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FirefoxWeb browser · Mozilla

CVE-2026-12294

CRITICAL · 9.6 CVSS v3.1 Published 2026-06-16
Fix available
A fix is available. Upgrade to 115.37.0 / 140.12.0 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 in the DOM: Workers component. This vulnerability was fixed in Firefox 152, Firefox ESR 140.12, Firefox ESR 115.37, Thunderbird 152, and Thunderbird 140.12.

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 in the DOM Workers component allows an attacker to break out of the browser's security sandbox and potentially execute arbitrary code or access sensitive resources. This affects Firefox and Thunderbird browsers.

MitigationUpdate affected systems to Firefox 152+, Firefox ESR 140.12+, Firefox ESR 115.37+, Thunderbird 152+, or Thunderbird 140.12+ to remediate the vulnerability.

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:< 115.37.0< 152.0.0>= 128.0, < 140.12.0
ThunderbirdApplication
Affected:< 140.12.0< 152.0.0

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. 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 The version is less than 115.37.0, or between 128.0.0 and 140.12.0, or between 140.12.0 and 152.0.0 (any version below 152.0.0 is potentially affected)
  2. Check installed Thunderbird version
    Open Thunderbird, go to Menu > Help > About Thunderbird. Note the version number displayed.
    Affected if The version is less than 140.12.0 or less than 152.0.0 (any version below 152.0.0 is potentially affected)
  3. Confirm DOM Workers usage
    In Firefox, open the Web Developer Tools (F12), go to the Console tab, and check for any worker scripts loaded by visiting websites or web applications. Alternatively, search for Worker objects in loaded JavaScript files.
    Affected if DOM Workers are actively used by any website or web application accessed through the browser; the vulnerability requires Worker functionality to be exploitable

You are affected if you run Firefox or Thunderbird versions below 152.0.0 (or below 140.12.0 for specific release branches) and use the browser to access content that employs DOM Workers.

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 115.37.0 / 140.12.0 / 152.0.0 or later
Fixed in 115.37.0140.12.0152.0.0
Interim mitigation

Update affected systems to Firefox 152+, Firefox ESR 140.12+, Firefox ESR 115.37+, Thunderbird 152+, or Thunderbird 140.12+ to remediate the vulnerability.

Recommended fix High confidence

Firefox 152 / Firefox ESR 140.12 / Firefox ESR 115.37 / Thunderbird 152 / Thunderbird 140.12 (depending on current branch)

  1. 1. Determine current Firefox or Thunderbird version by navigating to Help > About (or typing 'about:support' in the address bar)
  2. 2. If using Firefox 128 or later: upgrade to Firefox 140.12 or later (Firefox 152 recommended)
  3. 3. If using Firefox 115.x - 127.x: upgrade to Firefox ESR 115.37
  4. 4. If using Thunderbird: upgrade to Thunderbird 140.12 or later (Thunderbird 152 recommended)
  5. 5. After upgrade, verify the version matches or exceeds the fixed release (Help > About)
  6. 6. Restart the application to complete the update
Caveat Standard minor version upgrade - minimal risk; usual browser data (bookmarks, passwords, history) preserved

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

Fix this in Firefox Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing2.0 h
  • Review / QA1.0 h
9.0 hours of engineering $1,600
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,560.

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-2026-12294 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