Free software Exim servers up to and including 4.92.1 have a serious flaw (CVE-2019-15846) that could allow a remote, unauthenticated attacker to take full control of them. The vulnerability ranks 9.8 out of 10 on the CVSS scale, making it critical in severity. Exim is the most used mail transfer agent globally and has over 5 million internet-facing hosts, meaning the attack surface for the flaw is massive. No public exploit of the vulnerabilities have yet been reported, but a rudimentary proof-of-concept does exist.
Source: https://threatpost.com/critical-exim-flaw-opens-millions-of-servers-to-takeover/148108/