CVE-2021-21261

HIGHNVD 7.37.3—
EchelonGraph scoreMEDIUM confidence

This high-severity CVE scores 7.3 under NVD CVSS v3. EPSS exploit probability: 0.6%, top 56% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, nvd
Trending — 3 sources updated this week
7.3
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS: 1%CVSS: 7.3Exploit: NoneExposed: 0

A fix is available — apply it.

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.

CVSS v3
7.3
EG Score
7.3(medium)
EG Risk
38(Track)
EG Risk 38/100SSVC: Track

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity73% × 45%
Exploitation1% × 40%
Automatability30% × 15%
Action: Routine — remediate on your standard cadence.
EPSS
43.7%
KEV
Not listed

Published

January 14, 2021

Last Modified

November 21, 2024

References (16)

Patch Availability(5)

Vendor / EcosystemFixed in / PatchReleasedSource
ubuntulibflatpak0 (1.8.2-1ubuntu0.1) @ groovy2026-05-26ubuntu
redhatflatpak-0:1.0.9-10.el7_92021-02-04redhat
redhatflatpak-0:1.6.2-5.el8_32021-02-01redhat
redhatflatpak-0:1.0.9-3.el8_12021-02-01redhat
redhatflatpak-0:1.6.2-4.el8_22021-02-01redhat

Patches are aggregated from vendor advisories (Red Hat, Microsoft, Cisco, GitHub) and package ecosystems (OSV, GHSA). Multiple rows for the same upstream release have been deduplicated.

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

All Vendor Advisories

(5)

Every vendor that published an advisory referencing this CVE — pulled from our cve_vendor_advisories aggregation. Click any row for the vendor's original advisory page.

Data Freshness Timeline

(refreshed 11× in last 7d / 50× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

Showing the most recent 100 of 105 total refreshes for this CVE.

  1. 2026-07-23 14:15 UTCEPSS rescore
  2. 2026-07-23 01:57 UTCEG score recompute
  3. 2026-07-22 22:43 UTCEG score recompute
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  74. 2026-05-26 22:05 UTCVendor advisory
  75. 2026-05-26 13:42 UTCEPSS rescore

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Frequently asked(5)

What is CVE-2021-21261?
CVE-2021-21261 is a high vulnerability published on January 14, 2021. 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…
When was CVE-2021-21261 disclosed?
CVE-2021-21261 was first published in the National Vulnerability Database on January 14, 2021, with the most recent update on November 21, 2024. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-21261 actively exploited?
CVE-2021-21261 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 43.7% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2021-21261?
CVE-2021-21261 has a CVSS v3 base score of 7.3 (NVD).
How do I remediate CVE-2021-21261?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-21261, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

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