FirefoxWeb browser · Mozilla
CVE-2020-15654
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 · uneditedWhen in an endless loop, a website specifying a custom cursor using CSS could make it look like the user is interacting with the user interface, when they are not. This could lead to a perceived broken state, especially when interactions with existing browser dialogs and warnings do not work. This vulnerability affects Firefox ESR < 78.1, Firefox < 79, and Thunderbird < 78.1.
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 confidenceThis is a UI spoofing vulnerability in Firefox and Thunderbird where a malicious webpage using CSS custom cursors combined with an endless JavaScript loop can make the browser appear frozen or unresponsive while the cursor still moves. This creates a perceived broken state where browser dialogs and warnings become non-functional, potentially enabling social engineering attacks against users.
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
< 79.0
Firefox EsrWeb browser
< 78.1
Ubuntu LinuxOperating system
= 16.04= 18.04= 20.04< 78.1CVSS 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
- None
- Integrity
- High
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/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 versionIn Firefox, navigate to Menu > Help > About Firefox, or type 'about:support' in the address bar and look at the 'Version' field in the 'Application Basics' sectionAffected if The version number is less than 79.0 (for example, 78.0 or earlier)
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Identify installed Firefox ESR versionIn Firefox ESR, navigate to Menu > Help > About Firefox, or type 'about:support' in the address bar and look at the 'Version' field. ESR versions typically include 'ESR' in the version string (for example, 78.0 ESR)Affected if The version number is less than 78.1 (for example, 78.0 ESR or earlier)
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Identify installed Thunderbird versionIn Thunderbird, navigate to Menu > Help > About Thunderbird, or go to Help > Troubleshooting Information and look at the 'Version' fieldAffected if The version number is less than 78.1 (for example, 78.0 or earlier)
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Check Ubuntu system browser package versionOn Ubuntu Linux systems, run 'dpkg -l | grep firefox' or 'apt list --installed | grep firefox' in terminal to see the installed Firefox package versionAffected if The installed Firefox package version corresponds to a Firefox release earlier than 79.0
A user is affected if they have Firefox version below 79.0, Firefox ESR version below 78.1, or Thunderbird version below 78.1 running on any operating system including Ubuntu Linux.
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 data78.179.0
Update affected browsers to Firefox ESR 78.1+, Firefox 79+, or Thunderbird 78.1+ to patch the vulnerability. Organizations should deploy patched browser versions via their patch management systems.
Fix this in Firefox
Scoped from the published advisory
- Consultation2.0 h
- Implementation4.0 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 $2,560.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2020-15654 — 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-2020-15654 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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