ILIAS deserialises stored session data for an unauthenticated caller. The Shibboleth back-channel endpoint at components/ILIAS/AuthShibboleth/resources/shib_logout.php runs in a context that ilInitialisation exempts from authentication, and its logout-notification handler locates the session to terminate by reading every live row of the session table and passing each row's stored data to a hand-written parser that calls unserialize without restricting which classes may be constructed. Any serialised object present in any session row is therefore instantiated on behalf of an anonymous request, and object destructors run when those objects are discarded. A serialised object can be placed into a session row without logging in, because the LTI authentication entry point stores request parameters into the session and is reachable on a path the same initialisation code exempts from authentication. A class bundled with the application writes a JSON-encoded structure to a file named by one of its own properties when it is destroyed, which places attacker-controlled content at an attacker-chosen path below the web root and results in code execution as the web server user. Versions 9.22, 10.10 and 11.3 remove the endpoint's logout-notification implementation.