Contao is an Open Source CMS. From version 4.0.0 to before 4.13.57, before 5.3.42, and before 5.6.5, it is possible to inject code into the template output that will be executed in the browser in the front end and back end. This issue has been patched in versions 4.13.57, 5.3.42, and 5.6.5. A workaround for this issue involves not using the affected templates or patch them manually.

Contao is vulnerable to cross-site scripting in templates