HMAC-derived authentication material is cryptographic data created during an authentication handshake from a shared secret or password and a protocol-specific challenge. It is not the password itself, but it can be related enough to let an attacker test password guesses offline if the material is captured.
In cyber security, this matters because a system may leak useful login data before an account is actually accessed. Attackers can record the handshake, then compare candidate passwords against the captured HMAC-based value without repeatedly contacting the service. Defenders reduce this risk by limiting exposure of management interfaces, enforcing strong unique passwords, using modern authentication protocols, and isolating administrative access so handshake data cannot be harvested from public networks.