CVE-2021-21381

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.

Bugs

• <https://bugs.launchpad.net/ubuntu/+source/flatpak/+bug/1918482&gt;
• <http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=984859&gt;