A debian-archive-keyring update has been released for Debian GNU/Linux 9 LTS to update the GnuPG archive keys of the Debian archive.

UPDATE: DLA 2948-1: debian-archive-keyring update

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Debian LTS Advisory DLA-2948-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Anton Gladky
March 13, 2022   https://wiki.debian.org/LTS
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Package : debian-archive-keyring
Version : 2017.5+deb9u2

debian-archive-keyring is a package containing GnuPG archive keys of the Debian
archive. New GPG-keys are being constantly added with every new Debian release.

For Debian 9 stretch, GPG-keys for 11/bullseye Debian release are added
in the version 2017.5+deb9u2.

We recommend that you upgrade your debian-archive-keyring packages only if you
need to work with packages from 11/bullseye release.

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