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FirefoxWeb browser · Mozilla

CVE-2026-4690

HIGH · 8.6 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 →
95/100
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 due to incorrect boundary conditions, integer overflow in the XPCOM 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 · high confidence

This is a client-side sandbox escape vulnerability in Mozilla Firefox and Thunderbird caused by an integer overflow in the XPCOM (Cross-Platform Component Object Model) component due to incorrect boundary conditions. The flaw allows a malicious web page or email content to escape the sandbox security boundary and potentially execute arbitrary code.

MitigationUpgrade to Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, or Thunderbird 140.9 (or later) to patch the vulnerability. Organizations should deploy these updates through their patch management systems.

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

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
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/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:firefox in the address bar. Alternatively, run 'firefox --version' from command line.
    Affected if The version displayed is before 115.34.0, between 115.34.0 and 149.0 (exclusive), or between 128.0 and 140.9.0 (exclusive). Versions 115.34.0+, 140.9.0+, and 149.0+ are not affected.
  2. Verify XPCOM component presence
    The XPCOM component is a core part of Firefox. Confirm Firefox is installed and operational. On Linux, check for libxul.so in the Firefox installation directory. On Windows, check for xul.dll in the program folder.
    Affected if Firefox is installed with the XPCOM component, which is present in all standard Firefox installations.
  3. Confirm Firefox is used in the environment
    Determine if Mozilla Firefox is the primary or any browser in use. Check for running firefox.exe (Windows) or firefox (Linux) processes via Task Manager or 'ps aux | grep firefox'.
    Affected if Firefox is installed and running, exposing the XPCOM component with the integer overflow vulnerability in the affected version ranges.

You are affected if the installed Firefox version falls into any of these ranges: before 115.34.0, 115.34.0 to 149.0 (exclusive), or 128.0 to 140.9.0 (exclusive). You are NOT affected if running version 115.34.0+, 140.9.0+, or 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

Upgrade to Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, or Thunderbird 140.9 (or later) to patch the vulnerability. Organizations should deploy these updates through their patch management systems.

Recommended fix High confidence

Firefox 149 (standard), Firefox ESR 115.34 or Firefox ESR 140.9 (ESR track), Thunderbird 149 or Thunderbird 140.9

  1. Check current Firefox version: Open Firefox, click Help > About Firefox
  2. Download the appropriate fixed release from the official Mozilla website: https://www.mozilla.org/firefox/download/
  3. For standard Firefox users: Upgrade to Firefox 149 or later
  4. For Enterprise deployments: If using Firefox ESR 128.x, upgrade to Firefox ESR 140.9; if using Firefox ESR 115.x, upgrade to Firefox ESR 115.34
  5. If using Thunderbird: Upgrade to Thunderbird 149 or Thunderbird 140.9 depending on your release track
  6. After installation, verify the version by checking Help > About to confirm the upgrade was successful
Caveat Minor version upgrades typically have no breaking changes; however, ESR upgrades may have compatibility considerations for enterprise policies and add-ons

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

Fix this in Firefox Scoped from the published advisory
  • Consultation4.0 h
  • Implementation2.0 h
  • Testing1.0 h
  • Review / QA1.0 h
8.0 hours of engineering $1,490
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,384.

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-2026-4690 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