A buffer overflow attack occurs when more data is written to a buffer than it is designed to hold. This excess data can overwrite adjacent memory locations, leading to the execution of malicious code or crashing the system. Buffer overflows are a common vulnerability that attackers exploit to gain unauthorized access to systems.

During the negotiation phase of the TLS handshake, the client sends a "ClientHello" message to the server which includes information about TLS versions it supports, cipher-suites it supports and suggested compression methods. This initiates communication protocols for secure connection. Reference:= Cisco Cybersecurity source documents or study guide