FirefoxWeb browser · Mozilla
CVE-2025-6430
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 · uneditedWhen a file download is specified via the `Content-Disposition` header, that directive would be ignored if the file was included via a `<embed>` or `<object>` tag, potentially making a website vulnerable to a cross-site scripting attack. This vulnerability was fixed in Firefox 140, 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 confidenceWhen a file download is specified via the Content-Disposition header (e.g., Content-Disposition: attachment), this directive is ignored when the file is embedded using <embed> or <object> tags. This allows potentially malicious content (e.g., HTML, SVG) to be rendered in the browser instead of downloaded, enabling cross-site scripting attacks.
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
< 128.12.0< 140.0CVSS 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
- Required
- Scope
- Changed
- Confidentiality
- Low
- Integrity
- Low
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
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 checksWork through these to decide whether this CVE applies to you.
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Check installed Firefox versionNavigate to Firefox, click the menu (three horizontal lines), select Help, then About Firefox. Alternatively, enter 'about:support' in the address bar.Affected if The displayed version is less than 128.12.0 (for ESR versions) or less than 140.0 (for regular versions)
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Verify Content-Disposition handling for embed/object tagsHost a test page with an <embed> or <object> tag referencing a file served with 'Content-Disposition: attachment' header (e.g., an SVG file). Use browser developer tools (Network tab) to inspect the response headers.Affected if The browser attempts to render the embedded content instead of treating it as a download, indicating the header is being ignored
A user is affected if their Firefox version is below 128.12.0 (ESR) or below 140.0 (regular) AND they encounter web content that uses embed/object tags with Content-Disposition: attachment headers.
Generated from the published advisory. Verify against your own configuration.
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 · scoped128.12.0140.0
Update affected browsers to Firefox 140+, Firefox ESR 128.12+, Thunderbird 140+, or Thunderbird 128.12+ to receive the vendor patch.
Firefox 140 (or Firefox ESR 128.12 for extended support users)
- Identify current Firefox version via Menu > Help > About Firefox (or about:support)
- If current version is < 128.12.0, upgrade to Firefox ESR 128.12 or later
- If current version is between 128.12.0 and 139.x, upgrade to Firefox 140 or later
- Restart Firefox after upgrade
- Verify version at Menu > Help > About Firefox shows 128.12.0+ or 140.0+
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Firefox
Scoped from the published advisory
- Consultation2.0 h
- Implementation2.0 h
- Testing1.0 h
- Review / QA1.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,744.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2025-6430 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2025-6430 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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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.
- 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
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