In Apache SpamAssassin before 3.4.5, malicious rule configuration (.cf) files can be configured to run system commands without any output or errors. With this, exploits can be injected in a number of scenarios. In addition to upgrading to SA version 3.4.5, users should only use update channels or 3rd party .cf files from trusted places.
lists.debian.org/debian-lts-announce/2021/04/msg00000.html
lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/7V2SBVTKVLFFT36ECJQ7TQ7KAQCQZDRZ/
lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/JFBFRIG5TX23NF4ND6OAKKY7I6TLRCCP/
lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/NKAXYBKBMQOLIW6UKASJCAZRBOIYS4RL/
s.apache.org/3r1wh
security.gentoo.org/glsa/202105-26
www.debian.org/security/2021/dsa-4879