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FirefoxWeb browser · Mozilla

CVE-2026-4716

CRITICAL · 9.1 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
Incorrect boundary conditions, uninitialized memory 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 · high confidence

This is a memory corruption vulnerability in the SpiderMonkey JavaScript Engine involving incorrect boundary condition checks combined with uninitialized memory usage. These defects can be exploited to achieve arbitrary code execution through malicious JavaScript code, as the boundary errors allow out-of-bounds memory access while uninitialized memory can provide controlled data for exploitation.

MitigationApply the vendor-supplied updates: upgrade to Firefox 149+, Firefox ESR 140.9+, Thunderbird 149+, or Thunderbird 140.9+ to remediate the JavaScript Engine memory safety flaws.

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
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/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, click Help > About Firefox, or type 'about:support' in the address bar, or run 'firefox --version' from command line
    Affected if Version displayed is below 140.9.0 or below 149.0 (for example 140.8.x, 148.x, etc.)
  2. Confirm SpiderMonkey engine is in use
    Firefox uses SpiderMonkey for JavaScript execution by default - no additional configuration needed; the vulnerability exists in the js::frontend::Parser module
    Affected if Firefox is running and JavaScript execution is permitted in the browser (SpiderMonkey is the default JS engine)
  3. Verify JavaScript is enabled in the browser
    Go to Firefox Settings > Privacy & Security > Permissions, or type 'about:preferences#privacy' and check JavaScript permissions
    Affected if JavaScript is allowed to execute in any context (local pages, remote sites, or extensions)
  4. Check if running ESR version and its exact build
    In about:support, look for 'Version' field; ESR builds will show 'ESR' in the version string (e.g., 140.8.x ESR)
    Affected if Running Firefox ESR below 140.9.0

You are affected if Firefox version is below 140.9.0 or below 149.0 AND JavaScript execution is enabled in the browser, since the SpiderMonkey memory corruption triggers during JS parsing.

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

Apply the vendor-supplied updates: upgrade to Firefox 149+, Firefox ESR 140.9+, Thunderbird 149+, or Thunderbird 140.9+ to remediate the JavaScript Engine memory safety flaws.

Recommended fix High confidence

Firefox 149 (or Firefox ESR 140.9 for enterprise deployments)

  1. Open Firefox and navigate to Help > About Firefox (or Firefox > About Firefox on macOS)
  2. In the About window, click 'Check for Updates' or wait for the automatic update check
  3. If an update is available, click 'Download the update' and wait for it to complete
  4. Click 'Restart to Update Firefox' to apply the update
  5. Alternatively, download the latest version directly from https://www.mozilla.org/firefox/ or https://www.mozilla.org/en-US/firefox/all/

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
  • Testing3.0 h
  • Review / QA1.0 h
10.0 hours of engineering $1,750
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,800.

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-2026-4716 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