CVE-2026-55663mediasoup is a WebRTC video conferencing system. From version 3.20.0 until 3.20.6 for the npm package and from 0.22.0 until 0.22.5 for the Rust crate, mediasoup's built-in SCTP stack authenticates state cookies using only the hardcoded msworker and 0xAD81 magic values instead of a per-instance secret and HMAC, contrary to RFC 9260 Section 5.1.3. The cookie structure and validation in worker/include/RTC/SCTP/association/StateCookie.hpp and worker/src/RTC/SCTP/association/StateCookie.cpp allow an on-path attacker targeting PlainTransport or PipeTransport with SCTP enabled and without DTLS protection to forge a COOKIE-ECHO whose packet verification tag matches the attacker-controlled localVerificationTag. The forged cookie passes StateCookie::IsMediasoupStateCookie() and Association::HandleReceivedCookieEchoChunk(), establishes an unauthorized SCTP association, and permits DataChannel message injection as a trusted peer. WebRtcTransport is not affected because its SCTP runs inside DTLS. This issue is fixed in npm version 3.20.6 and Rust crate version 0.22.5.