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FirefoxWeb browser · Mozilla

CVE-2026-2776

CRITICAL · 10.0 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
Sandbox escape due to incorrect boundary conditions in the Telemetry component in External Software. 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 · high confidence

A critical sandbox escape vulnerability exists in the Telemetry component of Firefox and Thunderbird due to incorrect boundary conditions. This flaw allows an attacker to escape the browser's security sandbox and potentially execute arbitrary code outside the protected environment.

MitigationImmediately update all Firefox and Thunderbird installations to the fixed versions (Firefox 148, Firefox ESR 115.33/140.8, Thunderbird 148, or Thunderbird 140.8) to remediate this critical sandbox escape 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:< 115.33.0< 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
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 the installed Mozilla product
    Check if Firefox or Thunderbird is installed by looking for the application in the system. On Windows, check Program Files for 'Mozilla Firefox' or 'Mozilla Thunderbird'. On Linux, check for 'firefox' or 'thunderbird' binaries using 'which firefox' or 'which thunderbird'. On macOS, check /Applications for Firefox or Thunderbird.
    Affected if The product is Mozilla Firefox or Mozilla Thunderbird
  2. Determine the installed Firefox version
    For Firefox: Type 'about:Firefox' in the address bar or go to Help > About Firefox. Alternatively, check the executable file version: on Windows right-click firefox.exe > Properties > Details, on Linux run 'firefox --version'.
    Affected if Firefox version is less than 115.33.0, or between 128.0 and 140.8.0 (excluding 140.8.0), or less than 148.0
  3. Determine the installed Thunderbird version
    For Thunderbird: Type 'about:Thunderbird' in the address bar or go to Help > About Thunderbird. Alternatively, check the executable file version: on Windows right-click thunderbird.exe > Properties > Details, on Linux run 'thunderbird --version'.
    Affected if Thunderbird version is less than 140.8.0 or less than 148.0
  4. Verify Telemetry component status
    In Firefox, go to Settings > Privacy & Security > Scroll to Firefox Data Collection and Use. In Thunderbird, go to Config Editor (about:config) and search for 'toolkit.telemetry.enabled'. Check if Telemetry is enabled.
    Affected if Telemetry is enabled (this is the default setting in both Firefox and Thunderbird)

The environment is affected if Mozilla Firefox version is less than 115.33.0, or between 128.0 and 140.8.0, or less than 148.0; OR Mozilla Thunderbird version is less than 140.8.0 or less than 148.0, AND Telemetry is enabled (default).

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

Immediately update all Firefox and Thunderbird installations to the fixed versions (Firefox 148, Firefox ESR 115.33/140.8, Thunderbird 148, or Thunderbird 140.8) to remediate this critical sandbox escape vulnerability.

Recommended fix High confidence

Firefox 148, Firefox ESR 115.33, Firefox ESR 140.8, Thunderbird 148, or Thunderbird 140.8 (depending on edition and channel in use)

  1. Check current Firefox or Thunderbird version via Help > About (or about:support in Firefox, about:support in Thunderbird)
  2. Close all Firefox/Thunderbird windows and instances
  3. Download the appropriate fixed version from the official Mozilla website (www.mozilla.org) based on your edition: Firefox 148 (standard), Firefox ESR 115.33 (ESR), Firefox ESR 140.8 (ESR), Thunderbird 148, or Thunderbird 140.8
  4. Run the installer and follow the prompts to complete the upgrade
  5. After installation, restart the application and verify the version via Help > About to confirm the fix is applied
  6. Ensure any automated update mechanisms in your environment are configured to pull the fixed versions
Caveat Minimal; this is a critical security fix with no expected functional breaking changes

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

Fix this in Firefox Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA1.0 h
10.0 hours of engineering $1,750
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,800.

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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-2026-2776 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