Newly discovered critical vulnerabilities in Exim mail transfer agent software allow unauthenticated remote attackers to execute arbitrary code and gain root privilege on mail servers with default or common configurations. The security flaws collectively known as a 21Nails vulnerability are collectively known by Qualys Research Team. Exim is the default MTA on Debian Linux distros and currently the most popular mail server on the world’s most popular distros. All versions released before Exim 4.94 are vulnerable to attacks attempting to exploit the vulnerabilities.
Source: https://www.bleepingcomputer.com/news/security/critical-21nails-exim-bugs-expose-millions-of-servers-to-attacks/