A bind9 security update has been released for Debian GNU/Linux 10 and 11 to address a denial of service issue in the lame server cache in BIND.

DSA 4994-1: bind9 security update

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Debian Security Advisory DSA-4994-1 security@debian.org
  https://www.debian.org/security/ Salvatore Bonaccorso
October 28, 2021   https://www.debian.org/security/faq
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Package : bind9
CVE ID : CVE-2021-25219

Kishore Kumar Kothapalli discovered that the lame server cache in BIND,
a DNS server implementation, can be abused by an attacker to
significantly degrade resolver performance, resulting in denial of
service (large delays for responses for client queries and DNS timeouts
on client hosts).

For the oldstable distribution (buster), this problem has been fixed
in version 1:9.11.5.P4+dfsg-5.1+deb10u6.

For the stable distribution (bullseye), this problem has been fixed in
version 1:9.16.22-1~deb11u1.

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