A postgresql-9.6 security update has been released for Debian GNU/Linux 9 LTS to disallow SSL renegotiation completely.

DLA 2751-1: postgresql-9.6 security update

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Debian LTS Advisory DLA-2751-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Christoph Berg
August 31, 2021   https://wiki.debian.org/LTS
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Package : postgresql-9.6
Version : 9.6.23-0+deb9u1
CVE ID : CVE-2021-3449

PostgreSQL 9.6.23 fixes this security issue:

Disallow SSL renegotiation more completely (Michael Paquier)

SSL renegotiation has been disabled for some time, but the server would
still cooperate with a client-initiated renegotiation request. A
maliciously crafted renegotiation request could result in a server crash
(see OpenSSL issue CVE-2021-3449). Disable the feature altogether on
OpenSSL versions that permit doing so, which are 1.1.0h and newer.

For Debian 9 stretch, this problem has been fixed in version
9.6.23-0+deb9u1.

We recommend that you upgrade your postgresql-9.6 packages.

For the detailed security status of postgresql-9.6 please refer to
its security tracker page at:
  https://security-tracker.debian.org/tracker/postgresql-9.6

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