FirefoxWeb browser · Mozilla
CVE-2021-23954
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 · uneditedUsing the new logical assignment operators in a JavaScript switch statement could have caused a type confusion, leading to a memory corruption and a potentially exploitable crash. This vulnerability affects Firefox < 85, Thunderbird < 78.7, and Firefox ESR < 78.7.
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 type confusion vulnerability exists in the SpiderMonkey JavaScript engine where using new logical assignment operators (||=, &&=, ??=) within switch statements causes incorrect type handling, leading to memory corruption and potentially exploitable crashes. This is a memory safety issue in the JavaScript parsing logic.
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
< 85.0
Firefox EsrWeb browser
< 78.7< 78.7CVSS 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
- Required
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/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 productCheck if Firefox or Thunderbird is installed. In Firefox, type 'about:support' in the address bar. In Thunderbird, go to Help > About.Affected if The product is Mozilla Firefox or Thunderbird and the version is below the fixed releases.
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Determine the exact version numberLook for the 'Version' field on the about:support page (Firefox) or the About dialog (Thunderbird). Record the full version string.Affected if Version is below 85.0 for Firefox, below 78.7 for Firefox ESR, or below 78.7 for Thunderbird.
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Confirm the product typeOn about:support, check the 'Application Basics > Build Configuration > Build ID' or similar field to determine if this is Firefox release, Firefox ESR, or Thunderbird.Affected if The product is Firefox < 85.0, Firefox ESR < 78.7, or Thunderbird < 78.7.
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Identify vulnerable JavaScript usage (optional)Search JavaScript source files for the pattern: logical assignment operators (||=, &&=, ??=) used inside switch statement bodies. Search for 'switch' followed by blocks containing these operators.Affected if Code uses ||=, &&=, or ??= operators within switch statement blocks while running an affected browser version.
You are affected if Mozilla Firefox is below version 85.0, or Firefox ESR/Thunderbird is below version 78.7, and you execute JavaScript containing logical assignment operators within switch statements.
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 data78.785.0
Upgrade to Firefox 85+, Thunderbird 78.7+, or Firefox ESR 78.7+ to obtain the patched JavaScript engine. No custom code changes are required.
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-2021-23954 — 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-2021-23954 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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