FirefoxWeb browser · Mozilla
CVE-2026-8949
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 · uneditedInteger overflow in the Widget: Win32 component. This vulnerability was fixed in Firefox 151, Firefox ESR 140.11, Thunderbird 151, and Thunderbird 140.11.
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 confidenceInteger overflow vulnerability in the Widget: Win32 component of Firefox and Thunderbird on Windows. This could allow an attacker to cause a denial of service or potentially achieve arbitrary code execution via integer overflow leading to memory corruption.
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
< 140.11.0< 151.0.0< 140.11< 151.0.0CVSS 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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Confirm Windows environmentVerify the operating system is Windows since the vulnerability specifically affects the Win32 componentAffected if Running on Windows with Firefox or Thunderbird installed
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Check Firefox version on WindowsOpen Firefox, click the menu button (three horizontal lines), select Help, then About Firefox. Alternatively, run 'firefox --version' or check the executable properties of firefox.exeAffected if Installed Firefox version is less than 140.11.0 OR less than 151.0.0
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Check Thunderbird version on WindowsOpen Thunderbird, click the menu button, select Help, then About Thunderbird. Alternatively, run 'thunderbird --version' or check the executable properties of thunderbird.exeAffected if Installed Thunderbird version is less than 140.11 OR less than 151.0.0
A user is affected if running Firefox or Thunderbird on Windows with a version below 140.11/140.11.0 or below 151.0.0 in either the regular or ESR release tracks
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 · scoped140.11140.11.0151.0.0
Update to Firefox 151 or later, Firefox ESR 140.11 or later, Thunderbird 151 or later, or Thunderbird 140.11 or later to apply the vendor patch.
Firefox 151 / Firefox ESR 140.11 / Thunderbird 151 / Thunderbird 140.11
- 1. Check current Firefox version by clicking Help > About Firefox
- 2. Check current Thunderbird version by clicking Help > About Thunderbird
- 3. For Firefox: Click 'Check for Updates' in the About dialog or wait for automatic update notification, then install version 151 or ESR 140.11
- 4. For Thunderbird: Click 'Check for Updates' in the About dialog or wait for automatic update notification, then install version 151 or 140.11
- 5. Alternatively, download the appropriate version from https://www.mozilla.org/firefox/releases/ or https://www.mozilla.org/thunderbird/releases/
- 6. Restart the application after the update completes
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Firefox
Scoped from the published advisory
- Consultation1.0 h
- Implementation1.0 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 $1,200.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-8949 — 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-2026-8949 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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