FirefoxWeb browser · Mozilla
CVE-2021-23974
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 · uneditedThe DOMParser API did not properly process '<noscript>' elements for escaping. This could be used as an mXSS vector to bypass an HTML Sanitizer. This vulnerability affects Firefox < 86.
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 confidenceThe DOMParser API in Firefox versions prior to 86 failed to properly escape noscript elements during HTML parsing, creating an mXSS (mutation cross-site scripting) vector that could bypass HTML sanitizers.
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
< 86.0CVSS 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
- Required
- Scope
- Changed
- Confidentiality
- Low
- Integrity
- Low
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
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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Determine Firefox versionOpen Firefox, click the menu button (three horizontal lines), select Help, then About Firefox. The version number is displayed at the top of the window.Affected if The displayed version is less than 86.0 (e.g., 85.x, 84.x, etc.)
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Check for DOMParser usage in web applicationsSearch source code files for 'DOMParser' or 'new DOMParser()' to identify if the application parses HTML strings server-side or client-side.Affected if DOMParser is used and parses HTML content from untrusted sources
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Identify noscript element handlingReview code that uses DOMParser to see if it processes or could process HTML containing noscript tags.Affected if The application parses HTML that may include noscript elements from user input
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Verify HTML sanitizer bypass riskTest parsed HTML output to confirm whether noscript content is properly escaped when re-inserted into the DOM.Affected if The noscript content is not properly escaped and can inject script tags after DOM mutation
If Firefox version is below 86.0 and the application uses DOMParser to parse HTML containing noscript elements from untrusted sources, the environment is vulnerable to mXSS attacks.
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 data86.0
Update Firefox to version 86 or later to patch the DOMParser vulnerability, or implement additional input validation in sanitization logic to handle noscript edge cases.
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-2021-23974 — 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-2021-23974 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
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