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FirefoxWeb browser · Mozilla

CVE-2026-0877

HIGH · 8.1 CVSS v3.1 Published 2026-01-13
Fix available
A fix is available. Upgrade to 115.32.0 / 140.7.0 or later.
See remediation →
88/100
Remediation priority · High
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
Mitigation bypass in the DOM: Security component. This vulnerability was fixed in Firefox 147, Firefox ESR 115.32, 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

This is a mitigation bypass vulnerability in a DOM security component affecting Firefox and Thunderbird. The vulnerability allows attackers to circumvent existing security protections in the browser's Document Object Model. It was patched in Firefox 147, Firefox ESR 115.32, Firefox ESR 140.7, Thunderbird 147, and Thunderbird 140.7.

MitigationUpgrade to Firefox 147 (or ESR 115.32/140.7 for enterprise releases) and Thunderbird 140.7 to apply the security fix. Users on older versions should update immediately.

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:< 115.32.0< 147.0>= 128.0, < 140.7.0
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
Required
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
None

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

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
    Type 'about:support' in the Firefox address bar and look for the 'Version' field under 'Application Basics', or go to Help > About Firefox
    Affected if The version displayed is less than 115.32.0, or is between 128.0.0 and 140.6.x, or is less than 147.0.0 (excluding ESR 115.32+ and ESR 140.7+)
  2. Check Thunderbird version
    Type 'about:support' in the Thunderbird address bar and look for the 'Version' field, or go to Help > About Thunderbird
    Affected if The version displayed is less than 140.7.0 or less than 147.0
  3. Confirm Firefox ESR status
    On the about:support page, look for the 'Version' string. ESR releases contain 'ESR' in the version number (for example, '115.32.0esr')
    Affected if Running Firefox ESR and the version is less than 115.32.0 or less than 140.7.0

You are affected if you are running any unpatched version of Firefox or Thunderbird, specifically any Firefox version below 115.32.0, below 147.0, or between 128.x and 140.6.x, or any Thunderbird version below 140.7.0 or 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 115.32.0 / 140.7.0 / 147.0 or later
Fixed in 115.32.0140.7.0147.0
Interim mitigation

Upgrade to Firefox 147 (or ESR 115.32/140.7 for enterprise releases) and Thunderbird 140.7 to apply the security fix. Users on older versions should update immediately.

Recommended fix High confidence

Firefox 147 (or Firefox ESR 115.32/140.7); Thunderbird 140.7 or 147

  1. 1. Check the current installed version of Firefox or Thunderbird by navigating to the application's menu (three horizontal lines) and selecting 'Help' > 'About Firefox' or 'About Thunderbird'
  2. 2. If the installed version is less than 115.32.0, upgrade to Firefox ESR 115.32.0 or later
  3. 3. If the installed version is 128.x or higher but less than 140.7.0, upgrade to Firefox ESR 140.7.0 or later
  4. 4. If the installed version is less than 147.0, upgrade to Firefox 147.0 or later
  5. 5. For Thunderbird users: upgrade to Thunderbird 140.7.0 or later, or Thunderbird 147.0 or later
  6. 6. After upgrade, verify the version in 'About' to confirm the fix is applied

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
  • Testing2.0 h
  • Review / QA1.0 h
5.0 hours of engineering $860
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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-2026-0877 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