A zsh security update has been released for Debian GNU/Linux 9 LTS to address several security vulnerabilities.

DLA 2470-1: zsh security update

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Debian LTS Advisory DLA-2470-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Markus Koschany
November 30, 2020   https://wiki.debian.org/LTS
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Package : zsh
Version : 5.3.1-4+deb9u4
CVE ID : CVE-2017-18206 CVE-2018-0502 CVE-2018-1071 CVE-2018-1083
CVE-2018-1100 CVE-2018-13259 CVE-2019-20044
Debian Bug : 908000 894044 894043 895225 951458

Several security vulnerabilities were found and corrected in zsh, a powerful
shell and scripting language. Off-by-one errors, wrong parsing of shebang lines
and buffer overflows may lead to unexpected behavior. A local, unprivileged
user can create a specially crafted message file or directory path. If the
receiving user is privileged or traverses the aforementioned path, this leads
to privilege escalation.

For Debian 9 stretch, these problems have been fixed in version
5.3.1-4+deb9u4.

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