CVE-2025-8671: 
lighttpd vulnerability analysis and mitigation

A mismatch caused by client-triggered server-sent stream resets between HTTP/2 specifications and the internal architectures of some HTTP/2 implementations may result in excessive server resource consumption leading to denial-of-service (DoS). This vulnerability, tracked as CVE-2025-8671 and known as 'MadeYouReset', was discovered by researchers from Tel Aviv University and publicly disclosed on August 13, 2025. The vulnerability affects multiple HTTP/2 server implementations including Apache Tomcat, F5 BIG-IP, and Netty (CERT VU, Deepness Lab).

MadeYouReset exploits a discrepancy between HTTP/2 stream accounting and server processing. By opening streams and then rapidly triggering the server to reset them using malformed frames or flow control errors, an attacker can cause the server to handle an unbounded number of concurrent streams on a single connection. The attack uses six primitives to trigger server-side resets: WINDOW_UPDATE frame with zero increment, invalid PRIORITY frame lengths, self-dependent PRIORITY frames, excessive window size increments, and sending HEADERS or DATA frames after stream closure. This bypasses typical server-imposed limits of 100 concurrent HTTP/2 requests per TCP connection (Imperva Blog, Gal Blog).

The primary impact is denial-of-service through resource exhaustion. When targeted, servers either exhaust their memory or processing capabilities, potentially leading to complete service outage or severely limited connection handling capacity. The attack can cause the server to process an extremely high number of concurrent requests, resulting in high CPU overload or memory exhaustion depending on the HTTP/2 implementation (CERT VU, Imperva Blog).

Various vendors have released patches to address the vulnerability. For unpatched systems, temporary mitigations include disabling HTTP/2 support or implementing rate limiting on protocol-level errors. Recommended security measures include stricter protocol validation, rigorous stream state enforcement, and deployment of anomaly detection tools to flag unusual patterns of protocol violations. Organizations are urged to keep their HTTP/2 server implementations up to date with the latest security patches (Varnish Security, Fastly Status).

The security community has responded quickly to the disclosure, with major infrastructure providers implementing various mitigation techniques. Fastly implemented a fix in release 25.17 of their internal H2O fork and deployed it across their network by June 2, 2025. Varnish Cache released patches in versions 7.6.5, 7.7.3, and 6.0.16. The IETF HTTP Working Group clarified that this is not a vulnerability in the HTTP protocol itself, but rather an implementation issue (Fastly Status, Varnish Security).