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FirefoxWeb browser · Mozilla

CVE-2025-6424

CRITICAL · 9.8 CVSS v3.1 Published 2025-06-24
Fix available
A fix is available. Upgrade to 115.25.0 / 128.12.0 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
A use-after-free in FontFaceSet resulted in a potentially exploitable crash. This vulnerability was fixed in Firefox 140, Firefox ESR 115.25, Firefox ESR 128.12, Thunderbird 140, and Thunderbird 128.12.

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 in Firefox's and Thunderbird's FontFaceSet implementation (the API for loading custom fonts via JavaScript/CSS) allows memory to be accessed after being freed, potentially leading to arbitrary code execution. With a CVSS score of 9.8, this is a critical memory safety flaw.

MitigationApply the vendor-supplied updates: upgrade to Firefox 140, Firefox ESR 115.25, Firefox ESR 128.12, Thunderbird 140, or Thunderbird 128.12 (or later 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:< 115.25.0< 140.0>= 116.0, < 128.12.0

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. Identify installed Firefox version
    In Firefox, navigate to Menu > Help > About Firefox, or enter 'about:support' in the address bar. The version number is displayed at the top.
    Affected if The version is earlier than 115.25.0, OR falls between 116.0 and 128.12.0 (exclusive), OR is earlier than 140.0
  2. Identify installed Thunderbird version
    In Thunderbird, go to Menu > Help > About Thunderbird. The version number is displayed at the top.
    Affected if The version is earlier than 115.25.0, OR falls between 116.0 and 128.12.0 (exclusive), OR is earlier than 140.0
  3. Check for custom font usage
    Search web application source code and CSS files for '@font-face' rules or JavaScript usage of the FontFace API (e.g., 'new FontFace()' calls). Inspect loaded web pages for custom font loading.
    Affected if Custom fonts are loaded via CSS @font-face rules or the JavaScript FontFace API in any page loaded in the browser

You are affected if your installed Firefox or Thunderbird version falls within any of the vulnerable ranges AND you use custom fonts via @font-face or the FontFace API in your browser environment.

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 115.25.0 / 128.12.0 / 140.0 or later
Fixed in 115.25.0128.12.0140.0
Interim mitigation

Apply the vendor-supplied updates: upgrade to Firefox 140, Firefox ESR 115.25, Firefox ESR 128.12, Thunderbird 140, or Thunderbird 128.12 (or later versions).

Recommended fix High confidence

Firefox 140 (or Firefox ESR 115.25 / Firefox ESR 128.12 depending on current version branch)

  1. Identify the current installed Firefox version by navigating to Help > About Firefox
  2. For Firefox ESR 115.x users: Upgrade to Firefox ESR 115.25.0 or later
  3. For Firefox 116.x through 128.x users: Upgrade to Firefox ESR 128.12.0 or later
  4. For Firefox 139.x and earlier users: Upgrade to Firefox 140.0 or later
  5. Alternatively, for users on modern Firefox releases: Enable automatic updates or download the latest version from https://www.mozilla.org/firefox/
  6. Verify the update was successful by checking Help > About Firefox shows the patched version
Caveat Users on very old systems may need to verify system requirements for Firefox 140; some legacy extensions or themes may need updates

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
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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-2025-6424 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