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FirefoxWeb browser · Mozilla

CVE-2026-0884

CRITICAL · 9.8 CVSS v3.1 Published 2026-01-13
Fix available
A fix is available. Upgrade to 140.7.0 / 147.0 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
Use-after-free in the JavaScript Engine component. This vulnerability was fixed in Firefox 147, Firefox ESR 140.7, Thunderbird 147, and Thunderbird 140.7.

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

A use-after-free vulnerability in the Mozilla JavaScript Engine allows memory to be accessed after being freed, potentially enabling arbitrary code execution or denial of service. This is a memory corruption issue in the engine that parses and executes JavaScript code within Firefox and Thunderbird applications.

MitigationApply the security updates (Firefox 147/Firefox ESR 140.7 or Thunderbird 147/Thunderbird 140.7) to remediate the vulnerability. Organizations should prioritize patching given the critical CVSS score.

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:< 140.7.0< 147.0
ThunderbirdApplication
Affected:< 140.7.0< 147.0

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. Identify installed Firefox version
    Open Firefox, click the menu button (three horizontal lines), select Help, then About Firefox. The version number displays on this page.
    Affected if The displayed version is lower than 140.7.0, or falls between 140.7.0 and 146.x (i.e., less than 147.0)
  2. Identify installed Thunderbird version
    Open Thunderbird, click the menu button (three horizontal lines), select Help, then About Thunderbird. The version number displays on this page.
    Affected if The displayed version is lower than 140.7.0, or falls between 140.7.0 and 146.x (i.e., less than 147.0)
  3. Check Firefox version via command line (optional)
    Run 'firefox --version' or on macOS '/Applications/Firefox.app/Contents/MacOS/firefox --version' to retrieve the version from the terminal.
    Affected if The output shows a version lower than 140.7.0 or between 140.7.0 and 146.x
  4. Check Thunderbird version via command line (optional)
    Run 'thunderbird --version' or on macOS '/Applications/Thunderbird.app/Contents/MacOS/thunderbird --version' to retrieve the version from the terminal.
    Affected if The output shows a version lower than 140.7.0 or between 140.7.0 and 146.x

You are affected if either Firefox or Thunderbird is installed and its version falls below 140.7.0 or is between 140.7.0 and 146.x (less than 147.0).

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.

dbcve · scoped
Upgrade available Upgrade to 140.7.0 / 147.0 or later
Fixed in 140.7.0147.0
Interim mitigation

Apply the security updates (Firefox 147/Firefox ESR 140.7 or Thunderbird 147/Thunderbird 140.7) to remediate the vulnerability. Organizations should prioritize patching given the critical CVSS score.

Recommended fix High confidence

Firefox 147 (or Firefox ESR 140.7); Thunderbird 147 (or Thunderbird 140.7)

  1. Check the current version of Firefox or Thunderbird by navigating to the application menu and selecting 'About Firefox' or 'About Thunderbird'
  2. Download Firefox 147 or later (or Firefox ESR 140.7 or later for enterprise deployments) from the official Mozilla website
  3. Download Thunderbird 147 or later (or Thunderbird 140.7 or later for enterprise deployments) from the official Mozilla website
  4. Install the downloaded version of Firefox or Thunderbird
  5. Restart the application to complete the update

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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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-2026-0884 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