Cesanta Mongoose before 7.22 contains an out-of-bounds read in the built-in TLS server function mg tls server recv hello(), which uses an attacker-controlled session id len byte from a TLS ClientHello as a buffer index without validating it against the length of received data. A remote, unauthenticated attacker can send a single crafted ClientHello with an oversized session id length to read past the receive buffer, crashing any HTTPS, MQTTS, or WSS service built on MG TLS BUILTIN.