In what type of attack does the attacker take over an established session between two parties and then interacts with the remaining party as if the attacker were the party that has been disconnected?

Active Session Hijacking

Explanation:

Based on the information provided within the question it can be said that the type of attack being described is called an Active Session Hijacking. Like mentioned in the question this refers to an attack on an active TCP/IP communication session, where the attacker gains access to the communication and assumes the identity of the user that has been hijacked and disconnected.

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