OpenSMTPD has released version 6.6.4p1 to address a critical vulnerability. A remote attacker could exploit this vulnerability to take control of an affected server. OpenSMTPD is an open-source server-side implementation of the Simple Mail Transfer Protocol (SMTP) that is part of the OpenBSD Project. CISA encourages users and administrators to apply the necessary update. For OpenBSD implementations, binary patches are available through syspatch; see OpenSMTPD’s Message 04888 for further instruction. For other systems, the update is available at OpenSMTPD’s GitHub release page. Read the advisory at CISA.