A bind9 security update has been released for Debian GNU/Linux 9 LTS to address a vulnerable to cache poisoning.

DLA 2955-1: bind9 security update

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Debian LTS Advisory DLA-2955-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Markus Koschany
March 18, 2022   https://wiki.debian.org/LTS
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Package : bind9
Version : 1:9.10.3.dfsg.P4-12.3+deb9u11
CVE ID : CVE-2021-25220

It was found that bind9, an internet domain name server, was vulnerable to
cache poisoning. When using forwarders, bogus NS records supplied by, or via,
those forwarders may be cached and used by named if it needs to recurse for any
reason, causing it to obtain and pass on potentially incorrect answers.

For Debian 9 stretch, this problem has been fixed in version
1:9.10.3.dfsg.P4-12.3+deb9u11.

We recommend that you upgrade your bind9 packages.

For the detailed security status of bind9 please refer to
its security tracker page at:
  https://security-tracker.debian.org/tracker/bind9

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