# CVE-2021-21261: Flatpak sandbox escape via spawn portal

> A malicious Flatpak app can escape its sandbox and execute arbitrary code on the host system by manipulating environment variables passed to the portal service.

- **Severity:** High 7.3 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 14 Jan 2021
- **Updated:** 3 Aug 2024
- **Also known as:** CLEANSTART-2024-PO02295, DEBIAN-CVE-2021-21261, DSA-4830-1, GHSA-4ppf-fxf6-vxg2, OESA-2021-1043, RHSA-2021:0304, RHSA-2021:0306, RHSA-2021:0307, RHSA-2021:0411, SUSE-SU-2021:1094-1, UBUNTU-CVE-2021-21261, USN-4721-1, openSUSE-SU-2021:0520-1

## What is CVE-2021-21261?

CVE-2021-21261 is a high-severity code injection vulnerability in Debian Debian Linux, published on 14 Jan 2021. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

Flatpak is a system for building, distributing, and running sandboxed desktop applications on Linux. A bug was discovered in the `flatpak-portal` service that can allow sandboxed applications to execute arbitrary code on the host system (a sandbox escape). This sandbox-escape bug is present in versions from 0.11.4 and before fixed versions 1.8.5 and 1.10.0. The Flatpak portal D-Bus service (`flatpak-portal`, also known by its D-Bus service name `org.freedesktop.portal.Flatpak`) allows apps in a Flatpak sandbox to launch their own subprocesses in a new sandbox instance, either with the same security settings as the caller or with more restrictive security settings. For example, this is used in Flatpak-packaged web browsers such as Chromium to launch subprocesses that will process untrusted web content, and give those subprocesses a more restrictive sandbox than the browser itself. In vulnerable versions, the Flatpak portal service passes caller-specified environment variables to non-sandboxed processes on the host system, and in particular to the `flatpak run` command that is used to launch the new sandbox instance. A malicious or compromised Flatpak app could set environment variables that are trusted by the `flatpak run` command, and use them to execute arbitrary code that is not in a sandbox. As a workaround, this vulnerability can be mitigated by preventing the `flatpak-portal` service from starting, but that mitigation will prevent many Flatpak apps from working correctly. This is fixed in versions 1.8.5 and 1.10.0.

