Port sweeps allow attackers to gather information that could be used to discover exploitable vulnerabilities.
Attackers often look for vulnerable services using port sweep programs that connect to several ports. Many network service daemons respond to a connection with a text banner describing their program name and version number. Attackers can use these responses to identify services that may have vulnerabilities.
The existence of a "listener" on a port can also be useful information that an intruder can use to plan future attacks.
Cisco Systems Cisco Intrusion Prevention System (IPS) 5.1
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