A vulnerability classified as critical was found in Zope up to 4.6.0/5.20 (Application Server Software). Affected by this vulnerability is an unknown functionality of the component TAL Expression Handler. Upgrading to version 4.6.1 or 5.21 eliminates this vulnerability. Applying a patch is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.