CAPEC Details
Name UDP Scan
Likelyhood of attack Typical severity
Low Low
Summary 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. During a UDP scan, a datagram is sent to a target port. If an 'ICMP Type 3 Port unreachable' error message is returned then the port is considered closed. Different types of ICMP messages can indicate a filtered port. UDP scanning is slower than TCP scanning. The protocol characteristics of UDP make port scanning inherently more difficult than with TCP, as well as dependent upon ICMP for accurate scanning. Due to ambiguities that can arise between open ports and filtered ports, UDP scanning results often require a high degree of interpretation and further testing to refine. In general, UDP scanning results are less reliable or accurate than TCP-based scanning.
Prerequisites The ability to send UDP datagrams to a host and receive ICMP error messages from that host. In cases where particular types of ICMP messaging is disallowed, the reliability of UDP scanning drops off sharply.
Execution Flow
Step Phase Description Techniques
1 Experiment An adversary sends UDP packets to target ports.
2 Experiment An adversary uses the response from the target to determine the port's state. Whether a port responds to a UDP packet is dependant on what application is listening on that port. No response does not indicate the port is not open.
Solutions 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.
Related Weaknesses
CWE ID Description
CWE-200 Exposure of Sensitive Information to an Unauthorized Actor
Related CAPECS
CAPEC ID Description
CAPEC-300 An adversary uses a combination of techniques to determine the state of the ports on a remote target. Any service or application available for TCP or UDP networking will have a port open for communications over the network. Although common services have assigned port numbers, services and applications can run on arbitrary ports. Additionally, port scanning is complicated by the potential for any machine to have up to 65535 possible UDP or TCP services. The goal of port scanning is often broader than identifying open ports, but also give the adversary information concerning the firewall configuration. Depending upon the method of scanning that is used, the process can be stealthy or more obtrusive, the latter being more easily detectable due to the volume of packets involved, anomalous packet traits, or system logging. Typical port scanning activity involves sending probes to a range of ports and observing the responses. There are four port statuses that this type of attack aims to identify: open, closed, filtered, and unfiltered. For strategic purposes it is useful for an adversary to distinguish between an open port that is protected by a filter vs. a closed port that is not protected by a filter. Making these fine grained distinctions is requires certain scan types. Collecting this type of information tells the adversary which ports can be attacked directly, which must be attacked with filter evasion techniques like fragmentation, source port scans, and which ports are unprotected (i.e. not firewalled) but aren't hosting a network service. An adversary often combines various techniques in order to gain a more complete picture of the firewall filtering mechanisms in place for a host.