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FirefoxWeb browser · Mozilla

CVE-2026-12304

CRITICAL · 9.1 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 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
Same-origin policy bypass in the Networking: Cookies 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 · moderate confidence

A same-origin policy bypass in the Firefox/Thunderbird Networking: Cookies component allows web content from one origin to improperly access cookies or resources belonging to a different origin. This critical flaw enables cross-origin attacks such as session hijacking, data theft, or authentication bypass.

MitigationEnsure all Firefox installations are updated to version 152 or later (or ESR 140.12+), and Thunderbird to version 152 or 140.12. Organizations should deploy browser updates through their patch management systems and verify completion across all endpoints.

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
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

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 Firefox version
    Open Firefox, go to Menu > Help > About Firefox (or navigate to about:support). Note the version number displayed.
    Affected if The version is below 140.12.0, or between 140.12.0 and 151.x (inclusive)
  2. Identify installed Thunderbird version
    Open Thunderbird, go to Menu > Help > About Thunderbird. Note the version number displayed.
    Affected if The version is below 140.12.0, or between 140.12.0 and 151.x (inclusive)
  3. Check if the Cookies component is active
    In Firefox, navigate to about:preferences#privacy or inspect cookie storage via Developer Tools (Application tab > Cookies). This is a browser component that handles cookie processing for all web requests.
    Affected if The browser is actively used for web browsing, which enables cookie handling across origins. The vulnerability affects any Firefox or Thunderbird installation used for HTTP/HTTPS connections.
  4. Confirm same-origin policy is in use
    This is a default browser security mechanism. To verify, check that the browser has not been configured with overly permissive CORS or same-origin settings (about:config flags like security.fileuri.strict_origin_policy, or network.cookie.cookieBehavior).
    Affected if Default browser configurations with standard same-origin policy enforcement are affected by this bypass vulnerability.

A user is affected if they run Firefox or Thunderbird versions below 140.12.0, or versions 140.12.0 through 151.x, and use the browser for web activity that involves cookies across different origins.

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

Ensure all Firefox installations are updated to version 152 or later (or ESR 140.12+), and Thunderbird to version 152 or 140.12. Organizations should deploy browser updates through their patch management systems and verify completion across all endpoints.

Recommended fix High confidence

Firefox 152+ / Firefox ESR 140.12+ / Thunderbird 152+ / Thunderbird 140.12+

  1. Check the current version of Firefox or Thunderbird by navigating to Menu > Help > About (or using the 'firefox --version' or 'thunderbird --version' command in terminal)
  2. Open Firefox or Thunderbird and trigger an update check: Menu > Help > About > 'Check for Updates' (the application will automatically download and prompt to restart when 140.12 or 152+ is available)
  3. Alternatively, download the fixed version directly from the official Mozilla website: https://www.mozilla.org/en-US/firefox/releases/ for Firefox or https://www.mozilla.org/en-US/thunderbird/releases/ for Thunderbird
  4. For enterprise deployments using Firefox ESR, ensure the update is pushed through your management system to version 140.12 (ESR) or later
  5. After installation, restart the application completely and verify the version shows 140.12.0, 152.0, or higher
Caveat Mozilla regular releases typically have minimal user-facing breaking changes; ESR versions are recommended for organizations requiring extended stability

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

Fix this in Firefox Scoped from the published advisory
  • Consultation2.0 h
  • Implementation1.0 h
  • Testing2.0 h
  • Review / QA1.0 h
6.0 hours of engineering $1,060
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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-2026-12304 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