FirefoxWeb browser · Mozilla
CVE-2026-4726
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 · uneditedDenial-of-service in the XML component. This vulnerability was fixed in Firefox 149 and Thunderbird 149.
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 · moderate confidenceA denial-of-service vulnerability exists in the XML component, likely involving malformed XML input causing excessive resource consumption or application crash. The vulnerability was addressed in Firefox 149 and Thunderbird 149.
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
< 149.0< 149.0CVSS 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
- 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 checksWork through these to decide whether this CVE applies to you.
-
Check Firefox versionOpen Firefox and navigate to Menu > Help > About Firefox, or run 'firefox --version' from command lineAffected if The displayed version is less than 149.0 (e.g., 148.x or earlier)
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Check Thunderbird versionOpen Thunderbird and navigate to Menu > Help > About Thunderbird, or run 'thunderbird --version' from command lineAffected if The displayed version is less than 149.0 (e.g., 148.x or earlier)
If either Firefox or Thunderbird is installed with a version lower than 149.0, the environment is affected by this XML denial-of-service vulnerability.
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 · scoped149.0
Update to Firefox 149 or Thunderbird 149. For other applications using similar XML parsing libraries, conduct a security audit and apply vendor patches or updates.
Firefox 149.0 and Thunderbird 149.0
- Open Firefox and navigate to Help > About Firefox (or Menu > Help > About Firefox)
- In the About Firefox window, click 'Check for updates' or wait for the automatic check to complete
- Download and install Firefox 149.0 or later when prompted
- Restart Firefox to complete the installation
- For Thunderbird: Open Thunderbird and navigate to Help > About Thunderbird
- In the About Thunderbird window, click 'Check for updates' or wait for the automatic check to complete
- Download and install Thunderbird 149.0 or later when prompted
- Restart Thunderbird to complete the installation
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Firefox
Scoped from the published advisory
- Consultation1.0 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 $1,424.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-4726 — 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-2026-4726 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