FirefoxWeb browser · Mozilla
CVE-2020-26951
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 parsing and event loading mismatch in Firefox's SVG code could have allowed load events to fire, even after sanitization. An attacker already capable of exploiting an XSS vulnerability in privileged internal pages could have used this attack to bypass our built-in sanitizer. This vulnerability affects Firefox < 83, Firefox ESR < 78.5, and Thunderbird < 78.5.
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 confidenceFirefox's SVG parsing and event loading mechanisms had a mismatch where load events could fire even after sanitization was applied. An attacker who already had XSS capability in privileged internal pages could exploit this to bypass the built-in sanitizer.
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
< 83.0
Firefox EsrWeb browser
< 78.5< 78.5CVSS 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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Identify installed Mozilla productOpen terminal or command prompt and run: firefox --version (or thunderbird --version) to determine if Firefox or Thunderbird is installed and note the product name.Affected if Product is Firefox or Thunderbird and version has not been determined yet
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Check Firefox versionEnter 'about:Firefox' in the Firefox address bar, or run 'firefox --version' from command line. The version number appears as 'Firefox XX.X' where XX.X is the version.Affected if Firefox version is below 83.0 (e.g., 82.x, 81.x, etc.)
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Check Firefox ESR versionEnter 'about:Firefox' in the Firefox ESR address bar, or run 'firefox --version' from command line. Look for 'ESR' designation in the about dialog.Affected if Firefox ESR version is below 78.5
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Check Thunderbird versionEnter 'about:Thunderbird' in the Thunderbird address bar, or run 'thunderbird --version' from command line. The version number appears as 'Thunderbird XX.X'.Affected if Thunderbird version is below 78.5
User is affected if Mozilla Firefox (any channel) is below 83.0, Firefox ESR is below 78.5, or Thunderbird is below 78.5.
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 data78.583.0
Update to Firefox 83, Firefox ESR 78.5, Thunderbird 78.5 or later to obtain the vendor patch for this vulnerability.
Fix this in Firefox
Scoped from the published advisory
- Consultation1.0 h
- Implementation2.0 h
- Testing2.0 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 $1,664.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2020-26951 — 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-2020-26951 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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