What does an attacker need to consider when attempting an IP spoofing attack that relies on guessing Initial Sequence Numbers (ISNs)?

When a server is handling little to no other traffic, its TCP Initial Sequence Numbers are easier to predict or enumerate, and there is less competing traffic that could cause the server to reset or invalidate a forged session. Active servers with many simultaneous connections generate more unpredictable ISNs and increased legitimate traffic that interferes with the blind injection. The attacker must time and guess correctly within a narrow window, which is only feasible when the server state is quiet and stable.

ISN-guessing attacks are conducted using a standard or minimally modified network stack to send crafted packets; they rely on predicting sequence numbers, not on a fundamentally altered TCP/IP implementation.

Because the attack uses a spoofed source IP address, it is extremely difficult to trace back to the actual attacker, making traceability the opposite of a concern - it is one of the attack's advantages.

TCP ISN prediction attacks are not OS-specific; any host with a predictable ISN generation algorithm, regardless of operating system, is potentially vulnerable.