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

## Which versions are affected by CVE-2021-21261?

It affects 2 products.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `flatpak` | Red Hat:enterprise_linux:7::client | All versions | 0:1.0.9-10.el7_9 |
| `flatpak-builder` | Red Hat:enterprise_linux:7::client | All versions | 0:1.0.0-10.el7_9 |
| `flatpak-debuginfo` | Red Hat:enterprise_linux:7::client | All versions | 0:1.0.9-10.el7_9 |
| `flatpak-devel` | Red Hat:enterprise_linux:7::client | All versions | 0:1.0.9-10.el7_9 |
| `flatpak-libs` | Red Hat:enterprise_linux:7::client | All versions | 0:1.0.9-10.el7_9 |
| `flatpak` | Red Hat:enterprise_linux:7::computenode | All versions | 0:1.0.9-10.el7_9 |
| `flatpak-builder` | Red Hat:enterprise_linux:7::computenode | All versions | 0:1.0.0-10.el7_9 |
| `flatpak-debuginfo` | Red Hat:enterprise_linux:7::computenode | All versions | 0:1.0.9-10.el7_9 |
| `flatpak-devel` | Red Hat:enterprise_linux:7::computenode | All versions | 0:1.0.9-10.el7_9 |
| `flatpak-libs` | Red Hat:enterprise_linux:7::computenode | All versions | 0:1.0.9-10.el7_9 |
| `flatpak` | Red Hat:enterprise_linux:7::server | All versions | 0:1.0.9-10.el7_9 |
| `flatpak-builder` | Red Hat:enterprise_linux:7::server | All versions | 0:1.0.0-10.el7_9 |
| `flatpak-debuginfo` | Red Hat:enterprise_linux:7::server | All versions | 0:1.0.9-10.el7_9 |
| `flatpak-devel` | Red Hat:enterprise_linux:7::server | All versions | 0:1.0.9-10.el7_9 |
| `flatpak-libs` | Red Hat:enterprise_linux:7::server | All versions | 0:1.0.9-10.el7_9 |
| `flatpak` | Red Hat:enterprise_linux:7::workstation | All versions | 0:1.0.9-10.el7_9 |
| `flatpak-builder` | Red Hat:enterprise_linux:7::workstation | All versions | 0:1.0.0-10.el7_9 |
| `flatpak-debuginfo` | Red Hat:enterprise_linux:7::workstation | All versions | 0:1.0.9-10.el7_9 |
| `flatpak-devel` | Red Hat:enterprise_linux:7::workstation | All versions | 0:1.0.9-10.el7_9 |
| `flatpak-libs` | Red Hat:enterprise_linux:7::workstation | All versions | 0:1.0.9-10.el7_9 |
| `flatpak` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.6.2-5.el8_3 |
| `flatpak-debuginfo` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.6.2-5.el8_3 |
| `flatpak-debugsource` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.6.2-5.el8_3 |
| `flatpak-libs` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.6.2-5.el8_3 |
| `flatpak-libs-debuginfo` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.6.2-5.el8_3 |
| `flatpak-selinux` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.6.2-5.el8_3 |
| `flatpak-session-helper` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.6.2-5.el8_3 |
| `flatpak-session-helper-debuginfo` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.6.2-5.el8_3 |
| `flatpak-tests-debuginfo` | Red Hat:enterprise_linux:8::appstream | All versions | 0:1.6.2-5.el8_3 |
| `flatpak` | Red Hat:rhel_eus:8.1::appstream | All versions | 0:1.0.9-3.el8_1 |
| `flatpak-debuginfo` | Red Hat:rhel_eus:8.1::appstream | All versions | 0:1.0.9-3.el8_1 |
| `flatpak-debugsource` | Red Hat:rhel_eus:8.1::appstream | All versions | 0:1.0.9-3.el8_1 |
| `flatpak-libs` | Red Hat:rhel_eus:8.1::appstream | All versions | 0:1.0.9-3.el8_1 |
| `flatpak-libs-debuginfo` | Red Hat:rhel_eus:8.1::appstream | All versions | 0:1.0.9-3.el8_1 |
| `flatpak` | Red Hat:rhel_eus:8.2::appstream | All versions | 0:1.6.2-4.el8_2 |
| `flatpak-debuginfo` | Red Hat:rhel_eus:8.2::appstream | All versions | 0:1.6.2-4.el8_2 |
| `flatpak-debugsource` | Red Hat:rhel_eus:8.2::appstream | All versions | 0:1.6.2-4.el8_2 |
| `flatpak-libs` | Red Hat:rhel_eus:8.2::appstream | All versions | 0:1.6.2-4.el8_2 |
| `flatpak-libs-debuginfo` | Red Hat:rhel_eus:8.2::appstream | All versions | 0:1.6.2-4.el8_2 |
| `flatpak-selinux` | Red Hat:rhel_eus:8.2::appstream | All versions | 0:1.6.2-4.el8_2 |

Products: Debian Debian Linux, Flatpak Flatpak.

## How do I fix CVE-2021-21261?

Upgrade to a fixed version.

## Is CVE-2021-21261 being exploited?

There is no known exploitation of CVE-2021-21261 in the wild and no public exploit code.

## Other vulnerabilities in Debian Debian Linux

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- [CVE-2025-49113](https://insights.sbomify.com/CVE-2025-49113/): RoundCube Webmail Deserialization of Untrusted Data (critical 9.9, exploited in the wild)
- [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-2021-21261 and fix it if it is affected.

What it is: CVE-2021-21261 is a high-severity code injection vulnerability in Debian Debian Linux, published on 14 Jan 2021. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-21261 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-2021-21261/ (Markdown: https://insights.sbomify.com/CVE-2021-21261/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-21261
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-21261
- OSV: https://osv.dev/vulnerability/CVE-2021-21261

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-21261/
Summaries are generated and may contain mistakes; the official description above is the record.