Contao is an Open Source CMS. In versions starting from 5.0.0 and prior to 5.3.38 and 5.6.1, the table access voter in the back end doesn't check if a user is allowed to access the corresponding module. This issue has been patched in versions 5.3.38 and 5.6.1. A workaround involves not relying solely on the voter and additionally to check USER_CAN_ACCESS_MODULE.
https://github.com/contao/contao/security/advisories/GHSA-7m47-r75r-cx8v
https://github.com/contao/contao/commit/3f05c603e1c94d34819f837f060df5d66447d0d7
https://contao.org/en/security-advisories/improper-access-control-in-the-back-end-voters