A firejail security update has been released for Debian GNU/Linux 9 LTS to address a root privilege escalation.

DLA 2554-1: firejail security update

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Debian LTS Advisory DLA-2554-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Utkarsh Gupta
February 11, 2021   https://wiki.debian.org/LTS
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Package : firejail
Version : 0.9.44.8-2+deb9u2
CVE ID : CVE-2021-26910

Roman Fiedler discovered a vulnerability in the OverlayFS code in
firejail, a sandbox program to restrict the running environment of
untrusted applications, which could result in root privilege
escalation. This update disables OverlayFS support in firejail.

For Debian 9 stretch, this problem has been fixed in version
0.9.44.8-2+deb9u2.

We recommend that you upgrade your firejail packages.

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

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