Read the official Contao announcements.
Contao 3.5.28 is available
by Leo Feyer – Announcements
Contao version 3.5.28 is available. The bugfix release fixes an arbitrary PHP file inclusion vulnerability in the back end.
A logged in back end user can include arbitrary PHP files by manipulating an URL parameter. Since Contao does not allow to upload PHP files in the file manager, the attack is limited to the existing PHP files on the server.
The issue affects Contao 3.0.0 to 3.5.27 and Contao 4.0.0 to 4.4.0.
Although we do not consider the vulnerability to be critical, we strongly recommend to update to either Contao 3.5.28 or Contao 4.4.1.