CVE-2025-8671: 
lighttpd vulnerability analysis and mitigation

The MadeYouReset vulnerability (CVE-2025-8671) is a denial of service (DoS) vulnerability discovered in HTTP/2 implementations, publicly disclosed on August 13, 2025. The vulnerability exploits a mismatch between HTTP/2 specifications and internal architectures of some HTTP/2 implementations, allowing attackers to cause excessive server resource consumption. This vulnerability is similar to the HTTP/2 Rapid Reset vulnerability (CVE-2023-44487) disclosed in 2023, but with a different attack vector (CERT VU, Gal Blog).

The vulnerability stems from how HTTP/2 implementations handle stream resets. When streams are reset by the server due to malformed frames or flow control errors, they are considered closed at the protocol level, but backend processing continues. An attacker can exploit this discrepancy by opening streams and rapidly triggering the server to reset them, causing the server to handle an unbounded number of concurrent streams on a single connection. The HTTP/2 protocol's SETTINGSMAXCONCURRENT_STREAMS parameter, which should limit concurrent streams, becomes ineffective as reset streams are no longer counted against this limit (CERT VU, H2O Advisory).

The primary impact is the potential for denial of service attacks. Attackers can force targets offline or severely limit connection capabilities by making servers process an extremely high number of concurrent requests. Affected systems may experience either high CPU overload or memory exhaustion, depending on their HTTP/2 implementation (CERT VU, SUSE KB).

Various vendors have released patches to address the vulnerability. For systems that cannot be immediately patched, the primary mitigation is to disable HTTP/2 support. Some vendors recommend removing h2 from the ALPN protocol list if using a TLS terminator. Additionally, implementing rate limits on RST_STREAMs sent from the server and monitoring stream reset patterns can help mitigate the attack (Varnish Advisory, Fastly Status).

Multiple major vendors and organizations have responded to the vulnerability. Mozilla confirmed their websites and services were affected but noted their client software like Firefox was not impacted. Fastly implemented fixes in their infrastructure by June 2025, ahead of public disclosure. The IETF HTTP Working Group clarified that this is not a protocol vulnerability but an implementation issue, emphasizing that HTTP/2 implementations need to properly handle denial of service scenarios (CERT VU).