CVE-2026-4700: Mozilla Firefox and Thunderbird mitigation bypass in Networking HTTP component

CVE-2026-4700 · Severity: critical · CVSS 9.8 · Published 2026-03-24

A security vulnerability has been identified in the HTTP networking component of Mozilla Firefox and Thunderbird. This flaw could allow an attacker to bypass built-in security mitigations, potentially leading to unauthorized access or data compromise. Users should update their browsers and email clients to the latest versions to protect their systems and data.

Technologies: Mozilla Firefox, Mozilla Firefox ESR, Mozilla Thunderbird, Mozilla Thunderbird ESR, Red Hat Enterprise Linux AppStream EUS. Vendors: Mozilla, Red Hat.

Quick answers

Is CVE-2026-4700 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-4700 being exploited in the wild.
How severe is CVE-2026-4700?
Critical severity, with a CVSS score of 9.8, published 24 March 2026.
Which products and versions are affected?
Mozilla Firefox < 149; Mozilla Firefox ESR < 140.9; Mozilla Thunderbird < 149; Mozilla Thunderbird ESR < 140.9; and 1 more.
Is there a fix for CVE-2026-4700?
A patch was published on 2026-03-24: Fixed versions released for Firefox and Thunderbird. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-4700 attacked?
It is attacked over the network, in a network appliance.
Am I affected by CVE-2026-4700?
Check whether you run Mozilla Firefox, Mozilla Firefox ESR, Mozilla Thunderbird or one of the others listed at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

A mitigation bypass vulnerability was identified in the Networking: HTTP component of Mozilla Firefox and Thunderbird. The vulnerability is characterized by inconsistent interpretation of HTTP requests (HTTP Request/Response Smuggling) or authentication bypass using an alternate path. An unauthenticated attacker can exploit this over the network to bypass security controls. While Mozilla rated the impact as 'moderate', NVD and CISA-ADP have assigned a CVSS score of 9.8 (Critical) due to the potential for high impact on confidentiality, integrity, and availability. The issue is resolved in Firefox 149, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

Affected products

  • Mozilla Firefox < 149
  • Mozilla Firefox ESR < 140.9
  • Mozilla Thunderbird < 149
  • Mozilla Thunderbird ESR < 140.9
  • Red Hat Enterprise Linux Server 7 ELS
  • Red Hat Enterprise Linux AppStream EUS 10.0

Timeline

  • 2026-03-24: advisory: Mozilla published security advisories MFSA2026-20 and MFSA2026-22.
  • 2026-03-24: patched: Fixed versions released for Firefox and Thunderbird.

References

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