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FirefoxWeb browser · Mozilla

CVE-2026-2771

CRITICAL · 9.8 CVSS v3.1 Published 2026-02-24
Fix available
A fix is available. Upgrade to 115.33.0 / 140.8.0 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
Undefined behavior in the DOM: Core & HTML component. This vulnerability was fixed in Firefox 148, Firefox ESR 115.33, Firefox ESR 140.8, Thunderbird 148, and Thunderbird 140.8.

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 critical undefined behavior vulnerability exists in the DOM: Core & HTML component of Firefox and Thunderbird browsers. The specific nature of the undefined behavior is not detailed in available documentation, but the critical CVSS score (9.8) indicates potential for remote code execution or severe memory corruption. The issue affects multiple browser and email client versions across both standard and Extended Support Release (ESR) channels.

MitigationApply the vendor-supplied updates: upgrade to Firefox 148 or later, Firefox ESR 115.33/140.8 or later, or Thunderbird 148/140.8 or later. Organizations should deploy these updates through their standard patch management processes.

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:< 115.33.0< 148.0>= 128.0, < 140.8.0
ThunderbirdApplication
Affected:< 140.8.0< 148.0

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
    In Firefox, navigate to Help > About Firefox (or enter 'about:support' in the address bar). The version number is displayed at the top.
    Affected if Version is below 115.33.0, or between 128.0 and 140.8.0 exclusive, or below 148.0 (for non-ESR releases)
  2. Check Thunderbird version
    In Thunderbird, navigate to Help > About Thunderbird. The version number is displayed at the top.
    Affected if Version is below 140.8.0 or below 148.0
  3. Confirm product is Firefox ESR if applicable
    In Firefox, go to about:support and look for 'Application Basics' > 'Build Configuration' - ESR builds are labeled as 'ESR' in the version string.
    Affected if Using Firefox ESR version below 115.33.0 or below 140.8.0 (ESR branch)
  4. Verify DOM:Core & HTML components are in use
    This vulnerability affects DOM manipulation. Any use of JavaScript to interact with HTML documents through document Object Model APIs (document.getElementById, innerHTML, createElement, etc.) triggers the affected code path.
    Affected if Browser or application uses DOM manipulation features commonly enabled by default in web content rendering

A system is affected if it runs Mozilla Firefox (any channel) below 115.33.0, between 128.0-140.8.0, or below 148.0, or Thunderbird below 140.8.0 or below 148.0.

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 115.33.0 / 140.8.0 / 148.0 or later
Fixed in 115.33.0140.8.0148.0
Interim mitigation

Apply the vendor-supplied updates: upgrade to Firefox 148 or later, Firefox ESR 115.33/140.8 or later, or Thunderbird 148/140.8 or later. Organizations should deploy these updates through their standard patch management processes.

Recommended fix High confidence

Firefox 148.0+ (or ESR 115.33+ / ESR 140.8+); Thunderbird 148.0+ (or 140.8+)

  1. Check current Firefox or Thunderbird version via Help > About (Firefox) or Help > About Thunderbird
  2. For Firefox: Upgrade to version 148.0 or later (or ESR 115.33+ / ESR 140.8+ for enterprise deployments)
  3. For Thunderbird: Upgrade to version 148.0 or later (or version 140.8+)
  4. Restart the application after upgrade
  5. Verify the installed version matches or exceeds the fixed release

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
  • Testing1.0 h
  • Review / QA1.0 h
5.0 hours of engineering $890
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,424.

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-2026-2771 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