An exim4 security update has been released for both Debian GNU/Linux 9 and 10 to address an authentication bypass vulnerability.

DSA 4687-1: exim4 security update

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Debian Security Advisory DSA-4687-1 security@debian.org
  https://www.debian.org/security/ Florian Weimer
May 16, 2020   https://www.debian.org/security/faq
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Package : exim4
CVE ID : CVE-2020-12783

It was discovered that exim4, a mail transport agent, suffers from a
authentication bypass vulnerability in the spa authentication driver.
The spa authentication driver is not enabled by default.

For the oldstable distribution (stretch), this problem has been fixed
in version 4.89-2+deb9u7.

For the stable distribution (buster), this problem has been fixed in
version 4.92-8+deb10u4.

We recommend that you upgrade your exim4 packages.

For the detailed security status of exim4 please refer to
its security tracker page at:
  https://security-tracker.debian.org/tracker/exim4

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/