We discovered a vulnerability in OpenSMTPD, OpenBSD’s mail server. This vulnerability, an out-of-bounds read introduced in December 2015 (commit 80c6a60c, “when peer outputs a multi-line response …”), is exploitable remotely and leads to the execution of arbitrary shell commands: either as root, after May 2018 (commit a8e22235, “switch smtpd to new grammar”); or as any non-root user, before May 2018.