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FirefoxWeb browser · Mozilla

CVE-2026-4689

CRITICAL · 10.0 CVSS v3.1 Published 2026-03-24
Fix available
A fix is available. Upgrade to 115.34.0 / 140.9.0 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
Sandbox escape due to incorrect boundary conditions, integer overflow in the XPCOM component. This vulnerability was fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

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 · high confidence

Integer overflow in the XPCOM component due to incorrect boundary conditions allows a remote attacker to escape the Firefox/Thunderbird sandbox and execute arbitrary code. This memory corruption vulnerability occurs when the XPCOM component improperly handles integer values during memory allocation, leading to out-of-bounds access.

MitigationUpgrade immediately to Firefox 149+, Firefox ESR 115.34+/140.9+, or Thunderbird 149+/140.9+ to patch the vulnerability. Until patched, minimize exposure by disabling JavaScript in untrusted contexts and avoiding untrusted websites.

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:< 115.34.0< 149.0>= 128.0, < 140.9.0
ThunderbirdApplication
Affected:< 140.9.0< 149.0

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
Changed
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/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 Mozilla product
    Check if Firefox or Thunderbird is installed. On Windows, look in Program Files. On macOS, check /Applications. On Linux, run: dpkg -l | grep -i firefox or dpkg -l | grep -i thunderbird
    Affected if Either Firefox or Thunderbird is present
  2. Get Firefox version number
    For Firefox: Open menu > Help > About Firefox, or run 'firefox --version' in terminal, or check the version file (Windows: C:\Program Files\Mozilla Firefox\firefox.exe or on macOS: /Applications/Firefox.app/Contents/MacOS/firefox --version)
    Affected if Version is < 115.34.0 OR (>= 128.0 AND < 140.9.0) OR < 149.0 (any version before 149.0 is likely affected given the ranges)
  3. Get Thunderbird version number
    For Thunderbird: Open menu > Help > About Thunderbird, or run 'thunderbird --version' in terminal, or check on macOS: /Applications/Thunderbird.app/Contents/MacOS/thunderbird --version
    Affected if Version is < 140.9.0 OR < 149.0 (any version before 149.0 is likely affected)
  4. Confirm XPCOM sandbox context
    This vulnerability affects the XPCOM component sandbox. The sandbox is typically enabled by default in Firefox and Thunderbird for web content. Check about:config for 'security.sandbox.content.enabled' (Firefox) or 'security.sandbox.enabled' (Thunderbird) - a value of true means sandbox is active
    Affected if Sandbox is enabled (default setting) and the product version falls within affected ranges

User is affected if Firefox version is below 115.34.0, between 128.0-140.9.0 inclusive, or below 149.0; or if Thunderbird version is below 140.9.0 or below 149.0, and the sandbox is in use (default enabled).

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 115.34.0 / 140.9.0 / 149.0 or later
Fixed in 115.34.0140.9.0149.0
Interim mitigation

Upgrade immediately to Firefox 149+, Firefox ESR 115.34+/140.9+, or Thunderbird 149+/140.9+ to patch the vulnerability. Until patched, minimize exposure by disabling JavaScript in untrusted contexts and avoiding untrusted websites.

Recommended fix High confidence

Firefox 149 (standard) or Firefox ESR 115.34/140.9; Thunderbird 149 or 140.9

  1. Check current Firefox or Thunderbird version via Help > About (or running 'firefox --version' / 'thunderbird --version' in terminal)
  2. For Firefox: Upgrade to Firefox 149 (standard release) or Firefox ESR 115.34 / 140.9 (if using ESR branch)
  3. For Thunderbird: Upgrade to Thunderbird 149 or Thunderbird 140.9
  4. Download the appropriate fixed version from the official Mozilla website (www.mozilla.org)
  5. Close all browser/mail instances and install the update
  6. Restart the application and verify the version has been updated
Caveat Standard minor version upgrade with security fixes; minimal risk of breaking changes in point releases

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-2026-4689 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