CVE-2021-21261
📋 TL;DR
This CVE allows malicious or compromised Flatpak applications to escape their sandbox and execute arbitrary code on the host Linux system. The vulnerability affects Flatpak versions from 0.11.4 up to (but not including) 1.8.5 and 1.10.0. Any Linux system running vulnerable Flatpak versions with sandboxed applications is at risk.
💻 Affected Systems
- Flatpak
📦 What is this software?
Flatpak by Flatpak
Flatpak by Flatpak
⚠️ Risk & Real-World Impact
Worst Case
Complete host system compromise allowing attackers to install persistent malware, steal sensitive data, or pivot to other systems.
Likely Case
Local privilege escalation where a compromised Flatpak app gains full host system access, potentially leading to data theft or further attacks.
If Mitigated
Limited to sandboxed application compromise without host system access if proper patching or workarounds are implemented.
🎯 Exploit Status
Exploitation requires a malicious or compromised Flatpak application already running in the sandbox.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 1.8.5 or 1.10.0
Vendor Advisory: https://github.com/flatpak/flatpak/releases/tag/1.8.5
Restart Required: Yes
Instructions:
1. Update Flatpak using your distribution's package manager. 2. Restart the flatpak-portal service. 3. Verify the update with 'flatpak --version'.
🔧 Temporary Workarounds
Disable flatpak-portal service
linuxPrevents the vulnerable service from running but will break many Flatpak applications.
sudo systemctl stop org.freedesktop.portal.Flatpak
sudo systemctl disable org.freedesktop.portal.Flatpak
🧯 If You Can't Patch
- Remove or disable all Flatpak applications to eliminate attack surface
- Implement strict application allowlisting and monitor for suspicious Flatpak activity
🔍 How to Verify
Check if Vulnerable:
Check Flatpak version with 'flatpak --version' and compare to affected ranges.Check Version:
flatpak --versionVerify Fix Applied:
Confirm version is 1.8.5 or higher, or 1.10.0 or higher with 'flatpak --version'.📡 Detection & Monitoring
Log Indicators:
- Unusual flatpak-portal service activity
- Suspicious environment variable manipulation in Flatpak context
Network Indicators:
- Unexpected outbound connections from Flatpak applications
SIEM Query:
process_name:"flatpak" AND (event_type:"process_execution" OR event_type:"sandbox_escape")🔗 References
- 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://github.com/flatpak/flatpak/releases/tag/1.8.5
- https://github.com/flatpak/flatpak/security/advisories/GHSA-4ppf-fxf6-vxg2
- https://security.gentoo.org/glsa/202101-21
- 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://github.com/flatpak/flatpak/releases/tag/1.8.5
- https://github.com/flatpak/flatpak/security/advisories/GHSA-4ppf-fxf6-vxg2
- https://security.gentoo.org/glsa/202101-21
- https://www.debian.org/security/2021/dsa-4830
