An avahi security update has been released for Debian GNU/Linux 9 LTS to address a denial of service issue.

DLA 3047-1: avahi security update

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Debian LTS Advisory DLA-3047-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Markus Koschany
June 07, 2022   https://wiki.debian.org/LTS
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Package : avahi
Version : 0.6.32-2+deb9u1
CVE ID : CVE-2021-3468 CVE-2021-26720
Debian Bug : 984938

It was discovered that the Debian package of Avahi, a framework for Multicast
DNS Service Discovery, executed the script avahi-daemon-check-dns.sh with root
privileges which would allow a local attacker to cause a denial of service or
create arbitrary empty files via a symlink attack on files under
/var/run/avahi-daemon. This script is now executed with the privileges of user
and group avahi and requires sudo in order to achieve that.

The aforementioned script has been removed from Debian 10 "Buster" onwards. The
workaround could not be implemented for Debian 9 "Stretch" because libnss-mdns
0.10 does not provide the required functionality to replace it.

Furthermore it was found (CVE-2021-3468) that the event used to signal the
termination of the client connection on the avahi Unix socket is not correctly
handled in the client_work function, allowing a local attacker to trigger an
infinite loop.

For Debian 9 stretch, these problems have been fixed in version
0.6.32-2+deb9u1.

We recommend that you upgrade your avahi packages.

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

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