Applications using @fastify/passport for user authentication, in combination with @fastify/session as the underlying session management mechanism, are vulnerable to session fixation attacks from network and same-site attackers.
Details
fastify applications rely on the @fastify/passport library for user authentication. The login and user validation are performed by the authenticate function. When executing this function, the sessionId is preserved between the pre-login and the authenticated session. Network and same-site attackers can hijack the victim's session by tossing a valid sessionId cookie in the victim's browser and waiting for the victim to log in on the website.
Fix
As a solution, newer versions of @fastify/passport regenerate sessionId upon login, preventing the attacker-controlled pre-session cookie from being upgraded to an authenticated session.