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FirefoxWeb browser · Mozilla

CVE-2026-4717

CRITICAL · 9.8 CVSS v3.1 Published 2026-03-24
Fix available
A fix is available. Upgrade to 140.9.0 / 149.0 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
Privilege escalation in the Netmonitor component. This vulnerability was fixed in Firefox 149, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

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 a privilege escalation vulnerability in the Netmonitor component of Firefox and Thunderbird. Netmonitor is a developer tool for inspecting network requests. The vulnerability allows an attacker to escalate privileges, likely by exploiting the developer tool's privileged access to network data. The critical CVSS score of 9.8 indicates trivial exploitability with complete impact.

MitigationApply the security updates: upgrade to Firefox 149, Firefox ESR 140.9, Thunderbird 149, or Thunderbird 140.9. Organizations should deploy these updates via their standard patch management processes.

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:< 140.9.0< 149.0

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
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/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
    Check if Firefox or Thunderbird is installed on the system. On Windows, look in Program Files or Program Files (x86). On macOS, check /Applications. On Linux, check package manager listings or /usr/lib/firefox or /usr/lib/thunderbird.
    Affected if Either Firefox or Thunderbird is installed
  2. Check Firefox version
    Open Firefox, click the menu (three lines or three dots), go to Help > About Firefox. The version number will be displayed. Alternatively, navigate to about:support in the address bar.
    Affected if The displayed version is less than 140.9.0 or between 141.0 and 148.x (any version less than 149.0)
  3. Check Thunderbird version
    Open Thunderbird, click the menu (three lines), go to Help > About Thunderbird. The version number will be displayed.
    Affected if The displayed version is less than 140.9.0 or between 141.0 and 148.x (any version less than 149.0)
  4. Verify Netmonitor component presence
    In Firefox or Thunderbird, open Developer Tools (press F12 or Ctrl+Shift+I / Cmd+Opt+I), then click the Network tab. This confirms the Netmonitor component is present in the installation.
    Affected if The Netmonitor/Network tab is accessible in DevTools on a vulnerable version

You are affected if Firefox or Thunderbird is installed with any version below 140.9.0 or any version from 141.0 through 148.x, as these contain the vulnerable Netmonitor component.

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 140.9.0 / 149.0 or later
Fixed in 140.9.0149.0
Interim mitigation

Apply the security updates: upgrade to Firefox 149, Firefox ESR 140.9, Thunderbird 149, or Thunderbird 140.9. Organizations should deploy these updates via their standard patch management processes.

Recommended fix High confidence

Firefox 149 (standard) or Firefox ESR 140.9 (extended support release)

  1. 1. Open Firefox and navigate to Help > About Firefox to check the current version
  2. 2. If an update is available, click 'Update to next version' or 'Restart to update'
  3. 3. Alternatively, download Firefox 149 (standard) or Firefox ESR 140.9 (extended support) from the official Mozilla website at www.mozilla.org
  4. 4. Run the installer and complete the installation process
  5. 5. Restart Firefox after the update completes
  6. 6. Verify the update by checking Help > About Firefox shows version 149.x or ESR 140.9.x
Caveat Firefox upgrades typically have minimal breaking changes; however, some older extensions or plugins may be incompatible with version 149

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

Fix this in Firefox Scoped from the published advisory
  • Consultation1.0 h
  • Implementation2.0 h
  • Testing1.0 h
  • Review / QA1.0 h
5.0 hours of engineering $890
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,424.

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-2026-4717 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