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FirefoxWeb browser · Mozilla

CVE-2025-6436

HIGH · 8.1 CVSS v3.1 Published 2025-06-24
Fix available
A fix is available. Upgrade to 140.0 or later.
See remediation →
90/100
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
Memory safety bugs present in Firefox 139 and Thunderbird 139. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. 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 · moderate confidence

Memory safety bugs in Firefox 139 and Thunderbird 139 that showed evidence of memory corruption. Mozilla expects these could be exploited with sufficient effort to run arbitrary code. Fixed in version 140.

MitigationUpdate to Firefox 140 or Thunderbird 140 or later to remediate the 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:< 140.0
ThunderbirdApplication
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
High
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:H/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 on Windows
    Open Firefox, click the menu (three horizontal lines), select Help, then About Firefox. The version number is displayed in the window that opens.
    Affected if The displayed version is below 140.0 (e.g., 139.x or earlier)
  2. Check Firefox version on Linux
    Run the command: firefox --version
    Affected if The output shows a version number below 140.0 (e.g., 139.x or earlier)
  3. Check Firefox version on macOS
    Run the command: /Applications/Firefox.app/Contents/MacOS/firefox --version or click Firefox in the menu bar, select About Firefox.
    Affected if The displayed version is below 140.0 (e.g., 139.x or earlier)
  4. Check Thunderbird version on Windows
    Open Thunderbird, click the menu (three horizontal lines), select Help, then About Thunderbird. The version number is displayed in the window that opens.
    Affected if The displayed version is below 140.0 (e.g., 139.x or earlier)
  5. Check Thunderbird version on Linux
    Run the command: thunderbird --version
    Affected if The output shows a version number below 140.0 (e.g., 139.x or earlier)
  6. Check Thunderbird version on macOS
    Run the command: /Applications/Thunderbird.app/Contents/MacOS/thunderbird --version or click Thunderbird in the menu bar, select About Thunderbird.
    Affected if The displayed version is below 140.0 (e.g., 139.x or earlier)

You are affected if either Firefox or Thunderbird is installed with a version number lower than 140.0 on your system.

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 to Firefox 140 or Thunderbird 140 or later to remediate the vulnerability.

Recommended fix High confidence

Firefox 140.0 and Thunderbird 140.0

  1. Open Firefox or Thunderbird
  2. Navigate to the menu and select 'Help' > 'About Firefox' or 'About Thunderbird'
  3. The application will check for updates and prompt to update to version 140.0
  4. Click 'Update' to download and install the update
  5. Restart the application after the update completes

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

Fix this in Firefox Scoped from the published advisory
  • Consultation1.0 h
  • Implementation2.0 h
  • Testing2.0 h
  • Review / QA1.0 h
6.0 hours of engineering $1,040
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,664.

Scan for this in your stack

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dbcve dependency scanner

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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-2025-6436 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