CAPEC Details
Name TCP Null Scan
Likelyhood of attack Typical severity
Low Low
Summary An adversary uses a TCP NULL scan to determine if ports are closed on the target machine. This scan type is accomplished by sending TCP segments with no flags in the packet header, generating packets that are illegal based on RFC 793. The RFC 793 expected behavior is that any TCP segment with an out-of-state Flag sent to an open port is discarded, whereas segments with out-of-state flags sent to closed ports should be handled with a RST in response. This behavior should allow an attacker to scan for closed ports by sending certain types of rule-breaking packets (out of sync or disallowed by the TCB) and detect closed ports via RST packets.
Prerequisites The adversary requires logical access to the target network. NULL scanning requires the use of raw sockets, and thus cannot be performed from some Windows systems (Windows XP SP 2, for example). On Unix and Linux, raw socket manipulations require root privileges.
Execution Flow
Step Phase Description Techniques
1 Experiment An adversary sends TCP packets with no flags set and that are not associated with an existing connection to target ports.
2 Experiment An adversary uses the response from the target to determine the port's state. If no response is received the port is open. If a RST packet is received then the port is closed.
Solutions Employ a robust network defensive posture that includes a managed IDS/IPS.
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.