CVE-2021-21381

Published: 11 March 2021

Flatpak is a system for building, distributing, and running sandboxed desktop applications on Linux. In Flatpack since version 0.9.4 and before version 1.10.2 has a vulnerability in the "file forwarding" feature which can be used by an attacker to gain access to files that would not ordinarily be allowed by the app's permissions. By putting the special tokens `@@` and/or `@@u` in the Exec field of a Flatpak app's .desktop file, a malicious app publisher can trick flatpak into behaving as though the user had chosen to open a target file with their Flatpak app, which automatically makes that file available to the Flatpak app. This is fixed in version 1.10.2. A minimal solution is the first commit "`Disallow @@ and @@U usage in desktop files`". The follow-up commits "`dir: Reserve the whole @@ prefix`" and "`dir: Refuse to export .desktop files with suspicious uses of @@ tokens`" are recommended, but not strictly required. As a workaround, avoid installing Flatpak apps from untrusted sources, or check the contents of the exported `.desktop` files in `exports/share/applications/*.desktop` (typically `~/.local/share/flatpak/exports/share/applications/*.desktop` and `/var/lib/flatpak/exports/share/applications/*.desktop`) to make sure that literal filenames do not follow `@@` or `@@u`.

From the Ubuntu security team

Anton Lydike discovered that Flatpak did not properly handle special tokens in desktop files. An attacker could use this to specially craft a Flatpak application that could escape sandbox confinement.

Priority

Medium

CVSS 3 base score: 8.2

Status

Package Release Status
flatpak
Launchpad, Ubuntu, Debian
Upstream
Released (1.10.1-4)
Ubuntu 21.04 (Hirsute Hippo) Not vulnerable
(1.10.2-1)
Ubuntu 20.04 LTS (Focal Fossa)
Released (1.6.5-0ubuntu0.3)
Ubuntu 18.04 LTS (Bionic Beaver)
Released (1.0.9-0ubuntu0.3)
Ubuntu 16.04 ESM (Xenial Xerus) Does not exist

Ubuntu 14.04 ESM (Trusty Tahr) Does not exist