What hacking attack is challenge/response authentication used to prevent?

In a challenge/response scheme, the server sends a random nonce (the challenge) to the client, who computes a response by combining the nonce with their credentials via a hash or cryptographic function. Because each challenge is unique and single-use, an attacker who captures a valid authentication exchange cannot resubmit the same response to gain access - the server will issue a different challenge, making the captured response worthless.

Scanning attacks involve network reconnaissance such as port scanning; they are not an authentication-layer threat that challenge/response is designed to mitigate.

Session hijacking targets an already-established authenticated session by stealing session tokens, which occurs after authentication completes - challenge/response protects the authentication phase itself, not the subsequent session.

Password cracking involves offline brute-force or dictionary attacks against captured password hashes; while challenge/response avoids transmitting the plaintext password, its primary design goal is preventing replay, not stopping offline cracking attempts.