mod/chat/chat_ajax.php in Moodle through 2.3.11, 2.4.x before 2.4.9, 2.5.x before 2.5.5, and 2.6.x before 2.6.2 does not properly check for the mod/chat:chat capability during chat sessions, which allows remote authenticated users to bypass intended access restrictions in opportunistic circumstances by remaining in a chat session after an intra-session capability removal by an administrator.