Read the official Contao announcements.
Contao 3.5.35 is available
by Leo Feyer – Announcements
Contao version 3.5.35 is available. The bugfix release fixes an XSS vulnerability in the system log of the back end (CVE-2018-10125).
With a manipulated request, an attacker can implant a script which is executed when a logged in back end user opens the system log. The attacker themselves does not have to be logged in.
The problem affects Contao 3.0.0 to 3.5.34, 4.0.0 to 4.4.17 and 4.5.0 to 4.5.7. We highly recommend you to update.