WWBN AVideo is an open source video platform. In versions up to and including 26.0, AVideo's "_session_start()" function accepts arbitrary session IDs via the "PHPSESSID" GET parameter and sets them as the active PHP session. A session regeneration bypass exists for specific blacklisted endpoints when the request originates from the same domain. Combined with the explicitly disabled session regeneration in "User::login()", this allows a classic session fixation attack where an attacker can fix a victim's session ID before authentication and then hijack the authenticated session. Commit 5647a94d79bf69a972a86653fe02144079948785 contains a patch.