Contao: Cross-site scripting in the comments bundle

An unauthenticated front end visitor can post a comment containing a script that runs as whichever back end user opens the Comments module, without further interaction. It can act with that user's access, including reading accessible modules, creating an administrator or editing a template, which can lead to code execution. Comment moderation does not prevent this: unpublished comments are stored, and opening the list to moderate them triggers exposure.

Check composer.lock for contao/comments-bundle. Affected versions are >= 4.0.0, < 5.3.50 and >= 5.4.0-RC1, < 5.7.12; the fixes are 5.3.50 and 5.7.12. Check whether front end visitors can submit comments and back end users open the Comments module. Moderation does not protect those users, and the advisory states that the Contao back end sends no Content-Security-Policy header.

An unauthenticated front end visitor can post a comment containing a XSS injection. The victim is any back end user who opens the Comments module, and moderation makes exposure certain rather than preventing it. No Content-Security-Policy header is sent on the Contao back end, so nothing mitigates the inline handler.

Impact

Anyone on the internet can inject script that executes in the Contao back end origin, under the session of whichever user opens the Comments module, with no click and no hover. From there the payload acts as that user through the back end: reading any module they can reach, creating a new administrator, or editing a template, which on Contao is a documented path from back end write access to code execution. Moderation does not help and in fact guarantees exposure, since unpublished comments are still inserted and a moderator has to open the list to act on them.