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FirefoxWeb browser · Mozilla

CVE-2026-6776

HIGH · 7.8 CVSS v3.1 Published 2026-04-21
Fix available
A fix is available. Upgrade to 140.10.0 / 150.0 or later.
See remediation →
81/100
Remediation priority · High
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
Incorrect boundary conditions in the WebRTC: Networking component. This vulnerability was fixed in Firefox 150, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10.

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

This is a boundary condition vulnerability in Mozilla's WebRTC networking stack. Incorrect boundary conditions typically indicate improper bounds checking during memory operations (such as buffer reads/writes), which could allow memory corruption and potentially remote code execution. The issue affects Firefox and Thunderbird browsers.

MitigationUpdate to Firefox 150+, Firefox ESR 140.10+, Thunderbird 150+, or Thunderbird 140.10+ to obtain the patched versions. For embedded WebRTC implementations, review the specific boundary condition fix in Mozilla's codebase for similar patterns.

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:< 140.10.0< 150.0
ThunderbirdApplication
Affected:< 140.10.0

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
Local
Complexity
Low
Privileges
None
User interaction
Required
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:L/AC:L/PR:N/UI:R/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 Firefox version
    In Firefox, go to Help > About Firefox, or type 'about:support' in the address bar and look for the 'Version' field. On command line, run 'firefox --version' or check the application binary metadata.
    Affected if The installed version is less than 140.10.0 OR between 140.10.0 and 150.0 (exclusive of 150.0) - i.e., any version below 150.0 that is not 140.10.0 or higher.
  2. Identify installed Thunderbird version
    In Thunderbird, go to Help > About Thunderbird, or check the application binary metadata. On command line, run 'thunderbird --version' if available.
    Affected if The installed version is less than 140.10.0.
  3. Confirm WebRTC is in use
    WebRTC is built into Firefox and Thunderbird. To verify WebRTC is active in Firefox, visit a WebRTC test site or check 'about:webrtc' in the address bar which displays active WebRTC sessions.
    Affected if WebRTC is enabled and being used - the vulnerability exists in the WebRTC component regardless of active sessions, as the flaw is in the networking stack code itself.
  4. Cross-reference version against CVEaffected ranges
    Compare your identified Firefox version against < 140.10.0 and < 150.0. Compare your identified Thunderbird version against < 140.10.0. Note that Firefox versions 140.10.0 through 149.x are also affected; only 150.0+ is patched.
    Affected if Any Firefox version below 150.0 that is not exactly 140.10.0 or higher, OR any Thunderbird version below 140.10.0.

You are affected if you run Firefox any version below 150.0 (excluding 140.10.0+) or Thunderbird below 140.10.0, and the WebRTC component is present in your installation.

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 140.10.0 / 150.0 or later
Fixed in 140.10.0150.0
Interim mitigation

Update to Firefox 150+, Firefox ESR 140.10+, Thunderbird 150+, or Thunderbird 140.10+ to obtain the patched versions. For embedded WebRTC implementations, review the specific boundary condition fix in Mozilla's codebase for similar patterns.

Recommended fix High confidence

Firefox 150 or Firefox ESR 140.10; Thunderbird 150 or Thunderbird 140.10

  1. 1. Open Firefox and navigate to Menu > Help > About Firefox (or go to about:support)
  2. 2. Click 'Check for Updates' to download and install the latest version
  3. 3. Restart Firefox after the update completes
  4. 4. For Thunderbird users: Open Thunderbird and go to Menu > Help > About Thunderbird
  5. 5. Click 'Check for Updates' in Thunderbird to download version 140.10 or 150
  6. 6. Restart Thunderbird after the update completes
  7. 7. Verify the installed version by checking Menu > Help > About to confirm Firefox 150 or Thunderbird 140.10/150 is installed

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
  • Testing4.0 h
  • Review / QA2.0 h
16.0 hours of engineering $2,800
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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-2026-6776 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