Session Hijacking

Session hijacking is when a hacker takes control of a user session after the user has successfully authenticated with a server. Session hijacking involves an attack identifying the current session IDs of a client/server communication and taking over the client's session. Session hijacking is made possible by tools that perform sequence-number prediction. The details of sequence-number prediction will be discussed later in this chapter in the sequence prediction section.

Spoofing attacks are different from hijacking attacks. In a spoofing attack, the hacker performs sniffing and listens to traffic as it's passed along the network from sender to receiver. The hacker then uses the information gathered to spoof or uses an address of a legitimate system. Hijacking involves actively taking another user offline to perform the attack. The attacker relies on the legitimate user to make a connection and authenticate. After that, the attacker takes over the session, and the valid user's session is disconnected.

Session hijacking involves the following three steps to perpetuate an attack:

• Tracking the Session The hacker identifies an open session and predicts the sequence number of the next packet.
• Desynchronizing the Connection The hacker sends the valid user's system a TCP reset (RST) or finish (FIN) packet to cause them to close their session.
• Injecting the Attacker's Packet The hacker sends the server a TCP packet with the predicted sequence number, and the server accepts it as the valid user's next packet.

Hackers can use two types of session hijacking: active and passive. The primary difference between active and passive hijacking is the hacker's level of involvement in the session. In an active attack, an attacker finds an active session and takes over the session by using tools that predict the next sequence number used in the TCP session.

In a passive attack, an attacker hijacks a session and then watches and records all the traffic that is being sent by the legitimate user. Passive session hijacking is really no more than sniffing. It gathers information such as passwords and then uses that information to authenticate as a separate session.

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TCP Concepts: Three-Way Handshake

Two of the key features of TCP are reliability and ordered delivery of packets. To accomplish these goals, TCP uses acknowledgment (ACK) packets and sequence numbers. Manipulating these numbers is the basis for TCP session hijacking. To understand session hijacking, let's review the TCP three-way handshake described in earlier chapters:

1. The valid user initiates a connection with the server. This is accomplished by the valid user sending a packet to the server with the SYN bit set and the user's initial sequence number (ISN).
2. The server receives this packet and sends back a packet with the SYN bit set and an ISN for the server, plus the ACK bit set identifying the user's ISN incremented by a value of 1.
3. The valid user acknowledges the server by returning a packet with the ACK bit set and incrementing the server's ISN by 1.

This connection can be closed from either side due to a timeout or upon receipt of a package with the FIN or RST flag set.

Upon receipt of a packet with the RST flag set, the receiving system closes the connection, and any incoming packets for the session are discarded. If the FIN flag is set in a packet, the receiving system goes through the process of closing the connection, and any packets received while closing the connection are still processed. Sending a packet with the FIN or RST flag set is the most common method hijackers use to close the client's session with the server and take over the session by acting as the client.