Products.ATContentTypes are the core content types for Plone 2.1 - 4.3. Versions of Plone that are dependent on Products.ATContentTypes prior to version 3.0.6 are vulnerable to reflected cross site scripting and open redirect when an attacker can get a compromised version of the image_view_fullscreen page in a cache, for example in Varnish. The technique is known as cache poisoning. Any later visitor can get redirected when clicking on a link on this page. Usually only anonymous users are affected, but this depends on the user's cache settings. Version 3.0.6 of Products.ATContentTypes has been released with a fix. This version works on Plone 5.2, Python 2 only. As a workaround, make sure the image_view_fullscreen page is not stored in the cache. More information about the vulnerability and cvmitigation measures is available in the GitHub Security Advisory.

References:

https://github.com/plone/Products.ATContentTypes/commit/fc793f88f35a15a68b52e4abed77af0da5fdbab8
https://github.com/plone/Products.ATContentTypes/security/advisories/GHSA-g4c2-ghfg-g5rh