Symfony is a PHP framework for web and console applications and a set of reusable PHP components. When authenticating users Symfony by default regenerates the session ID upon login, but preserves the rest of session attributes. Because this does not clear CSRF tokens upon login, this might enables same-site attackers to bypass the CSRF protection mechanism by performing an attack similar to a session-fixation. This issue has been fixed in the 4.4 branch.
Authenticating a user, or otherwise establishing a new user session, without invalidating any existing session identifier gives an attacker the opportunity to steal authenticated sessions.
According to WASC, "Insufficient Session Expiration is when a web site permits an attacker to reuse old session credentials or session IDs for authorization."