A gnutls28 alternate chains verification security update has been released for Debian GNU/Linux 9 LTS to address an issue where GnuTLS fails to validate alternate trust chains in some conditions.

DLA 2759-1: gnutls28 alternate chains verification

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Debian LTS Advisory DLA-2759-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Sylvain Beucler
September 17, 2021   https://wiki.debian.org/LTS
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Package : gnutls28
Version : 3.5.8-5+deb9u6
Debian Bug : 961889

GnuTLS, a portable cryptography library, fails to validate alternate
trust chains in some conditions. In particular this breaks connecting
to servers that use Let's Encrypt certificates, starting 2021-10-01.

For Debian 9 stretch, this problem has been fixed in version
3.5.8-5+deb9u6.

We recommend that you upgrade your gnutls28 packages.

For the detailed security status of gnutls28 please refer to
its security tracker page at:
  https://security-tracker.debian.org/tracker/gnutls28

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