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critical advisory

Firefox and Thunderbird Mitigation Bypass Vulnerability (CVE-2026-4700)

CVE-2026-4700 is a critical vulnerability in the Networking: HTTP component of Firefox, Firefox ESR, and Thunderbird, allowing a mitigation bypass in versions prior to Firefox 149, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

CVE-2026-4700 is a mitigation bypass vulnerability affecting Mozilla Firefox, Firefox ESR, and Thunderbird. The vulnerability resides within the Networking: HTTP component and impacts versions earlier than Firefox 149, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9. Successful exploitation could allow an attacker to bypass intended security mitigations, potentially leading to further compromise of the affected system. This vulnerability was disclosed on March 24, 2026, and poses a…

Detection coverage 2

Detect Suspicious HTTP Request Headers

medium

Detects potentially malicious HTTP requests based on unusual or suspicious header combinations. This could indicate attempts to exploit CVE-2026-4700 or other HTTP-related vulnerabilities.

sigma tactics: initial_access techniques: T1190 sources: webserver, linux

Detect High Number of HTTP Requests from Single IP

low

Detects a high number of HTTP requests originating from a single IP address within a short timeframe. This could be indicative of an attacker attempting to exploit a vulnerability or perform a brute-force attack.

sigma tactics: resource_development techniques: T1588 sources: webserver, linux

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