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FirefoxWeb browser · Mozilla

CVE-2026-4718

HIGH · 8.1 CVSS v3.1 Published 2026-03-24
Fix available
A fix is available. Upgrade to 140.9.0 / 149.0 or later.
See remediation →
88/100
Remediation priority · High
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
Undefined behavior in the WebRTC: Signaling 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 case of undefined behavior exists in the WebRTC Signaling component, which handles the setup and management of peer-to-peer connections in Firefox and Thunderbird. Undefined behavior in this context could potentially lead to memory corruption or unexpected program states during WebRTC session negotiation.

MitigationUpgrade to Firefox 149, Firefox ESR 140.9, Thunderbird 149, or Thunderbird 140.9 or later to obtain the patch for this 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:< 140.9.0< 149.0
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
Required
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N

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 the installed Mozilla product
    Check if Firefox or Thunderbird is installed by looking for the application in your system (e.g., on Windows check Program Files, on macOS check Applications, on Linux check your package manager or /usr/bin)
    Affected if The system has Firefox or Thunderbird installed
  2. Determine the Firefox version
    Open Firefox, go to Menu > Help > About Firefox (or navigate to about:support) to view the installed version number
    Affected if The version displayed is below 140.9.0 or below 149.0
  3. Determine the Thunderbird version
    Open Thunderbird, go to Menu > Help > About Thunderbird (or navigate to about:support) to view the installed version number
    Affected if The version displayed is below 140.9.0 or below 149.0
  4. Compare against affected version ranges
    Cross-reference your detected version with the affected ranges: versions < 140.9.0 and versions < 149.0 are both listed as affected
    Affected if Your installed version falls into either of the affected ranges (any version below 140.9.0 or any version below 149.0)

You are affected if you have Firefox or Thunderbird installed with a version below 140.9.0 or below 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 140.9.0 / 149.0 or later
Fixed in 140.9.0149.0
Interim mitigation

Upgrade to Firefox 149, Firefox ESR 140.9, Thunderbird 149, or Thunderbird 140.9 or later to obtain the patch for this vulnerability.

Recommended fix High confidence

Firefox 149, Firefox ESR 140.9, Thunderbird 149, or Thunderbird 140.9

  1. Check current Firefox version by navigating to about:support or Help > About Firefox
  2. Check current Thunderbird version by navigating to Help > About Thunderbird
  3. For Firefox users: Download and install Firefox 149 or later from www.mozilla.org/firefox/new/
  4. For Firefox ESR users: Download and install Firefox ESR 140.9 or later from www.mozilla.org/firefox/enterprise/
  5. For Thunderbird users: Download and install Thunderbird 149 or later from www.mozilla.org/thunderbird/
  6. For Thunderbird users on older release: Download and install Thunderbird 140.9 or later
  7. Restart the application after upgrade
  8. Verify the version by checking about:support (Firefox) or Help > About Thunderbird
Caveat Mozilla releases may include web compatibility changes, removed features, or extension compatibility updates; review release notes before upgrading

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

Fix this in Firefox Scoped from the published advisory
  • Consultation1.0 h
  • Implementation1.0 h
  • Testing2.0 h
  • Review / QA1.0 h
5.0 hours of engineering $860
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,376.

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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-2026-4718 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