Computer systems connected to networks, such as the Internet, are also vulnerable to sniffing: eavesdropping on communications between computers. Special software called packet sniffers can be connected to a computer network and extract all the packets of information used to pass data between computers.

This paper describes an active attack against the Transport Control Protocol (TCP) which allows a cracker to redirect the TCP stream through his machine thereby permitting him to bypass the protection offered by such a system as a one-time password [skey] or ticketing authentication [kerberos]. The TCP connection is vulnerable to anyone with a TCP packet sniffer and generator located on the path followed by the connection.

The TCP layer of the NeXT OS occasionally fails to ACK data packets….We have used an Ethernet packet sniffer to trace the problem to a missing NeXT ACK.