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

> A malicious Flatpak app can access unauthorized files by exploiting special tokens in .desktop files, bypassing sandbox permissions.

- **Severity:** High 7.1 (`CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 11 Mar 2021
- **Updated:** 3 Aug 2024
- **Also known as:** DEBIAN-CVE-2021-21381, DSA-4868-1, GHSA-xgh4-387p-hqpp, MGASA-2021-0145, RHSA-2021:1002, RHSA-2021:1068, RHSA-2021:1073, RHSA-2021:1074, UBUNTU-CVE-2021-21381, USN-4951-1

## What is CVE-2021-21381?

CVE-2021-21381 is a high-severity path traversal vulnerability in Debian Debian Linux, published on 11 Mar 2021. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.

### Official 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`.

Weakness: CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component)

## Which versions are affected by CVE-2021-21381?

It affects 3 products.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `flatpak` | Red Hat:enterprise_linux:7::client | All versions | 0:1.0.9-11.el7_9 |
| `flatpak-builder` | Red Hat:enterprise_linux:7::client | All versions | 0:1.0.0-11.el7_9 |
| `flatpak-debuginfo` | Red Hat:enterprise_linux:7::client | All versions | 0:1.0.9-11.el7_9 |
| `flatpak-devel` | Red Hat:enterprise_linux:7::client | All versions | 0:1.0.9-11.el7_9 |
| `flatpak-libs` | Red Hat:enterprise_linux:7::client | All versions | 0:1.0.9-11.el7_9 |
| `flatpak` | Red Hat:enterprise_linux:7::computenode | All versions | 0:1.0.9-11.el7_9 |
| `flatpak-builder` | Red Hat:enterprise_linux:7::computenode | All versions | 0:1.0.0-11.el7_9 |
| `flatpak-debuginfo` | Red Hat:enterprise_linux:7::computenode | All versions | 0:1.0.9-11.el7_9 |
| `flatpak-devel` | Red Hat:enterprise_linux:7::computenode | All versions | 0:1.0.9-11.el7_9 |
| `flatpak-libs` | Red Hat:enterprise_linux:7::computenode | All versions | 0:1.0.9-11.el7_9 |
| `flatpak` | Red Hat:enterprise_linux:7::server | All versions | 0:1.0.9-11.el7_9 |
| `flatpak-builder` | Red Hat:enterprise_linux:7::server | All versions | 0:1.0.0-11.el7_9 |
| `flatpak-debuginfo` | Red Hat:enterprise_linux:7::server | All versions | 0:1.0.9-11.el7_9 |
| `flatpak-devel` | Red Hat:enterprise_linux:7::server | All versions | 0:1.0.9-11.el7_9 |
| `flatpak-libs` | Red Hat:enterprise_linux:7::server | All versions | 0:1.0.9-11.el7_9 |
| `flatpak` | Red Hat:enterprise_linux:7::workstation | All versions | 0:1.0.9-11.el7_9 |
| `flatpak-builder` | Red Hat:enterprise_linux:7::workstation | All versions | 0:1.0.0-11.el7_9 |
| `flatpak-debuginfo` | Red Hat:enterprise_linux:7::workstation | All versions | 0:1.0.9-11.el7_9 |
| `flatpak-devel` | Red Hat:enterprise_linux:7::workstation | All versions | 0:1.0.9-11.el7_9 |
| `flatpak-libs` | Red Hat:enterprise_linux:7::workstation | All versions | 0:1.0.9-11.el7_9 |
| `flatpak` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.6.2-6.el8_3 |
| `flatpak-debuginfo` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.6.2-6.el8_3 |
| `flatpak-debugsource` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.6.2-6.el8_3 |
| `flatpak-libs` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.6.2-6.el8_3 |
| `flatpak-libs-debuginfo` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.6.2-6.el8_3 |
| `flatpak-selinux` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.6.2-6.el8_3 |
| `flatpak-session-helper` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.6.2-6.el8_3 |
| `flatpak-session-helper-debuginfo` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.6.2-6.el8_3 |
| `flatpak-tests-debuginfo` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.6.2-6.el8_3 |
| `flatpak` | Red Hat:rhel_eus:8.1::appstream | All versions | 0:1.0.9-4.el8_1 |
| `flatpak-debuginfo` | Red Hat:rhel_eus:8.1::appstream | All versions | 0:1.0.9-4.el8_1 |
| `flatpak-debugsource` | Red Hat:rhel_eus:8.1::appstream | All versions | 0:1.0.9-4.el8_1 |
| `flatpak-libs` | Red Hat:rhel_eus:8.1::appstream | All versions | 0:1.0.9-4.el8_1 |
| `flatpak-libs-debuginfo` | Red Hat:rhel_eus:8.1::appstream | All versions | 0:1.0.9-4.el8_1 |
| `flatpak` | Red Hat:rhel_eus:8.2::appstream | All versions | 0:1.6.2-5.el8_2 |
| `flatpak-debuginfo` | Red Hat:rhel_eus:8.2::appstream | All versions | 0:1.6.2-5.el8_2 |
| `flatpak-debugsource` | Red Hat:rhel_eus:8.2::appstream | All versions | 0:1.6.2-5.el8_2 |
| `flatpak-libs` | Red Hat:rhel_eus:8.2::appstream | All versions | 0:1.6.2-5.el8_2 |
| `flatpak-libs-debuginfo` | Red Hat:rhel_eus:8.2::appstream | All versions | 0:1.6.2-5.el8_2 |
| `flatpak-selinux` | Red Hat:rhel_eus:8.2::appstream | All versions | 0:1.6.2-5.el8_2 |

Products: Debian Debian Linux, Fedoraproject Fedora, Flatpak Flatpak.

## How do I fix CVE-2021-21381?

Upgrade to a fixed version.

## Is CVE-2021-21381 being exploited?

There is no known exploitation of CVE-2021-21381 in the wild and no public exploit code.

## Other vulnerabilities in Debian Debian Linux

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- [CVE-2026-24061](https://insights.sbomify.com/CVE-2026-24061/): GNU InetUtils Argument Injection (critical 9.8, exploited in the wild)
- [CVE-2025-39682](https://insights.sbomify.com/CVE-2025-39682/): Linux Kernel Improper Check for Unusual or Exceptional Conditions (critical 9.8, exploited in the wild)
- [CVE-2024-9680](https://insights.sbomify.com/CVE-2024-9680/): Mozilla Firefox Use-After-Free (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-21381 and fix it if it is affected.

What it is: CVE-2021-21381 is a high-severity path traversal vulnerability in Debian Debian Linux, published on 11 Mar 2021. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2021-21381 in the wild and no public exploit code.

Affected product: Debian Debian Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-21381/ (Markdown: https://insights.sbomify.com/CVE-2021-21381/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-21381
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-21381
- OSV: https://osv.dev/vulnerability/CVE-2021-21381

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-21381/
Summaries are generated and may contain mistakes; the official description above is the record.