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FirefoxWeb browser · Mozilla

CVE-2025-6433

CRITICAL · 9.8 CVSS v3.1 Published 2025-06-24
Fix available
A fix is available. Upgrade to 140.0 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click

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
If a user visited a webpage with an invalid TLS certificate, and granted an exception, the webpage was able to provide a WebAuthn challenge that the user would be prompted to complete. This is in violation of the WebAuthN spec which requires "a secure transport established without errors". This vulnerability was fixed in Firefox 140 and Thunderbird 140.

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 and Thunderbird allowed WebAuthn authentication challenges to be presented to users on webpages with invalid TLS certificates (after user-granted exceptions), bypassing the WebAuthn specification requirement for secure transport without errors. This could enable authentication bypass in man-in-the-middle scenarios.

MitigationUpdate Firefox to version 140 or later, and Thunderbird to version 140 or later. Alternatively, avoid granting certificate exceptions for sites requiring WebAuthn authentication.

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:< 140.0

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
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/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
    Open Firefox, go to Menu > Help > About Firefox, or navigate to about:support, and note the version number displayed.
    Affected if The version is below 140.0 (e.g., 139.x, 138.x, etc.)
  2. Check Thunderbird version
    Open Thunderbird, go to Menu > Help > About Thunderbird, and note the version number displayed.
    Affected if The version is below 140.0 (e.g., 139.x, 138.x, etc.) and you use WebAuthn authentication
  3. Verify WebAuthn usage in the environment
    Check if your organization or users employ WebAuthn/FIDO2 hardware tokens or browser-stored credentials for authentication to web applications.
    Affected if WebAuthn is actively used for authentication and the Firefox/Thunderbird version is below 140.0
  4. Check for certificate exceptions
    In Firefox, go to Menu > Privacy & Security > Certificates > View Certificates, and inspect the 'Exceptions' tab for sites where certificate errors were overridden.
    Affected if There are certificate exceptions configured AND WebAuthn is used, with a Firefox/Thunderbird version below 140.0

You are affected if you use Firefox or Thunderbird below version 140.0, employ WebAuthn authentication, and have configured any certificate exceptions for sites.

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.

dbcve · scoped
Upgrade available Upgrade to 140.0 or later
Fixed in 140.0
Interim mitigation

Update Firefox to version 140 or later, and Thunderbird to version 140 or later. Alternatively, avoid granting certificate exceptions for sites requiring WebAuthn authentication.

Recommended fix High confidence

Firefox 140.0

  1. Open Firefox browser
  2. Click the menu button (three horizontal lines) and select Help > About Firefox
  3. The About Firefox window will automatically check for updates
  4. If Firefox 140.0 or later is available, click 'Update to this version' or allow it to download automatically
  5. Restart Firefox when prompted to complete the installation
  6. After restart, verify the version by returning to Help > About Firefox - it should show version 140.0 or later

Generated from the published advisory — verify against the referenced sources before acting.

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.

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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-2025-6433 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