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FirefoxWeb browser · Mozilla

CVE-2026-4702

CRITICAL · 9.8 CVSS v3.1 Published 2026-03-24
Fix available
A fix is available. Upgrade to 140.9.0 / 149.0 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
JIT miscompilation in the JavaScript Engine component. This vulnerability was fixed in Firefox 149, 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 just-in-time (JIT) miscompilation vulnerability in the SpiderMonkey JavaScript engine allows malformed JavaScript to be compiled incorrectly, potentially leading to memory corruption and arbitrary code execution. This affects Firefox browsers and Thunderbird email client which share the same engine.

MitigationUpdate affected software to Firefox 149+, Firefox ESR 140.9+, Thunderbird 149+, or Thunderbird 140.9+ to apply the patched JavaScript engine.

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:< 140.9.0< 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
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/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. Identify installed Mozilla product
    Check if Firefox or Thunderbird is installed by looking for their executables or checking package manager: which firefox, which thunderbird, or inspect /Applications folder on macOS, Start Menu on Windows, or package manager on Linux
    Affected if Either Firefox or Thunderbird is installed
  2. Determine Firefox version
    Run firefox --version from command line, or in Firefox go to Help > About Firefox, or check the application binary metadata
    Affected if Firefox version is less than 140.9.0 or less than 149.0 (the affected ranges are versions before 149.0 AND before 140.9.0)
  3. Determine Thunderbird version
    Run thunderbird --version from command line, or in Thunderbird go to Help > About Thunderbird, or check the application binary metadata
    Affected if Thunderbird version is less than 140.9.0 or less than 149.0 (the affected ranges are versions before 149.0 AND before 140.9.0)
  4. Verify SpiderMonkey JIT compiler status
    In the browser address bar, enter about:config and search for javascript.options.jit; confirm the value is true (JIT is typically enabled by default)
    Affected if JIT compilation is enabled (this is the default and required for the vulnerability to be exploitable)

A user is affected if they have Firefox or Thunderbird installed with a version below 149.0 (or below 140.9.0 for the ESR branch) and the JavaScript JIT compiler is enabled (default state).

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 140.9.0 / 149.0 or later
Fixed in 140.9.0149.0
Interim mitigation

Update affected software to Firefox 149+, Firefox ESR 140.9+, Thunderbird 149+, or Thunderbird 140.9+ to apply the patched JavaScript engine.

Recommended fix High confidence

Firefox 149 (or later) / Firefox ESR 140.9 (or later)

  1. Check current Firefox version by navigating to about:settings or using Help > About Firefox
  2. For regular Firefox users: Download Firefox 149 or later from the official Mozilla website (www.mozilla.org)
  3. For enterprise/ESR users: Download Firefox ESR 140.9 or later from the official Mozilla website
  4. Close all Firefox instances and install the downloaded update
  5. Restart Firefox and verify the version at about:settings shows 149.x or later (or 140.9.x for ESR)
  6. Ensure automatic updates are enabled (Preferences > Privacy & Security > Firefox Updates > Check for updates but let me decide to install)
Caveat Standard browser upgrade - verify that essential browser extensions and plugins are compatible with the new version

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
  • Testing2.0 h
  • Review / QA1.0 h
6.0 hours of engineering $1,040
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,664.

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-2026-4702 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