A squashfs-tools security update has been released for Debian GNU/Linux 9 LTS to address an issue where unsquashfs did not validate all filepaths.

DLA 2752-1: squashfs-tools security update

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Debian LTS Advisory DLA-2752-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Thorsten Alteholz
August 31, 2021   https://wiki.debian.org/LTS
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Package : squashfs-tools
Version : 1:4.3-3+deb9u2
CVE ID : CVE-2021-40153

An issue has been found in squashfs-tools, a tool to create and append to
squashfs filesystems.
As unsquashfs did not validate all filepaths, it would allow writing
outside of the original destination.

For Debian 9 stretch, this problem has been fixed in version
1:4.3-3+deb9u2.

We recommend that you upgrade your squashfs-tools packages.

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

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