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.
https://github.com/redaxo/core/security/advisories/GHSA-w998-qmw9-mf4m
https://github.com/redaxo/core/releases/tag/5.21.2
https://github.com/redaxo/core/pull/6581
https://github.com/redaxo/core/commit/2daaa3a30570bc76a82f63fd21fb8c9c2cd5dc7c