FirefoxWeb browser · Mozilla
CVE-2020-15676
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 · uneditedFirefox sometimes ran the onload handler for SVG elements that the DOM sanitizer decided to remove, resulting in JavaScript being executed after pasting attacker-controlled data into a contenteditable element. This vulnerability affects Firefox < 81, Thunderbird < 78.3, and Firefox ESR < 78.3.
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 DOM sanitizer in Firefox and Thunderbird failed to properly prevent onload handler execution when removing SVG elements. When user-controlled data containing malicious SVG was pasted into a contenteditable element, the SVG element would be removed but the onload handler would still fire, leading to arbitrary JavaScript execution (XSS).
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
< 81.0
Firefox EsrWeb browser
< 78.3
Debian LinuxOperating system
= 9.0= 10.0< 78.3= 15.1= 15.2CVSS 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 Firefox versionRun 'firefox --version' or check Help > About Firefox to get the exact version numberAffected if Version is below 81.0 for regular Firefox or below 78.3 for Firefox ESR
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Identify installed Thunderbird versionRun 'thunderbird --version' or check Help > About Thunderbird to get the exact version numberAffected if Version is below 78.3
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Check if Debian package manager knows the versionRun 'dpkg -l | grep firefox' or 'dpkg -l | grep thunderbird' on Debian-based systems to see installed package versionsAffected if Installed package version matches Debian 9.0 or 10.0 for Firefox, or the system is running the listed OpenSUSE Leap versions (15.1, 15.2) with vulnerable browser packages
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Determine if contenteditable paste is usedThis is a usage pattern check: identify whether users or automated processes paste HTML/SVG content from untrusted sources into web forms, email composer, or other contenteditable fields in Firefox or ThunderbirdAffected if Users paste untrusted HTML/SVG content into contenteditable areas (such as email compose windows, web form text areas, or rich text editors) in a vulnerable browser version
You are affected if you run Firefox below 81.0 (or ESR below 78.3) or Thunderbird below 78.3, AND you or your users paste untrusted SVG or HTML content into contenteditable areas like email compose fields or web forms.
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 data78.381.0
Update Firefox to version 81 or later, Thunderbird to 78.3 or later, or Firefox ESR to 78.3 or later. Until patched, avoid pasting untrusted content into contenteditable areas in affected browsers.
Fix this in Firefox
Scoped from the published advisory
- Consultation2.0 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 $1,984.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2020-15676 — 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-2020-15676 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
- No spam, self-promotion, credentials, or personal data