flatpak since 0.9.4 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.

Impact

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.

Patches

Available as #4156.

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.

Workarounds

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.

References

#4146

Acknowledgements

Thanks to @AntonLydike for reporting this issue, and @refi64 for providing the initial solution.