A TCP null scan attempt is a technique used in network scanning where the TCP packet sent has no flags set. This type of scan can sometimes penetrate through routers and firewalls that filter incoming packets based on certain flags because the absence of flags can prevent the packet from being filtered out. The TCP null scan is particularly useful for identifying open ports on a target system. If a port is open, the target system will not respond to the null scan, but if the port is closed, the system will send a TCP RST packet in response. This scanning method is not supported by Windows because Windows systems typically respond with a RST packet regardless of whether the port is open or closed, making it ineffective for distinguishing between the two states on those systems.

References: The TCP null scan’s ability to bypass certain types of filters and its behavior in response to open and closed ports are documented in various network security resources, including the Nmap documentation and other network security analysis articles12. These sources confirm the effectiveness of TCP null scans in penetrating through filters set up by routers and firewalls and their unsupported status on Windows systems.