ZSA-2021-03
There is a denial of service (DOS) issue, when a mail with a special crafted url is received.
This can lead to a maxout of the available server-CPU(s) and can reduce the quality of service
or even bring the system to a halt.
The issue is fixed in the current stable release Znuny LTS 6.0.33. An update of Znuny LTS to the latest version is recommended.