FirefoxWeb browser · Mozilla
CVE-2026-4727
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 Libraries component in NSS. 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 NSS (Network Security Services) library. The issue can be exploited to cause service disruption, likely through malformed input processing. The vulnerability is addressed in Firefox 149 and Thunderbird 149, indicating the fix was shipped via those application updates which include the patched NSS library.
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.
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Check if Mozilla Firefox is installedOpen Firefox and navigate to '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 if Mozilla Thunderbird is installedOpen Thunderbird and navigate to '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)
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Verify NSS library version if directly referencedInspect the NSS library files included with the application or system-wide NSS installation; on Linux this may be found in /usr/lib/nss/ or similar paths depending on OSAffected if The NSS library version bundled with the application is older than the patched version shipped in Firefox/Thunderbird 149
You are affected if either Mozilla Firefox version is below 149.0 or Mozilla Thunderbird version is below 149.0, as these contain the vulnerable NSS library.
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
Apply the patched versions (Firefox 149+, Thunderbird 149+) or update the underlying NSS library to the fixed version to remediate the DoS condition.
Firefox 149.0 and Thunderbird 149.0
- Back up your Firefox or Thunderbird profile and important data before updating
- Download Firefox 149.0 or Thunderbird 149.0 from the official Mozilla website (www.mozilla.org)
- Close Firefox or Thunderbird completely if currently running
- Install the update (on Windows: run the downloaded installer; on macOS: drag the application to Applications folder; on Linux: use your package manager or install the downloaded package)
- Launch the application and verify the version by navigating to Help > About to confirm you are on version 149.0 or later
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
- Testing4.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,328.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-4727 — 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-2026-4727 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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