Errata patches for OpenSMTPD have been released for OpenBSD 6.5 and 6.6. An out of bounds read in smtpd allows an attacker to inject arbitrary commands into the envelope file which are then executed as root.
Separately, missing privilege revocation in smtpctl allows arbitrary commands to be run with the _smtpq group. Binary updates for the amd64, i386, and arm64 platforms are available via the syspatch utility. Source code patches can be found on the respective errata page: https://www.openbsd.org/errata65.html https://www.openbsd.org/errata66.html After patching, restart the smtpd service.