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FirefoxWeb browser · Mozilla

CVE-2026-4687

HIGH · 8.6 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 →
95/100
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 in the Telemetry 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

A sandbox escape vulnerability exists in the Telemetry component of Firefox and Thunderbird due to incorrect boundary conditions. This allows an attacker to bypass the sandbox security boundary, potentially executing arbitrary code outside the sandboxed environment.

MitigationUpdate affected installations to Firefox 149, Firefox ESR 115.34/140.9, or Thunderbird 140.9 or later 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:< 115.34.0< 149.0>= 128.0, < 140.9.0

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
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/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 installed Firefox version
    In Firefox, navigate to Help > About Firefox, or run 'firefox --version' in a terminal
    Affected if Version is below 115.34.0, below 149.0, or between 128.0 and 140.9.0 (exclusive)
  2. Check installed Thunderbird version
    In Thunderbird, navigate to Help > About Thunderbird, or run 'thunderbird --version' in a terminal
    Affected if Version is below the patched releases (149 or 140.9 depending on release channel)
  3. Confirm Telemetry component status
    In Firefox or Thunderbird, type 'about:config' in the address bar, then search for 'toolkit.telemetry.enabled'
    Affected if The telemetry.enabled preference is set to true (vulnerability requires Telemetry to be enabled)

User is affected if they are running an unpatched Firefox or Thunderbird version with the Telemetry feature 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

Update affected installations to Firefox 149, Firefox ESR 115.34/140.9, or Thunderbird 140.9 or later to remediate the vulnerability.

Recommended fix High confidence

Firefox 149 (or Firefox ESR 115.34 / Firefox ESR 140.9 depending on your release track)

  1. 1. Check current Firefox version by navigating to menu > Help > About Firefox
  2. 2. If running a version < 115.34.0, upgrade to Firefox ESR 115.34 or later
  3. 3. If running version 128.x through 140.8.x, upgrade to Firefox ESR 140.9 or later
  4. 4. If running version 141.x through 148.x, upgrade to Firefox 149 or later
  5. 5. After upgrade, verify the fix by checking Help > About Firefox shows the patched version
  6. 6. For organizations using Firefox ESR, ensure deployment scripts target ESR 115.34+ or ESR 140.9+ depending on your release track
Caveat ESR releases may have different support timelines; ensure compatibility with existing enterprise policies and add-on compatibility before upgrading

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
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,376.

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-2026-4687 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