An openssl1.0 security update has been released for Debian GNU/Linux 9 LTS to address an issue with Let’s Encrypt certificates.

DLA 2761-1: openssl1.0 security update

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Debian LTS Advisory DLA-2761-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Sylvain Beucler
September 18, 2021   https://wiki.debian.org/LTS
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Package : openssl1.0
Version : 1.0.2u-1~deb9u5

The legacy 1.0 version of OpenSSL, a cryptography library for secure
communication, 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
1.0.2u-1~deb9u5.

We recommend that you upgrade your openssl1.0 packages.

For the detailed security status of openssl1.0 please refer to
its security tracker page at:
  https://security-tracker.debian.org/tracker/openssl1.0

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