OpenSMTPD before 6.6.4 allows remote code execution because of an out-of-bounds read in mta_io in mta_session.c for multi-line replies. Although this vulnerability affects the client side of OpenSMTPD, it is possible to attack a server because the server code launches the client code during bounce handling.
packetstormsecurity.com/files/156633/OpenSMTPD-Out-Of-Bounds-Read-Local-Privilege-Escalation.html
seclists.org/fulldisclosure/2020/Feb/32
www.openwall.com/lists/oss-security/2020/02/26/1
www.openwall.com/lists/oss-security/2020/03/01/1
www.openwall.com/lists/oss-security/2020/03/01/2
www.openwall.com/lists/oss-security/2021/05/04/7
lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/OPH4QU4DNVHA7ACFXMYFCEP5PSXXPN4E/
usn.ubuntu.com/4294-1/
www.debian.org/security/2020/dsa-4634
www.openbsd.org/security.html
www.openwall.com/lists/oss-security/2020/02/24/5