The vulnerability stems from the lack of session termination after user permissions are modified. The patch introduced a new method terminateSessionId() in the User class and called it in phpmyfaq/admin/user.php after updating user rights. The absence of this session invalidation in previous versions meant that existing sessions remained valid even after permissions were changed, leading to insufficient session expiration. The vulnerable code is located in the user permission update logic within admin/user.php, which did not include the necessary session termination step before the patch. The high confidence comes from the direct correlation between the patch's addition of session termination and the vulnerability description.