Exim 4.85 through 4.92 (fixed in 4.92.1) allows remote code execution as root in some unusual configurations that use the ${sort } expansion for items that can be controlled by an attacker (e.g., $local_part or $domain).
https://www.debian.org/security/2019/dsa-4488
https://security.gentoo.org/glsa/201909-06
https://seclists.org/bugtraq/2019/Jul/51