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

DSA 4849-1: firejail security update

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Debian Security Advisory DSA-4849-1 security@debian.org
  https://www.debian.org/security/ Salvatore Bonaccorso
February 09, 2021   https://www.debian.org/security/faq
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Package : firejail
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 the stable distribution (buster), this problem has been fixed in
version 0.9.58.2-2+deb10u2.

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 Security Advisories, how to apply
these updates to your system and frequently asked questions can be
found at:   https://www.debian.org/security/