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FirefoxWeb browser · Mozilla

CVE-2026-8955

HIGH · 8.8 CVSS v3.1 Published 2026-05-19
Fix available
A fix is available. Upgrade to 140.11 / 140.11.0 or later.
See remediation →
95/100
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
Privilege escalation in the DOM: Workers 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 · moderate confidence

A privilege escalation vulnerability exists in the DOM Workers component of Firefox and Thunderbird. Web Workers run scripts in background threads and should be subject to same-origin restrictions; this vulnerability allows a worker to potentially escape its security context and access privileged resources or APIs it shouldn't have access to.

MitigationUpdate affected installations to Firefox 151+, Firefox ESR 140.11+, Thunderbird 151+, or Thunderbird 140.11+ to obtain the patched version.

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
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/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. Check installed Firefox version
    Open Firefox, go to Menu > Help > About Firefox, or navigate to about:support in the address bar. Note the version number displayed.
    Affected if Version is below 140.11.0 OR between 141.0.0 and 150.x (inclusive)
  2. Check installed Thunderbird version
    Open Thunderbird, go to Menu > Help > About Thunderbird, or navigate to about:support in the address bar. Note the version number displayed.
    Affected if Version is below 140.11 OR between 141.0.0 and 150.x (inclusive)
  3. Identify if DOM Workers are in use
    Review your browser extensions, add-ons, or web applications that use web workers. Check for JavaScript code containing 'new Worker()' or Worker API calls in your local environment or installed extensions.
    Affected if Any extension, add-on, or application uses the Web Worker API (DOM Workers) in an affected Firefox or Thunderbird version

You are affected if Firefox is installed at version < 140.11.0 or >= 141.0.0 but < 151.0.0, or if Thunderbird is installed at version < 140.11 or >= 141.0.0 but < 151.0.0, and you use any web workers or have extensions that rely on the Worker API.

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

Update affected installations to Firefox 151+, Firefox ESR 140.11+, Thunderbird 151+, or Thunderbird 140.11+ to obtain the patched version.

Recommended fix High confidence

Firefox 151 (regular) or Firefox ESR 140.11; Thunderbird 151 or Thunderbird 140.11

  1. Check current Firefox or Thunderbird version via Menu > Help > About (or running 'firefox --version' / 'thunderbird --version' in terminal)
  2. For Firefox regular users: Download and install Firefox 151 from www.mozilla.org/firefox
  3. For Firefox ESR users: Download and install Firefox ESR 140.11 from www.mozilla.org/firefox/all
  4. For Thunderbird regular users: Download and install Thunderbird 151 from www.mozilla.org/thunderbird
  5. For Thunderbird ESR users: Download and install Thunderbird 140.11 from www.mozilla.org/thunderbird/all
  6. Restart the application after upgrade
Caveat Minor: Some legacy extension APIs may have minor behavioral changes in Firefox 151

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

Fix this in Firefox Scoped from the published advisory
  • Consultation1.0 h
  • Implementation1.0 h
  • Testing2.0 h
  • Review / QA1.0 h
5.0 hours of engineering $860
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,376.

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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-2026-8955 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