A btrbk security update has been released for Debian GNU/Linux 9 LTS to address an arbitrary code execution issue.

DLA 2755-1: btrbk security update

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Debian LTS Advisory DLA-2755-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Thorsten Alteholz
September 05, 2021   https://wiki.debian.org/LTS
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Package : btrbk
Version : 0.24.0-1+deb9u1
CVE ID : CVE-2021-38173

An issue has been found in btrbk, a backup tool for btrfs subvolumes.
Due to mishandling of remote hosts filtering SSH commands using
ssh_filter_btrbk.sh in authorized_keys an arbitrary code execution would
have been allowed.

For Debian 9 stretch, this problem has been fixed in version
0.24.0-1+deb9u1.

We recommend that you upgrade your btrbk packages.

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

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