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A man-in-the-middle (MITM) attack is a form of eavesdropping where communication between two users is monitored and modified by an unauthorized party. Generally, the attacker actively eavesdrops by intercepting a public key message exchange and retransmits the message while replacing the requested key with his own.
In the process, the two original parties appear to communicate normally. The message sender does not recognize that the receiver is an unknown attacker trying to access or modify the message before retransmitting to the receiver. Thus, the attacker controls the entire communication.
This term is also known as a janus attack or a fire brigade attack.
MITM is named for a ball game where two people play catch while a third person in the middle attempts to intercept the ball. MITM is also known as a fire brigade attack, a term derived from the emergency process of passing water buckets to put out a fire.
The MITM intercepts communications between two systems and is performed when the attacker is in control of a router along normal point of traffic. The attacker in almost all cases is located on the same broadcast domain as the victim. For instance, in an HTTP transaction, a TCP connection exists between client and server. The attacker splits the TCP connection into two connections – one between the victim and the attacker and the other between attacker and the server. On intercepting the TCP connection, the attacker acts as a proxy reading, altering and inserting data in intercepted communication. The session cookie reading the HTTP header can easily be captured by the intruder.
In an HTTPS connection, two independent SSL connections are established over each TCP connection. An MITM attack takes advantage of the weakness in network communication protocol, convincing the victim to route traffic through the attacker instead of normal router and is generally referred to as ARP spoofing.