TCP/IP hijacking is a technique that uses spoofed packets to take over a connection between a victim and a host machine. It is similar to a man-in-the-middle attack, except that the rogue agent sends a reset request to the client so that the client loses contact with the server while the rogue system assumes the role of the legitimate client, continuing the session. This technique is especially useful when the victim uses a one-time password to connect to the host machine. A one-time password can, as its name implies, be used to authenticate once and only once; thus, sniffing the authentication is useless for the attacker.
To carry out a TCP/IP hijacking attack, the attacker must be on the same network as the victim. This gives the attacker the ability to sniff the local network segment and, as a result, all the details of open TCP connections can be pulled from the headers. Each TCP packet contains a sequence number in its header. This sequence number is incremented with each packet sent to ensure that packets are received in the correct order. While sniffing packets, the attacker has access to the sequence numbers for a connection between a victim and a host machine. Then the attacker sends a spoofed packet from the victim’s IP address to the host machine, using the sniffed sequence number to provide the proper acknowledgment number. The host machine will receive the spoofed packet with the correct acknowledgment number and will have no reason to believe the packet did not come from the victim’s machine; thus the TCP/IP hijacking attempt will be successful.
Forms of TCP/IP Hijacking
One form of TCP/IP hijacking is to inject an authentic-looking reset (RST) packet. If the source is spoofed and the acknowledgment number is correct, the receiving side will believe that the source actually sent the reset packet, and the connection will be reset. The attacker could perform such an attack using a program that uses the libpcap and libnet libraries. libpcap would sniff the packets, and libnet would inject RST packets. The program does not need to look at every packet, but only established TCP connections to a target IP, so the libcpap function calls would be structured accordingly. It is relatively easy to come up with a filter rule for packets that have a certain destination IP. It is somewhat more difficult to filter for established connections, but since all established connections will have the ACK flag in the TCP header TCP flags, the program can look for that.
Another type of TCP/IP hijacking is continued hijacking. The spoofed packet does not need to be an RST packet; the spoof packet can contain data. When the host receives the spoofed packet, it will increment the sequence number and responds to the victim’s IP. Since the victim’s machine does not know about the spoofed packet, the host machine’s response has an incorrect sequence number, so the victim ignores that response packet. And since the victim’s machine ignored the host machine’s response packet, the victim’s sequence number count is off. Therefore, any packet the victim tries to send to the host machine will have an incorrect sequence number as well, causing the host machine to ignore it. In this instance, both legitimate sides of the connection have incorrect sequence numbers, resulting in a desynchronized state. And since the attacker sent out the first spoofed packet that caused all this chaos, it can keep track of sequence numbers and continue spoofing packets from the victim’s IP address to the host machine. This lets the attacker continue communicating with the host machine while the victim’s connection hangs.