COMMON ATTACK PATTERN ENUMERATION AND CLASSIFICATIONLive MITRE record

CAPEC-304

TCP Null Scan

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.

Abstraction

Detailed

Typical severity

Low

Likelihood of attack

Medium

Understand the attack

What is this attack pattern?

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.

Extended description

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.

Execution flow

How does the attack proceed, step by step?

1

Experiment

Attack step

An adversary sends TCP packets with no flags set and that are not associated with an existing connection to target ports.

Attack step

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.

Requirements

What does the attack require?

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.

Skills required

Not stated in the MITRE record.

Resources required

  • 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.

Impact

What does a successful attack lead to?

Other

Affected scopes: Confidentiality

Bypass Protection MechanismHide Activities

Affected scopes: Confidentiality, Access Control, Authorization

Defence

How is it prevented and mitigated?

1
Employ a robust network defensive posture that includes a managed IDS/IPS.
CAPEC-304: TCP Null Scan — DayBreach Saldırı Sözlüğü · DayBreach