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FirefoxWeb browser · Mozilla

CVE-2026-6748

CRITICAL · 9.8 CVSS v3.1 Published 2026-04-21
Fix available
A fix is available. Upgrade to 140.10.0 / 150.0 or later.
See remediation →
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
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 · high confidence

Uninitialized memory vulnerability in the Web Codecs component (used for audio/video encoding/decoding) allows reading sensitive data from memory or potentially executing arbitrary code. This is a critical memory safety flaw in Firefox and Thunderbird.

MitigationUpdate affected installations to Firefox 150+, Firefox ESR 140.10+, Thunderbird 150+, or Thunderbird 140.10+ to obtain the patched versions.

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
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. Check Firefox version
    Open Firefox and navigate to Help > About Firefox, or go to about:support in the address bar. Note the version number displayed.
    Affected if The installed version is below 140.10.0 or below 150.0 (any version less than 150.0, since both 140.10.0 and 150.0 are listed as fix points)
  2. Check Thunderbird version
    Open Thunderbird and navigate to Help > About Thunderbird. Note the version number displayed.
    Affected if The installed version is below 140.10.0
  3. Confirm product type
    Verify whether the affected installation is Firefox or Thunderbird, as they have different version requirements for this vulnerability.
    Affected if Running Firefox below version 150.0 or Thunderbird below version 140.10.0

If Firefox is installed at any version below 150.0, or Thunderbird is installed at any version below 140.10.0, the environment is affected by this uninitialized memory vulnerability 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 affected installations to Firefox 150+, Firefox ESR 140.10+, Thunderbird 150+, or Thunderbird 140.10+ to obtain the patched versions.

Recommended fix High confidence

Firefox 150 / Firefox ESR 140.10 / Thunderbird 150 / Thunderbird 140.10

  1. Identify the currently installed Firefox or Thunderbird version from the application menu (Help > About Firefox/Thunderbird)
  2. For Firefox users on version 139.x or earlier: Upgrade to Firefox 150 or later
  3. For Firefox users on version 140.x (non-ESR): Upgrade to Firefox 150 or later
  4. For Firefox ESR users on version 140.x: Upgrade to Firefox ESR 140.10 or later
  5. For Thunderbird users on version 139.x or earlier: Upgrade to Thunderbird 150 or later
  6. For Thunderbird users on version 140.x: Upgrade to Thunderbird 140.10 or later
  7. Restart the application after updating
  8. Verify the installed version matches or exceeds the fixed release
Caveat Mozilla stable releases typically have minimal breaking changes; however, some extensions or legacy features may require testing after major version upgrades

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
  • Testing2.0 h
  • Review / QA1.0 h
9.0 hours of engineering $1,600
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,560.

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-2026-6748 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