The receive_msg function in receive.c in the SMTP daemon in Exim 4.88 and 4.89 allows remote attackers to execute arbitrary code or cause a denial of service (use-after-free) via vectors involving BDAT commands.
http://openwall.com/lists/oss-security/2017/11/25/1
http://openwall.com/lists/oss-security/2017/11/25/2
http://openwall.com/lists/oss-security/2017/11/25/3
https://github.com/LetUsFsck/PoC-Exploit-Mirror/tree/master/CVE-2017-16944
https://www.debian.org/security/2017/dsa-4053