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FirefoxWeb browser · Mozilla

CVE-2009-4129

MEDIUM · 5.8 CVSS v2.0 Published 2009-12-14
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
67/100
Remediation priority · Elevated
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
Race condition in Mozilla Firefox allows remote attackers to produce a JavaScript message with a spoofed domain association by writing the message in between the document request and document load for a web page in a different domain.

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 race condition in Mozilla Firefox allows remote attackers to send JavaScript messages with a spoofed domain association by timing the message between the document request and document load phases for a page in a different domain. This enables cross-origin message spoofing attacks.

MitigationImplement strict origin validation in JavaScript message event handlers using the event.origin property, and update Firefox to the latest patched version.

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:all versions

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
M
Authentication
None
Confidentiality
None
Integrity
Partial
Availability
Partial

AV:N/AC:M/Au:N/C:N/I:P/A:P

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
    Navigate to Menu > Help > About Firefox, or type 'about:support' in the address bar, or run 'firefox --version' from command line
    Affected if Any version of Firefox is installed, as all versions are affected by this CVE
  2. Identify postMessage usage in local web applications
    Search web application source code for 'addEventListener("message"' or 'onmessage' handlers that process cross-origin messages
    Affected if Applications use postMessage without validating event.origin property, making them exploitable via this Firefox flaw
  3. Audit JavaScript message handlers for origin validation
    Review JavaScript code that handles window.postMessage events and verify it checks event.origin before processing messages
    Affected if Message handlers accept messages without verifying event.origin, allowing spoofed origins from the Firefox race condition

Any environment running Firefox with web applications that process postMessage events without strict event.origin validation is affected by this cross-origin message spoofing vulnerability.

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.

From vendor data
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Implement strict origin validation in JavaScript message event handlers using the event.origin property, and update Firefox to the latest patched version.

Fix this in Firefox Scoped from the published advisory
  • Consultation1.0 h
  • Implementation2.0 h
  • Testing1.0 h
  • Review / QA1.0 h
5.0 hours of engineering $890
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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-2009-4129 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