A handshake message is a protocol packet exchanged while two peers are establishing a secure session. In TLS and DTLS, these messages negotiate versions, algorithms, certificates, keys, and other state needed before protected traffic can flow. Each message has to be parsed, ordered, buffered, and sometimes fragmented or retransmitted.
Handshake messages matter because they are part of the most sensitive code path in a secure channel. If parsing is wrong, an attacker may crash the service, desynchronize state, or expose memory through malformed or replayed traffic. In DTLS, which runs over unreliable datagram networks, handshake messages can be resent after packet loss, so retry logic and buffer offsets become security-critical. Defenders focus on strict bounds checking, state validation, and careful retransmission handling to prevent leaks and denial of service.