dbcve.org
FirefoxWeb browser · Mozilla

CVE-2026-6759

HIGH · 7.5 CVSS v3.1 Published 2026-04-21
Fix available
A fix is available. Upgrade to 140.10.0 / 150.0 or later.
See remediation →
84/100
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
Use-after-free in the Widget: Cocoa 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

A use-after-free vulnerability exists in the Widget: Cocoa component of Firefox and Thunderbird. This occurs when the program continues to reference memory after it has been freed, potentially allowing remote attackers to cause denial of service or execute arbitrary code via crafted web content. The vulnerability affects macOS builds using the Cocoa UI framework.

MitigationUpdate affected installations to Firefox 150, Firefox ESR 140.10, Thunderbird 150, or Thunderbird 140.10 or later. Organizations should verify deployment of patched versions across all endpoints.

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
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/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. Identify installed Mozilla product
    Run 'firefox --version' or check /Applications/Firebird.app or /Applications/Thunderbird.app, or check Help > About in the application
    Affected if Product is Firefox or Thunderbird on macOS
  2. Check operating system
    Run 'uname -s' or check System Preferences > About > System Report to confirm macOS
    Affected if Running macOS (the vulnerability only affects macOS builds using Cocoa UI framework)
  3. Determine installed Firefox version
    Run 'firefox --version' or check Firefox > Help > About Firefox. Compare the version number to the affected ranges: < 140.10.0 or < 150.0
    Affected if Firefox version is 139.x, 140.0-140.9.x, or any version below 140.10.0 (for ESR) or 150.0 (for standard)
  4. Determine installed Thunderbird version
    Run 'thunderbird --version' or check Thunderbird > Help > About Thunderbird. Compare the version number to the affected range: >= 140.0 and < 140.10.0
    Affected if Thunderbird version is 140.0 through 140.9.x (140.10.0 and later are fixed)
  5. Verify Cocoa UI framework is in use
    This is automatic on macOS Firefox/Thunderbird - the Widget: Cocoa component is used by default on macOS builds
    Affected if Running the affected product on macOS means the Cocoa UI framework is active and the vulnerability applies

Affected if running Firefox < 140.10.0 or < 150.0, or Thunderbird 140.0-140.9.x, on macOS with the default Cocoa UI framework.

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 or later. Organizations should verify deployment of patched versions across all endpoints.

Recommended fix High confidence

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

  1. 1. Check the current installed version of Firefox or Thunderbird by navigating to the application and selecting Help > About Firefox/Thunderbird.
  2. 2. For Firefox users: If currently on version 140.x ESR, upgrade to Firefox ESR 140.10 or later. If on version 149 or earlier (non-ESR), upgrade to Firefox 150 or later.
  3. 3. For Thunderbird users: If on version 140.x, upgrade to Thunderbird 140.10 or later. If on an earlier version, upgrade to Thunderbird 150 or later.
  4. 4. Download the latest version from the official Mozilla website at https://www.mozilla.org/ or use the application's built-in update mechanism (Help > Check for Updates).
  5. 5. Install the updated version and restart the application.
  6. 6. Verify the fix by checking the version again (Help > About) to confirm the installed version matches or exceeds the fixed release.
Caveat Standard Mozilla release upgrades typically have minimal breaking changes; however, some add-ons may be incompatible with 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
  • 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

Free · runs locally
dbcve dependency scanner

Check whether your project pulls in CVE-2026-6759 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.

Get it on GitHub Scan in the browser

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-2026-6759 in production — separate from our analysis above.

No notes yet

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

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