An opensmtpd security update has been released for both Debian GNU/Linux 9 and 10 to address am issue that can result in local privilege escalation or the execution of arbitrary code.

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Debian Security Advisory DSA-4634-1 security@debian.org
  https://www.debian.org/security/ Moritz Muehlenhoff
February 26, 2020   https://www.debian.org/security/faq
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Package : opensmtpd
CVE ID : CVE-2020-8794
Debian Bug : 952453

Qualys discovered that the OpenSMTPD SMTP server performed insufficient
validation of SMTP commands, which could result in local privilege
escalation or the execution of arbitrary code.

For the oldstable distribution (stretch), this problem has been fixed
in version 6.0.2p1-2+deb9u3.

For the stable distribution (buster), this problem has been fixed in
version 6.0.3p1-5+deb10u4.

We recommend that you upgrade your opensmtpd packages.

For the detailed security status of opensmtpd please refer to
its security tracker page at:
  https://security-tracker.debian.org/tracker/opensmtpd

Further information about Debian Security Advisories, how to apply
these updates to your system and frequently asked questions can be
found at:   https://www.debian.org/security/