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FirefoxWeb browser · Mozilla

CVE-2026-6751

HIGH · 7.3 CVSS v3.1 Published 2026-04-21
Fix available
A fix is available. Upgrade to 140.10.0 / 150.0 or later.
See remediation →
82/100
Remediation priority · High
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
Uninitialized memory in the Audio/Video: Web Codecs 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

Uninitialized memory in the Web Codecs component of Firefox and Thunderbird allows reading of sensitive data from memory. This occurs when the Audio/Video codec processing fails to properly initialize memory buffers before use, potentially exposing heap contents to the renderer process.

MitigationUpdate to Firefox 150+, Firefox ESR 140.10+, Thunderbird 150+, or Thunderbird 140.10+ to obtain the patched version. Apply via standard software update mechanisms or enterprise patch management.

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:>= 140.0, < 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
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
Low
Availability
Low

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

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 Firefox version
    In Firefox, go to Menu > Help > About Firefox, or enter 'about:support' in the address bar. The version is displayed at the top of the page.
    Affected if Version is below 150.0 (for example, 149.x, 140.x, or earlier)
  2. Check Thunderbird version
    In Thunderbird, go to Menu > Help > About Thunderbird. The version number is displayed on the dialog that opens.
    Affected if Version is 140.0 or higher but below 140.10.0 (for example, 140.0 through 140.9.x)
  3. Confirm Web Codecs usage (optional context)
    Web Codecs API (window.VideoDecoder, window.AudioDecoder) is a browser feature available in both Firefox and Thunderbird. No specific enable flag is required - it is built into the renderer. If the application processes web content with audio or video, this component is in use.
    Affected if The application processes any web content containing audio or video elements (standard browsing or email with media)

You are affected if you run Firefox below version 150.0 or Thunderbird between versions 140.0 and 140.9.x, as these versions contain the uninitialized memory flaw in the Web Codecs component.

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 version. Apply via standard software update mechanisms or enterprise patch management.

Recommended fix High confidence

Firefox 150 (or Firefox ESR 140.10); Thunderbird 150 (or Thunderbird 140.10)

  1. Check current Firefox version via Help > About Firefox menu
  2. Check current Thunderbird version via Help > About Thunderbird menu
  3. Download Firefox 150 from mozilla.org (or Firefox ESR 140.10 for enterprise/Long Term Support)
  4. Install the downloaded Firefox version, ensuring to close all browser windows first
  5. If using Thunderbird, download Thunderbird 150 (or 140.10 for standard support) from mozilla.org
  6. Install the Thunderbird update after downloading
  7. Restart the applications to complete the update process
  8. Verify the installed version matches the fixed release (Help > About should show 150 or 140.10)
Caveat Standard major version upgrade - review mozilla.org release notes for any add-on compatibility or feature changes

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

Fix this in Firefox Scoped from the published advisory
  • Consultation1.0 h
  • Testing1.0 h
  • Review / QA1.0 h
3.0 hours of engineering $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 $1,200.

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-2026-6751 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