FirefoxWeb browser · Mozilla
CVE-2020-12387
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 race condition when running shutdown code for Web Worker led to a use-after-free vulnerability. This resulted in a potentially exploitable crash. This vulnerability affects Firefox ESR < 68.8, Firefox < 76, and Thunderbird < 68.8.0.
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 race condition in Web Worker shutdown code causes a use-after-free vulnerability that can lead to a potentially exploitable crash.
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
< 76.0
Firefox EsrWeb browser
< 68.8.0< 68.8.0CVSS 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:3.1/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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Identify the installed Mozilla productDetermine whether Firefox (standard), Firefox ESR, or Thunderbird is installed. On macOS, check /Applications folder. On Windows, check Program Files. On Linux, run 'which firefox' or 'which thunderbird'. Within Firefox, navigate to about:support. Within Thunderbird, go to Help > Troubleshooting Information.Affected if Any of Firefox (any channel), Firefox ESR, or Thunderbird is present on the system
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Determine the exact version numberFor Firefox: type 'about:support' in the address bar and note the Version field. For Thunderbird: go to Help > Troubleshooting Information. From command line on Linux: 'firefox --version' or 'thunderbird --version'. On Windows, right-click the executable and select Properties > Details to view the version.Affected if Version is below 76.0 for Firefox, below 68.8.0 for Firefox ESR, or below 68.8.0 for Thunderbird
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Verify Web Worker functionality is accessibleIn Firefox, type 'about:config' in the address bar, search for 'dom.workers.enabled' and confirm it is set to true (default). In Thunderbird, the same config path applies. Web Workers are enabled by default in these products.Affected if Web Workers are enabled (default configuration)
The environment is affected if a vulnerable version is installed (Firefox below 76.0, Firefox ESR below 68.8.0, or Thunderbird below 68.8.0) and Web Workers remain in their default enabled state.
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 data68.8.076.0
Update Firefox ESR to version 68.8 or later, Firefox to version 76 or later, or Thunderbird to version 68.8.0 or later.
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-2020-12387 — 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-2020-12387 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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