Admidio versions before 5.0.11 contain an authentication bypass vulnerability in the forum module. When the application is configured in login-only mode, the access control logic in modules/forum.php fails to properly validate the login-only configuration state. This flaw allows unauthenticated attackers to read forum topics and posts by directly accessing the module with read-only parameters. The vulnerability effectively bypasses the intended access restriction, exposing potentially sensitive forum content to unauthorized users. A fix is available in Admidio version 5.0.11 and later. The issue has been documented in the official GitHub security advisory and tracked by VulnCheck.