Stealth NULL Scan Port Probe

Darshan Thummar@Mighty

•updated Jun 10, 2026•0•0•6

This rule detects a Stealth NULL Scan Port Probe. A NULL scan is a type of port scan where all the TCP flags (URG, ACK, PSH, RST, SYN, FIN) are turned off. If the target port is closed, the target system should respond with an RST packet. If the port is open, the target system should not respond at all. This behavior can be used by attackers to map out open ports on a system while attempting to evade detection by some firewalls or intrusion detection systems that might be configured to only look for SYN packets.