What type of attack involves an intruder intercepting and altering communication between two parties in real-time?

The type of attack described is characterized by an intruder who intercepts and alters communication between two parties in real time, which is the defining characteristic of a man-in-the-middle attack. In this scenario, the attacker places themselves between the communicating parties—such as a user and a server—allowing them to observe, modify, or inject malicious content into the conversation without either party being aware of the intrusion.

This attack can occur over various communication channels, including but not limited to, web browsers, email transmissions, and network traffic. The attacker's capability to alter messages can lead to a variety of malicious outcomes, such as stealing sensitive information, redirecting users to fraudulent websites, or even impersonating one of the parties involved in the communication.

The other types of attacks mentioned, such as cross-site scripting, SQL injection, and denial-of-service, do not involve the real-time interception and alteration of communication between two parties. Instead, they target web applications in different ways—either by injecting malicious scripts, manipulating database queries, or overwhelming services to disrupt access. Thus, the nature of a man-in-the-middle attack distinctly sets it apart from the other options provided.