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FirefoxWeb browser · Mozilla

CVE-2026-8971

MEDIUM · 6.5 CVSS v3.1 Published 2026-05-19
Fix available
A fix is available. Upgrade to 151.0.0 or later.
See remediation →
74/100
Remediation priority · Elevated
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: JAR component. This vulnerability was fixed in Firefox 151 and Thunderbird 151.

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

Same-origin policy (SOP) bypass in Firefox/Thunderbird's Networking:JAR component. The JAR (Java Archive) handler likely failed to properly enforce origin restrictions when processing certain JAR file types, allowing malicious pages to access resources from different origins. This could enable data theft or unauthorized actions across origin boundaries.

MitigationUpdate affected Firefox installations to version 151 or later, and Thunderbird to version 151 or later. For enterprise deployments, use software distribution tools to ensure all client systems have the patched browser versions.

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:< 151.0.0
ThunderbirdApplication
Affected:< 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
Unchanged
Confidentiality
Low
Integrity
Low
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/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 Mozilla product
    Determine if Firefox or Thunderbird is installed on the system. On Windows, check Program Files or Program Files (x86). On macOS, check /Applications folder. On Linux, use package manager (dpkg -l | grep -i firefox or thunderbird) or check typical installation directories.
    Affected if Either Firefox or Thunderbird is present on the system
  2. Check Firefox version
    In Firefox, click the menu button (three horizontal lines in the top-right), select Help, then select About Firefox. Note the version number displayed in the window that opens.
    Affected if Firefox version is less than 151.0.0 (for example, 150.x, 150.0, 149.x, etc.)
  3. Check Thunderbird version
    In Thunderbird, click the menu button, select Help, then select About Thunderbird. Note the version number displayed in the window that opens.
    Affected if Thunderbird version is less than 151.0.0 (for example, 150.x, 150.0, 149.x, etc.)

The environment is affected if either Firefox or Thunderbird is installed with any version number below 151.0.0, as the same-origin policy bypass exists in the Networking:JAR component of those vulnerable versions.

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 151.0.0 or later
Fixed in 151.0.0
Interim mitigation

Update affected Firefox installations to version 151 or later, and Thunderbird to version 151 or later. For enterprise deployments, use software distribution tools to ensure all client systems have the patched browser versions.

Recommended fix High confidence

Firefox 151.0.0 / Thunderbird 151.0.0

  1. 1. Close all running instances of Firefox or Thunderbird
  2. 2. Open your system's software update mechanism (e.g., system update utility, or visit https://www.mozilla.org/firefox/ or https://www.mozilla.org/thunderbird/)
  3. 3. Check for available updates - Firefox and Thunderbird will typically prompt you to update automatically on startup
  4. 4. Ensure the update to version 151.0.0 or later is applied
  5. 5. Restart the application after the update completes
  6. 6. Verify the installed version by navigating to Help > About Firefox (or About Thunderbird) and confirming the version number is 151.0.0 or higher
Caveat Routine security update with no major breaking changes expected; standard Mozilla release notes should be reviewed for any minor compatibility notes

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
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,560.

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-2026-8971 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