REDAXO is a PHP-based content management system. Prior to 5.21.2, redaxo/src/addons/mediapool/pages/sync.php inserts filenames held in $diffFiles from the media filesystem into the Mediapool Sync page without rex_escape(). An attacker who can place an unregistered file with HTML metacharacters in the media directory can execute script in the browser of a backend user with media[sync] permission when that user opens the Sync page, enabling session theft or unauthorized backend actions. This issue is fixed in version 5.21.2.