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FirefoxWeb browser · Mozilla

CVE-2026-8958

HIGH · 8.6 CVSS v3.1 Published 2026-05-19
Fix available
A fix is available. Upgrade to 140.11 / 140.11.0 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
Information disclosure, sandbox escape in the Security: Process Sandboxing component. This vulnerability was fixed in Firefox 151, Firefox ESR 140.11, Thunderbird 151, and Thunderbird 140.11.

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

This is an information disclosure and sandbox escape vulnerability in Mozilla Firefox and Thunderbird's Security: Process Sandboxing component. A successful exploit could allow malicious code to break out of the browser's security sandbox and potentially access sensitive information or execute arbitrary code on the host system.

MitigationUpdate Firefox to version 151 or later (ESR 140.11 or later), and Thunderbird to version 151 or later (140.11 or later). Alternatively, apply the latest security updates from Mozilla.

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:< 140.11.0< 151.0.0
ThunderbirdApplication
Affected:< 140.11< 151.0.0

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
Changed
Confidentiality
High
Integrity
None
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/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. Check Firefox version on Windows
    Open Firefox, click the menu (three lines) > Help > About Firefox. The version number is displayed in the window that opens.
    Affected if The version shown is below 140.11.0 or below 151.0.0 (for example, 140.10.x, 150.x.x, or any 139.x release)
  2. Check Firefox version on macOS
    Open Firefox, click the Firefox menu > About Firefox. The version number is displayed in the window.
    Affected if The version shown is below 140.11.0 or below 151.0.0
  3. Check Firefox version on Linux
    Open a terminal and run: firefox --version
    Affected if The version output is below 140.11.0 or below 151.0.0
  4. Check Thunderbird version on Windows
    Open Thunderbird, click the menu (three lines) > Help > About Thunderbird. The version number is displayed.
    Affected if The version shown is below 140.11 or below 151.0.0
  5. Check Thunderbird version on Linux/macOS
    Open a terminal and run: thunderbird --version
    Affected if The version output is below 140.11 or below 151.0.0
  6. Verify if sandboxing is enabled (optional context)
    In Firefox or Thunderbird, type about:config in the address bar, search for 'sandbox', and check if security.sandbox.enabled is set to true (default state).
    Affected if This check confirms the vulnerable component is active - the sandbox is enabled by default in affected versions.

A user is affected if either Firefox version is below 140.11.0 or below 151.0.0, or if Thunderbird version is below 140.11 or below 151.0.0, with sandboxing 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 140.11 / 140.11.0 / 151.0.0 or later
Fixed in 140.11140.11.0151.0.0
Interim mitigation

Update Firefox to version 151 or later (ESR 140.11 or later), and Thunderbird to version 151 or later (140.11 or later). Alternatively, apply the latest security updates from Mozilla.

Recommended fix High confidence

Firefox 151 or Firefox ESR 140.11; Thunderbird 151 or Thunderbird 140.11

  1. Check current Firefox version by navigating to about:settings or going to Menu > Help > About Firefox
  2. Check current Thunderbird version by navigating to Menu > Help > About Thunderbird
  3. For Firefox: Download Firefox 151 (or Firefox ESR 140.11 for enterprise/long-term support) from the official Mozilla website
  4. For Thunderbird: Download Thunderbird 151 (or Thunderbird 140.11 for enterprise/long-term support) from the official Mozilla website
  5. Install the downloaded version, overwriting the existing installation
  6. Restart the browser/application after installation completes
  7. Verify the version by checking about:settings (Firefox) or Menu > Help > About Thunderbird to confirm the fix is applied
Caveat Standard minor version upgrade - should retain settings and extensions; ESR versions recommended for organizations needing stability

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
  • Testing1.0 h
  • Review / QA1.0 h
4.0 hours of engineering $750
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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-2026-8958 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