How does a man-in-the-middle attack work?

A man-in-the-middle attack occurs when an attacker intercepts the communication between two parties without their knowledge. This type of attack allows the attacker to eavesdrop on the conversation, gain access to sensitive information, or even manipulate the messages being exchanged. The attacker effectively inserts themselves between the sender and the receiver, which can happen in various scenarios, such as on unsecured Wi-Fi networks or through the compromise of a communication protocol.

This technique is distinct from the other options presented. Directly accessing the network server involves a different approach, often targeting the server itself rather than manipulating communications between users. Sending unsolicited emails relates to phishing attacks, which aim to trick users into revealing personal information but don't involve directly intercepting communications. A denial-of-service attack focuses on overwhelming a resource, making it unavailable to users, rather than intercepting any data or messages exchanged between them. Therefore, the correct choice highlights the essence of how a man-in-the-middle attack operates.