FirefoxWeb browser · Mozilla
CVE-2009-4630
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 · uneditedMozilla Necko, as used in Firefox, SeaMonkey, and other applications, performs DNS prefetching of domain names contained in links within local HTML documents, which makes it easier for remote attackers to determine the network location of the application's user by logging DNS requests. NOTE: the vendor disputes the significance of this issue, stating "I don't think we necessarily need to worry about that case."
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 confidenceMozilla Necko (the networking stack used in Firefox and SeaMonkey) performs DNS prefetching on hyperlinks within local HTML documents, causing the browser to resolve domain names before the user clicks. This allows remote attackers to potentially infer browsing patterns by monitoring DNS queries for domains referenced in locally-opened HTML files.
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
all versionsall versionsall versionsCVSS 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
- Authentication
- None
- Confidentiality
- Partial
- Integrity
- None
- Availability
- None
AV:N/AC:L/Au:N/C:P/I:N/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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Identify installed Mozilla productCheck if Firefox, SeaMonkey, or Thunderbird is installed by locating the application executable or checking system informationAffected if Any of these three products are present on the system
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Locate the browser profile or configurationFind the profile directory where preferences are stored (typically in %APPDATA%\Mozilla\Firefox\Profiles on Windows, ~/.mozilla/firefox on Linux, or ~/Library/Application Support/Firefox/Profiles on macOS)Affected if A profile exists, indicating the browser has been run
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Check DNS prefetch preferenceOpen the prefs.js file in the profile folder or type 'about:config' in the browser address bar and search for 'network.dns.disablePrefetch'Affected if The preference 'network.dns.disablePrefetch' is set to false, not present, or set to any value other than true
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Confirm preference state for local HTML filesOpen any local HTML file containing hyperlinks to external domains and observe network traffic with a packet sniffer to verify DNS queries are being made on page loadAffected if DNS queries for domains in the HTML file are resolved without user interaction
If Firefox, SeaMonkey, or Thunderbird is installed with DNS prefetch enabled (network.dns.disablePrefetch is not set to true), the environment is affected by this CVE.
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.
From vendor dataUsers can disable DNS prefetching in Firefox settings (network.dns.disablePrefetch), or organizations can deploy browser configurations/policies to disable this feature. The fundamental fix requires a patch from Mozilla to modify Necko behavior for local documents.
Fix this in Firefox
Scoped from the published advisory
- Consultation2.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2009-4630 — 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-2009-4630 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
- No spam, self-promotion, credentials, or personal data