A libssh security update has been released for Debian GNU/Linux 11 to address a buffer overflow.

DSA 4965-1: libssh security update

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Debian Security Advisory DSA-4965-1 security@debian.org
  https://www.debian.org/security/ Moritz Muehlenhoff
August 31, 2021   https://www.debian.org/security/faq
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Package : libssh
CVE ID : CVE-2021-3634
Debian Bug : 993046

It was discovered that a buffer overflow in rekeying in libssh could
result in denial of service or potentially the execution of arbitrary
code.

The oldstable distribution (buster) is not affected.

For the stable distribution (bullseye), this problem has been fixed in
version 0.9.5-1+deb11u1.

We recommend that you upgrade your libssh packages.

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

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/