The vulnerability stems from improper session management during FORM authentication when session IDs are URL-based. Session fixation occurs because the existing session (potentially provided by an attacker) isn't replaced after authentication. The FormAuthenticationMechanism class is central to FORM authentication handling in WildFly Elytron. The attemptAuthentication method would logically be responsible for session management during authentication. The lack of session invalidation/regeneration in this flow matches the described attack pattern. While no direct code is shown, the CWE-384 classification and mitigation guidance (switching to cookie-based sessions) strongly implicate session management in the FORM authentication handler.