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FirefoxWeb browser · Mozilla

CVE-2020-26967

MEDIUM · 6.5 CVSS v3.1 Published 2020-12-09
Fix available
A fix is available. Upgrade to 83.0 or later.
See remediation →
72/100
Remediation priority · Elevated
Remotely reachable No privileges

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 · unedited
When listening for page changes with a Mutation Observer, a malicious web page could confuse Firefox Screenshots into interacting with elements other than those that it injected into the page. This would lead to internal errors and unexpected behavior in the Screenshots code. This vulnerability affects Firefox < 83.

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 · moderate confidence

Firefox Screenshots uses Mutation Observer API to detect page changes. A malicious webpage can manipulate the DOM to cause Screenshots to interact with unintended elements (not just those it injected), leading to internal errors and unexpected behavior. This is a type confusion vulnerability via DOM manipulation.

MitigationUpgrade Firefox to version 83 or later, which contains the fix for this vulnerability.

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
Affected:< 83.0

CVSS 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 checks

Work through these to decide whether this CVE applies to you.

  1. Identify Firefox installation
    Check if Mozilla Firefox is installed by looking for firefox executable in standard locations (e.g., C:\Program Files\Mozilla Firefox\firefox.exe on Windows, /Applications/Firefox.app on macOS, or which firefox on Linux)
    Affected if Firefox is not installed or no Firefox browser is present in the environment
  2. Determine Firefox version
    Run 'firefox --version' from command line, or open Firefox and navigate to about:support, or check Help > About Firefox. Compare the version number to 83.0
    Affected if The installed Firefox version is less than 83.0 (for example, 82.0.3, 82.0, 81.0, etc.)
  3. Verify Screenshots add-on status
    Open Firefox, type 'about:addons' in the address bar, then go to the Extensions section. Look for 'Firefox Screenshots' listed as enabled, or check if the Screenshots icon (camera icon) appears in the toolbar
    Affected if Firefox Screenshots is installed and enabled in the browser

The environment is affected if Firefox version is below 83.0 AND the Firefox Screenshots feature is enabled, as the type confusion vulnerability requires both conditions to be exploitable.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

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 data
Upgrade available Upgrade to 83.0 or later
Fixed in 83.0
Interim mitigation

Upgrade Firefox to version 83 or later, which contains the fix for this vulnerability.

Fix this in Firefox Scoped from the published advisory
  • Consultation1.0 h
  • Implementation1.0 h
  • Testing1.0 h
  • Review / QA1.0 h
4.0 hours of engineering $750
Get the upgrade done

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

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2020-26967 in production — separate from our analysis above.

No notes yet

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What this is

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.

What belongs here
  • 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