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. Some schemes to detect this attack are presented as well as some methods of prevention and some interesting details of the TCP protocol behaviors.

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