Server-side exploitation is possible when the attacker connects to the OpenSMTPD server and sends an email that creates a bounce. When OpenSMTPD connects back to deliver the bounce, the attacker can take advantage of the client-side vulnerability.
Security researchers have discovered a new critical vulnerability in the OpenSMTPD email server. An attacker could exploit it remotely to run shell commands as root on the underlying operating system.
OpenSMTPD is present on many Unix-based systems, including FreeBSD, NetBSD, macOS, Linux (Alpine, Arch, Debian, Fedora, CentOS).