A c-ares security update has been released for Debian GNU/Linux 9 LTS to address a missing input validation of host names.

DLA 2738-1: c-ares security update

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Debian LTS Advisory DLA-2738-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Thorsten Alteholz
August 10, 2021   https://wiki.debian.org/LTS
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Package : c-ares
Version : 1.12.0-1+deb9u2
CVE ID : CVE-2021-3672

An issue has been found in c-ares, an asynchronous name resolver.
Missing input validation of host names returned by Domain Name Servers can
lead to output of wrong hostnames.

For Debian 9 stretch, this problem has been fixed in version
1.12.0-1+deb9u2.

We recommend that you upgrade your c-ares packages.

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

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