dbcve.org
FirefoxWeb browser · Mozilla

CVE-2021-23972

HIGH · 8.8 CVSS v3.1 Published 2021-02-26
Fix available
A fix is available. Upgrade to 86.0 or later.
See remediation →
95/100
Remediation priority · Urgent
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
One phishing tactic on the web is to provide a link with HTTP Auth. For example 'https://[email protected]'. To mitigate this type of attack, Firefox will display a warning dialog; however, this warning dialog would not have been displayed if evil.com used a redirect that was cached by the browser. 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 confidence

Firefox had a security feature to warn users about URLs containing embedded HTTP authentication credentials (e.g., 'https://[email protected]'). However, this warning was bypassed when the malicious site used an HTTP redirect that got cached by the browser, allowing phishing attacks to proceed without displaying the security warning.

MitigationUpgrade Firefox to version 86 or later, which contains the fix for this cached-redirect warning bypass. Organizations should ensure patch management systems deploy this update.

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:< 86.0

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
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

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. Check Firefox version number
    Open a new tab and navigate to 'about:support' or 'about:firefox'. Look for the version number displayed near the top of the page.
    Affected if The displayed version number is less than 86.0 (for example, 85.x, 85.0.1, 84.0, etc.)
  2. Alternative: Check version from application menu
    Click the menu button (three horizontal lines) in Firefox, then click Help > About Firefox. The version number appears in the window that opens.
    Affected if The version shown is below 86.0 (e.g., 85.2, 84.0.1, etc.)
  3. Check browser auto-update status (optional context)
    In 'about:support', look for the 'Update' section to see if automatic updates are enabled and if the browser is up to date.
    Affected if Automatic updates are disabled or have failed, and the installed version remains below 86.0

You are affected if your installed Firefox version is any release prior to version 86.0.

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 86.0 or later
Fixed in 86.0
Interim mitigation

Upgrade Firefox to version 86 or later, which contains the fix for this cached-redirect warning bypass. Organizations should ensure patch management systems deploy this update.

Fix this in Firefox Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing1.0 h
  • Review / QA1.0 h
6.0 hours of engineering $1,090
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,744.

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-2021-23972 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