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FirefoxWeb browser · Mozilla

CVE-2026-8959

CRITICAL · 9.6 CVSS v3.1 Published 2026-05-19
Fix available
A fix is available. Upgrade to 140.11 / 140.11.0 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component. This vulnerability was fixed in Firefox 151, Firefox ESR 140.11, Thunderbird 151, and Thunderbird 140.11.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

A sandbox escape vulnerability exists in Firefox, Firefox ESR, and Thunderbird due to incorrect boundary conditions in the Win32 Widget component. This allows an attacker to break out of the browser's sandbox protection and execute code with elevated privileges on the affected system.

MitigationApply the vendor patches by updating to Firefox 151, Firefox ESR 140.11, Thunderbird 151, or Thunderbird 140.11 or later.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
FirefoxWeb browser
Affected:< 140.11.0< 151.0.0
ThunderbirdApplication
Affected:< 140.11< 151.0.0

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
Required
Scope
Changed
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Confirm Windows platform
    Verify the operating system is Windows (e.g., run 'systeminfo' on Windows or check system properties). This vulnerability affects the Widget: Win32 component which is Windows-specific.
    Affected if System is running a non-Windows operating system (the Win32 widget component does not exist on macOS or Linux).
  2. Check Firefox version on Windows
    Open Firefox, go to Help > About Firefox, or navigate to about:support in the address bar, or run 'firefox --version' from command line.
    Affected if Firefox version is below 140.11.0, or between 140.11.0 and 151.0.0 (exclusive). Versions 151.0.0 and above are fixed.
  3. Check Thunderbird version on Windows
    Open Thunderbird, go to Help > About Thunderbird, or run 'thunderbird --version' from command line.
    Affected if Thunderbird version is below 140.11, or between 140.11 and 151.0.0 (exclusive). Versions 151.0.0 and above are fixed.

User is affected if running Windows with Firefox below 140.11.0 or between 140.11.0-151.0.0, or Thunderbird below 140.11 or between 140.11-151.0.0, as the sandbox escape in the Widget: Win32 component applies only on Windows within these version ranges.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 140.11 / 140.11.0 / 151.0.0 or later
Fixed in 140.11140.11.0151.0.0
Interim mitigation

Apply the vendor patches by updating to Firefox 151, Firefox ESR 140.11, Thunderbird 151, or Thunderbird 140.11 or later.

Recommended fix High confidence

Firefox 151, Firefox ESR 140.11, Thunderbird 151, or Thunderbird 140.11

  1. 1. Check the current version of Firefox or Thunderbird by navigating to Menu > Help > About Firefox/Thunderbird
  2. 2. If the version is below 140.11.0 (for Firefox) or below 140.11 (for Thunderbird), or below 151.0.0, an update is required
  3. 3. Download the latest stable version from the official Mozilla website at https://www.mozilla.org/ or use the built-in update mechanism via Menu > Help > About Firefox/Thunderbird > Check for Updates
  4. 4. For enterprise deployments, deploy Firefox ESR 140.11 or Thunderbird 140.11 as appropriate for your environment
  5. 5. Restart the application after the update completes
  6. 6. Verify the version by checking Menu > Help > About to confirm the installation of version 140.11 or higher, or 151 or higher

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Firefox Scoped from the published advisory
  • Consultation1.0 h
  • Implementation2.0 h
  • Testing1.0 h
  • Review / QA1.0 h
5.0 hours of engineering $890
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,424.

Scan for this in your stack

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-8959 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data