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FirefoxWeb browser · Mozilla

CVE-2026-16383

CRITICAL · 9.8 CVSS v3.1 Published 2026-07-21
Fix available
A fix is available. Upgrade to 140.13.0 / 153.0 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click 4 weeks old

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
Mitigation bypass in the DOM: Networking component. This vulnerability was fixed in Firefox 153, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13.

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 mitigation bypass vulnerability in the DOM: Networking component of Firefox and Thunderbird. This security flaw allowed attackers to circumvent existing protections in how the browser handles network requests within the Document Object Model. The vulnerability was addressed in Firefox 153, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13.

MitigationUpdate Firefox to version 153 or later (ESR 140.13+), and Thunderbird to version 153 or later (140.13+) to remediate this vulnerability. Organizations should verify all affected installations are patched.

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:< 140.13.0< 153.0.0
ThunderbirdApplication
Affected:< 140.13.0>= 141.0, < 153.0

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. Check Firefox version
    In Firefox, go to Menu > Help > About Firefox. The version number is displayed on this page. Alternatively, in Windows check Programs and Features or run: firefox --version in command prompt
    Affected if The installed version is any version before 153.0.0 (or before 140.13.0 if on an ESR release line)
  2. Check Thunderbird version
    In Thunderbird, go to Menu > Help > About Thunderbird. The version number is displayed on this page. Alternatively, in Windows check Programs and Features or run: thunderbird --version in command prompt
    Affected if The installed version is before 140.13.0, OR any version from 141.0.0 up to (but not including) 153.0.0
  3. Identify if DOM:Networking is in use
    This vulnerability is in the DOM:Networking component which handles network requests and Same-Origin Policy enforcement. Any browser profile that loads web content or renders HTML emails in Thunderbird will exercise this component. There is no config flag to disable it.
    Affected if The browser or email client is actively used to load web pages or render HTML content (the default behavior)
  4. Confirm platform and update channel
    Determine if running Firefox release, Firefox ESR, or Thunderbird. Check Help > About to see if ESR is indicated. Different version branches have different version numbers.
    Affected if Running any version that does not match the fixed releases: Firefox 153+, Firefox ESR 140.13+, Thunderbird 153+, or Thunderbird 140.13+

If the installed Firefox or Thunderbird version is below the fixed releases (153.0.0 or 140.13.0 depending on product and channel), the environment is affected.

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 140.13.0 / 153.0 / 153.0.0 or later
Fixed in 140.13.0153.0153.0.0
Interim mitigation

Update Firefox to version 153 or later (ESR 140.13+), and Thunderbird to version 153 or later (140.13+) to remediate this vulnerability. Organizations should verify all affected installations are patched.

Recommended fix High confidence

Firefox 153, Firefox ESR 140.13, Thunderbird 153, or Thunderbird 140.13

  1. Upgrade to Firefox 153 or later
  2. Upgrade to Firefox ESR 140.13 or later
  3. Upgrade to Thunderbird 153 or later
  4. Upgrade to Thunderbird 140.13 or later

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-2026-16383 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