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`.

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.

Status

Show unmaintained releases

Package	Ubuntu Release	Status
flatpak	21.04 hirsute	Not affected
	20.10 groovy	Fixed 1.8.2-1ubuntu0.2
	20.04 LTS focal	Fixed 1.6.5-0ubuntu0.3
	18.04 LTS bionic	Fixed 1.0.9-0ubuntu0.3
	16.04 LTS xenial	Not in release
	14.04 LTS trusty	Not in release

Severity score breakdown

Parameter	Value
Base score	8.2 · High
Attack vector	Local
Attack complexity	Low
Privileges required	None
User interaction	Required
Scope	Changed
Confidentiality	High
Integrity impact	High
Availability impact	None
Vector	CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:N

References

Related Ubuntu Security Notices (USN)

USN-4951-1
Flatpak vulnerability
12 May 2021