Typical severity: Low. An adversary engages in UDP scanning to gather information about UDP port status on the target system. UDP scanning methods involve sending a UDP datagram to the target port and looking for evidence that the port is closed. Open UDP ports usually do not respond to UDP datagrams as there is no stateful mechanism within the protocol that requires building or establishing a session. Responses to UDP datagrams are therefore application specific and cannot be relied upon as a method of detecting an open port. UDP scanning relies heavily upon ICMP diagnostic messages in order to determine the status of a remote port.
Firewalls or ACLs which block egress ICMP error types effectively prevent UDP scans from returning any useful information. | UDP scanning is complicated by rate limiting mechanisms governing ICMP error messages.