smtpd/lka_filter.c in OpenSMTPD before 6.8.0p1, in certain configurations, allows remote attackers to cause a denial of service (NULL pointer dereference and daemon crash) via a crafted pattern of client activity, because the filter state machine does not properly maintain the I/O channel between the SMTP engine and the filters layer.
https://www.mail-archive.com/misc%40opensmtpd.org/msg05188.html
https://security.gentoo.org/glsa/202105-12
https://github.com/openbsd/src/commit/6c3220444ed06b5796dedfd53a0f4becd903c0d1