What kind of attack occurs when a perpetrator positions himself in a conversation between a user and an application?

The scenario described involves a perpetrator inserting themselves into a communication stream between the user and an application, which is characteristic of a Man in the Middle (MitM) attack. In this type of attack, the attacker secretly intercepts and relays messages between two parties without their knowledge, allowing the attacker to eavesdrop, manipulate, or alter the communication. This method can lead to sensitive information being stolen or unauthorized actions taken on behalf of the user.

In contrast, a Denial of Service attack aims to disrupt service availability by overwhelming a system, so it does not fit the description of intercepting communication. Sniffing attacks involve capturing data packets transmitted over a network but do not necessarily imply active participation or manipulation of the conversation. Phishing attacks focus on deceiving the user into revealing personal information, typically through fraudulent communications, rather than inherently positioning between the user and an application for ongoing interaction. Therefore, the Man in the Middle attack is the precise identification of the situation described in the question.