A zsh security update has been released for Debian GNU/Linux 9 LTS to address an issue where an attacker can execute arbitrary commands into a user's shell.

DLA 2926-1: zsh security update

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Debian LTS Advisory DLA-2926-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Emilio Pozuelo Monfort
February 18, 2022   https://wiki.debian.org/LTS
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Package : zsh
Version : 5.3.1-4+deb9u5
CVE ID : CVE-2021-45444

It was discovered that zsh, a powerful shell and scripting language,
did not prevent recursive prompt expansion. This would allow an
attacker to execute arbitrary commands into a user's shell, for
instance by tricking a vcs_info user into checking out a git branch
with a specially crafted name.

For Debian 9 stretch, this problem has been fixed in version
5.3.1-4+deb9u5.

We recommend that you upgrade your zsh packages.

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

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