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FirefoxWeb browser · Mozilla

CVE-2026-12296

CRITICAL · 9.6 CVSS v3.1 Published 2026-06-16
Fix available
A fix is available. Upgrade to 140.12.0 / 152.0.0 or later.
See remediation →
100/100
Remediation priority · Urgent
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
Sandbox escape in the Security: Process Sandboxing component. This vulnerability was fixed in Firefox 152, Firefox ESR 140.12, Thunderbird 152, and Thunderbird 140.12.

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 in the Security: Process Sandboxing component allows an attacker to break out of the browser's security sandbox and potentially execute code with elevated privileges. This affects Firefox, Firefox ESR, and Thunderbird.

MitigationUpdate affected software to Firefox 152+, Firefox ESR 140.12+, Thunderbird 152+, or Thunderbird 140.12+ 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:< 140.12.0< 152.0.0
ThunderbirdApplication
Affected:< 140.12.0< 152.0.0

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
Changed
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/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
    Open terminal or command prompt and run: for Windows, check Start menu or Program Files; for Linux, run 'dpkg -l' or 'rpm -qa | grep -i mozilla'; for macOS, check /Applications folder. Look for Firefox or Thunderbird.
    Affected if Either Firefox or Thunderbird is installed
  2. Determine Firefox version
    In Firefox, type 'about:support' in the address bar and look for 'Version' in the first line, or run 'firefox --version' from command line.
    Affected if Version is less than 140.12.0 OR version is 140.x.x through 151.x.x (less than 152.0.0)
  3. Determine Thunderbird version
    In Thunderbird, go to Help > About Thunderbird and note the version number, or run 'thunderbird --version' from command line.
    Affected if Version is less than 140.12.0 OR version is 140.x.x through 151.x.x (less than 152.0.0)
  4. Verify Process Sandboxing is enabled
    In Firefox, go to about:preferences#privacy and scroll to 'Enhanced Tracking Protection' section, or check about:support for 'Sandbox' information. In Thunderbird, check Preferences > Security > Scripts. The vulnerability only applies when Process Sandboxing is actively enabled.
    Affected if Process Sandboxing feature is turned ON in the application settings

The environment is affected if either Firefox or Thunderbird is installed with a version below 140.12.0 or between 140.x.x and 151.x.x, AND the Process Sandboxing feature is enabled in the application settings.

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 140.12.0 / 152.0.0 or later
Fixed in 140.12.0152.0.0
Interim mitigation

Update affected software to Firefox 152+, Firefox ESR 140.12+, Thunderbird 152+, or Thunderbird 140.12+ to remediate the vulnerability.

Recommended fix High confidence

Firefox 152 (or Firefox ESR 140.12), Thunderbird 152 (or Thunderbird 140.12)

  1. 1. Back up any important data and bookmarks in the current Firefox or Thunderbird installation
  2. 2. Open the application and navigate to Menu > Help > About Firefox (or About Thunderbird)
  3. 3. Click 'Check for Updates' to download and install the latest version
  4. 4. Alternatively, download the appropriate version from the official Mozilla website: https://www.mozilla.org/firefox/releases/ or https://www.mozilla.org/thunderbird/releases/
  5. 5. For enterprise deployments, obtain Firefox ESR 140.12 from https://www.mozilla.org/firefox/enterprise/
  6. 6. Verify the version after installation by checking Menu > Help > About to confirm the version number matches the fixed release
  7. 7. Restart the application after the update completes
Caveat Minor: Security updates typically have minimal compatibility impact; however, some legacy extensions or add-ons may require verification for compatibility with version 152 or 140.12

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
  • Testing2.0 h
  • Review / QA1.0 h
9.0 hours of engineering $1,600
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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-2026-12296 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