FirefoxWeb browser · Mozilla
CVE-2019-9821
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 · uneditedA use-after-free vulnerability can occur in AssertWorkerThread due to a race condition with shared workers. This results in a potentially exploitable crash. This vulnerability affects Firefox < 67.
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 confidenceA use-after-free vulnerability in Firefox's AssertWorkerThread function due to a race condition with shared workers can lead to a potentially exploitable crash. The vulnerability occurs when the worker thread assertion encounters a race condition, causing memory to be accessed after being freed.
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
< 67.0CVSS 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
- High
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- High
CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/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 installed Firefox versionOpen Firefox, click the menu button (three horizontal lines), go to Help > About Firefox. The version number will be displayed.Affected if The displayed version is lower than 67.0 (e.g., 66.x, 65.x, etc.)
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Verify shared worker usageCheck if any web applications or browser-based tools are using Shared Workers. In Firefox, you can open the Web Developer Toolbox (F12), go to the Storage tab, and look for entries under Shared Workers.Affected if Shared Workers are actively in use while running an affected Firefox version below 67.0
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Review crash reports for worker-related crashesIn Firefox, go to about:crashes to view crash reports. Look for crashes mentioning 'AssertWorkerThread', 'shared worker', or 'WorkerThread'.Affected if There are crash reports related to worker threads on an affected Firefox version below 67.0
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Check browser console for worker errorsOpen the Web Developer Toolbox (F12), switch to the Console tab, and check for JavaScript errors related to Shared Workers or worker threads.Affected if Worker-related errors appear in the console while using an affected Firefox version below 67.0
A user is affected if they are running any Firefox version below 67.0 AND actively using or accessing web content that utilizes Shared Workers.
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 data67.0
Upgrade Firefox to version 67 or later to address this use-after-free vulnerability in the shared worker handling code.
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-2019-9821 — 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-2019-9821 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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- 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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