If TCP port 20 is open, then an attacker can assume that FTP is being used.

Name: If TCP port 20 is open, then an attacker can assume that FTP is being used.
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Quizplus · Accepted Answer

TCP port 20 is the default data transfer port for FTP (File Transfer Protocol), so if it is open, it is likely that FTP is being used. However, it is still possible that another protocol could be using port 20, so it is not a definitive indicator.

If TCP Port 20 Is Open, Then an Attacker Can