Cotonti Siena 0.9.26 and earlier contains a cross-site request forgery vulnerability that allows unauthenticated attackers to modify administrator configuration by tricking a logged-in administrator into submitting a forged POST request to the admin.php config update handler, which never invokes the application's CSRF validation function. Attackers can disable the PFS module's file extension whitelist by setting pfsfilecheck to 0, enabling any user with PFS access to upload and execute arbitrary PHP files on the server.