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FirefoxWeb browser · Mozilla

CVE-2026-4692

CRITICAL · 10.0 CVSS v3.1 Published 2026-03-24
Fix available
A fix is available. Upgrade to 115.34.0 / 140.9.0 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click

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 Responsive Design Mode component. This vulnerability was fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

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 sandbox escape vulnerability in Firefox/Thunderbird's Responsive Design Mode component allows an attacker to break out of the browser's security sandbox and potentially execute code with elevated privileges on the victim's system. This is a critical client-side vulnerability with CVSS 10, indicating complete compromise capability.

MitigationUpdate affected systems to Firefox 149+, Firefox ESR 115.34+, Firefox ESR 140.9+, Thunderbird 149+, or Thunderbird 140.9+ to apply the vendor patch. Deploy updated versions organization-wide and verify remediation.

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:< 115.34.0< 149.0>= 128.0, < 140.9.0
ThunderbirdApplication
Affected:< 140.9.0< 149.0

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
None
Scope
Changed
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/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. Identify installed Mozilla product
    Check if Firefox or Thunderbird is installed by locating the application binary or checking installed programs on the system
    Affected if Neither Firefox nor Thunderbird is installed - not applicable
  2. Get Firefox version
    Open Firefox and navigate to 'about:support', or check the application binary version via command line (e.g., 'firefox --version' or file properties of firefox.exe)
    Affected if Firefox version is < 115.34.0, or >= 128.0 and < 140.9.0, or < 149.0 (check against all affected ranges)
  3. Get Thunderbird version
    Open Thunderbird and go to Help > About Thunderbird, or check the application binary version via command line (e.g., 'thunderbird --version' or file properties of thunderbird.exe)
    Affected if Thunderbird version is < 140.9.0 or < 149.0 (check against both affected ranges)
  4. Verify Responsive Design Mode is present
    Open Developer Tools (F12 or Ctrl+Shift+I) and look for the Responsive Design Mode toggle icon, or check if the feature is accessible via Ctrl+Shift+M
    Affected if The Responsive Design Mode component exists in the installation (vulnerable code present regardless of whether the feature is actively used)

System is affected if Firefox or Thunderbird is installed with a version matching any of the vulnerable ranges: Firefox < 115.34.0, Firefox >= 128.0 and < 140.9.0, Firefox < 149.0, Thunderbird < 140.9.0, or Thunderbird < 149.0

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 115.34.0 / 140.9.0 / 149.0 or later
Fixed in 115.34.0140.9.0149.0
Interim mitigation

Update affected systems to Firefox 149+, Firefox ESR 115.34+, Firefox ESR 140.9+, Thunderbird 149+, or Thunderbird 140.9+ to apply the vendor patch. Deploy updated versions organization-wide and verify remediation.

Recommended fix High confidence

Firefox 149 (regular), Firefox ESR 115.34, Firefox ESR 140.9, or Thunderbird 140.9/149 depending on current release channel

  1. Check the currently installed Firefox or Thunderbird version via Help > About (or firefox/thunderbird --version from command line)
  2. Determine which release channel you are on: Firefox Regular, Firefox ESR 115.x, Firefox ESR 128.x-140.x, or Thunderbird
  3. For Firefox Regular users: Download and install Firefox 149 or later from https://www.mozilla.org/firefox/
  4. For Firefox ESR 115.x users: Download and install Firefox ESR 115.34 or later from https://www.mozilla.org/firefox/
  5. For Firefox ESR 128.x-140.x users: Download and install Firefox ESR 140.9 or later from https://www.mozilla.org/firefox/
  6. For Thunderbird users: Download and install Thunderbird 140.9 or 149 from https://www.thunderbird.net/
  7. Restart the browser/application after installation
  8. Verify the installed version matches or exceeds the fixed release

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

Fix this in Firefox Scoped from the published advisory
  • Consultation2.0 h
  • Implementation8.0 h
  • Testing2.0 h
  • Review / QA1.0 h
13.0 hours of engineering $2,320
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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-2026-4692 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