A Flatpak security update has been released for Ubuntu Linux 18.04 LTS, 20.04 LTS, and 20.10.

USN-4721-1: Flatpak vulnerability

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Ubuntu Security Notice USN-4721-1
February 04, 2021

flatpak vulnerability
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A security issue affects these releases of Ubuntu and its derivatives:

- Ubuntu 20.10
- Ubuntu 20.04 LTS
- Ubuntu 18.04 LTS

Summary:

Flatpak could be made to crash or run programs if it received
specially crafted input.

Software Description:
- flatpak: Application deployment framework for desktop apps

Details:

Simon McVittieg discovered that flatpak-portal service allowed sandboxed
applications to execute arbitrary code on the host system (a sandbox
escape). A malicious user could create a Flatpak application that set
environment variables, trusted by the Flatpak "run" command, and use it
to execute arbitrary code outside the sandbox.

Update instructions:

The problem can be corrected by updating your system to the following
package versions:

Ubuntu 20.10:
flatpak 1.8.2-1ubuntu0.1
libflatpak0 1.8.2-1ubuntu0.1

Ubuntu 20.04 LTS:
flatpak 1.6.5-0ubuntu0.2
libflatpak0 1.6.5-0ubuntu0.2

Ubuntu 18.04 LTS:
flatpak 1.0.9-0ubuntu0.2
libflatpak0 1.0.9-0ubuntu0.2

In general, a standard system update will make all the necessary changes.

References:
  https://usn.ubuntu.com/4721-1
CVE-2021-21261

Package Information:
  https://launchpad.net/ubuntu/+source/flatpak/1.8.2-1ubuntu0.1
  https://launchpad.net/ubuntu/+source/flatpak/1.6.5-0ubuntu0.2
  https://launchpad.net/ubuntu/+source/flatpak/1.0.9-0ubuntu0.2