

Fix

As a solution, newer versions of @fastify/passport include the configuration options

• clearSessionOnLogin (default: true) and
• clearSessionIgnoreFields (default: ['session'])

to clear all the session attributes by default, preserving those explicitly defined in clearSessionIgnoreFields.

Credits

• Pedro Adão (@pedromigueladao), Instituto Superior Técnico, University of Lisbon
• Marco Squarcina (@lavish), Security & Privacy Research Unit, TU Wien