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FirefoxWeb browser · Mozilla

CVE-2026-0891

HIGH · 8.1 CVSS v3.1 Published 2026-01-13
Fix available
A fix is available. Upgrade to 140.7.0 / 147.0 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 ESR 140.6, Thunderbird ESR 140.6, Firefox 146 and Thunderbird 146. 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 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

Memory safety bugs in Firefox ESR 140.6, Thunderbird ESR 140.6, Firefox 146, and Thunderbird 146 that could lead to memory corruption. Some bugs showed evidence of memory corruption that with sufficient effort could potentially be exploited for arbitrary code execution.

MitigationUpgrade to Firefox 147/Firefox ESR 140.7 or Thunderbird 147/Thunderbird 140.7 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:< 140.7.0< 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
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 if Mozilla Firefox is installed
    Open terminal and run: firefox --version OR check Help > About Firefox in the application menu
    Affected if Firefox version is less than 140.7.0 or falls between 140.x (if <140.7.0) and 147.0 (excluding 147.0 and above)
  2. Check if Mozilla Thunderbird is installed
    Open terminal and run: thunderbird --version OR check Help > About Thunderbird in the application menu
    Affected if Thunderbird version is less than 140.7.0 or falls between 140.x (if <140.7.0) and 147.0 (excluding 147.0 and above)
  3. Verify installed version against affected ranges
    Compare your reported version number to the affected ranges: Firefox < 140.7.0, Firefox < 147.0, Thunderbird < 140.7.0, Thunderbird < 147.0
    Affected if Your installed version matches any of these vulnerable ranges (both the 140.x line before 140.7.0 and the 146.x line before 147.0)

You are affected if you are running any version of Firefox or Thunderbird below 140.7.0 or below 147.0 (for the 146.x release line).

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

Upgrade to Firefox 147/Firefox ESR 140.7 or Thunderbird 147/Thunderbird 140.7 to remediate the vulnerability.

Recommended fix High confidence

Firefox 147, Firefox ESR 140.7, Thunderbird 147, or Thunderbird ESR 140.7 (or later versions)

  1. 1. Back up any important data, bookmarks, and browser profile settings before upgrading
  2. 2. Open Firefox or Thunderbird and navigate to the Help menu (or hamburger menu)
  3. 3. Select 'About Firefox' or 'About Thunderbird' to check the current version
  4. 4. Click 'Check for Updates' to automatically download and install the latest version
  5. 5. Alternatively, download the latest version from the official Mozilla website: https://www.mozilla.org/firefox/download/ or https://www.thunderbird.net/
  6. 6. Run the installer and follow the prompts to complete the upgrade
  7. 7. Restart the application after the update completes
  8. 8. Verify the version by checking 'About Firefox' or 'About Thunderbird' confirms version 140.7 or higher, or 147 or higher
Caveat Mozilla minor version upgrades typically have minimal breaking changes, but some older extensions or add-ons may need updates for compatibility with newer versions

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

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-2026-0891 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