The TCP sequence was found to be predictable. When the TCP sequence is predictable, an attacker can send packets that are forged to appear to come from a trusted computer. These forged packets can compromise services, such as rsh and rlogin, because their authentication is based on IP addresses.
Attackers can also perform session hijacking to gain access to unauthorized information.
Some Microsoft patches for this did not completely resolve the sequence predicability. Windows NT 4.0 SP4 through SP6 use a different algorithm to reduce sequence predictability, but the systems remain predictable. Microsoft released patch MS99-046, which uses the same algorithm as Windows 2000, to fully fix the problem. Windows 2000 is not TCP predictable. Note that some patches make sequence prediction more difficult, but still possible.