Alf.io is a free and open source event attendance management system. In versions prior to 2.0-M4-2402 users can access the admin area even after being invalidated/deleted. This issue has been addressed in version 2.0-M4-2402. All users are advised to upgrade. There are no known workarounds for this vulnerability.
According to WASC, “Insufficient Session Expiration is when a web site permits an attacker to reuse old session credentials or session IDs for authorization.”