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.

In addition to being fast, the major advantage of this scan type is its ability to scan through stateless firewall or ACL filters. Such filters are configured to block access to ports usually by preventing SYN packets, thus stopping any attempt to 'build' a connection. NULL packets, like out-of-state FIN or ACK packets, tend to pass through such devices undetected. Additionally, because open ports are inferred via no responses being generated, one cannot distinguish an open port from a filtered port without further analysis. For instance, NULL scanning a system protected by a stateful firewall may indicate all ports being open. Because of their obvious rule-breaking nature, NULL scans are flagged by almost all intrusion prevention or intrusion detection systems.

Low

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.

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.

This attack can be carried out via a network mapper/scanner, or via raw socket programming in a scripting language. Packet injection tools are also useful for this purpose. Depending upon the method used it may be necessary to sniff the network in order to see the response.

Confidentiality
Other

Confidentiality
Access Control
Authorization
Bypass Protection Mechanism
Hide Activities

Employ a robust network defensive posture that includes a managed IDS/IPS.

Many operating systems do not implement RFC 793 exactly and for this reason NULL scans do not work as expected against these devices. Some operating systems, like Microsoft Windows, send a RST packet in response to any out-of-sync (or malformed) TCP segments received by a listening socket (rather than dropping the packet via RFC 793), thus preventing the adversary from distinguishing between open and closed ports. NULL scans are limited by the range of platforms against which they work.

CAPEC Content Team
The MITRE Corporation
2014-06-23

CAPEC Content Team
The MITRE Corporation
2018-07-31
Updated Attack_Prerequisites, Description, Description Summary, References, Related_Weaknesses, Resources_Required

CAPEC Content Team
The MITRE Corporation
2020-12-17
Updated Description, Execution_Flow, Mitigations, Notes

CAPEC Content Team
The MITRE Corporation
2022-02-22
Updated Description, Extended_Description