Contao is an open source content management system. Starting in version 4.0.0 and prior to version 4.13.40 and 5.3.4, users can inject malicious code in filenames when uploading files (back end and front end), which is then executed in tooltips and popups in the back end. Contao versions 4.13.40 and 5.3.4 have a patch for this issue. As a workaround, remove upload fields from frontend forms and disable uploads for untrusted back end users.

Contao: Cross site scripting in the file manager