VSecLab - Capec-303

Description

An adversary uses a TCP XMAS scan to determine if ports are closed on the target machine. This scan type is accomplished by sending TCP segments with all possible flags set 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 its relative speed when compared with other types of scans, its major advantage 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. XMAS packets, like out-of-state FIN or ACK packets, tend to pass through such devices undetected. 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, XMAS scanning a system protected by a stateful firewall may indicate all ports being open. Because of their obvious rule- breaking nature, XMAS scans are flagged by almost all intrusion prevention or intrusion detection systems.

External References

External ID	Source	Link	Description
CAPEC-303	capec	https://capec.mitre.org/data/definitions/303.html
CWE-200	cwe	http://cwe.mitre.org/data/definitions/200.html
REF-33	reference_from_CAPEC		Stuart McClure, Joel Scambray, George Kurtz, Hacking Exposed: Network Security Secrets & Solutions (6th Edition), 2009, McGraw Hill
REF-128	reference_from_CAPEC	http://www.faqs.org/rfcs/rfc793.html	Defense Advanced Research Projects Agency Information Processing Techniques Office, Information Sciences Institute University of Southern California, RFC793 - Transmission Control Protocol, 1981--09, Defense Advanced Research Projects Agency (DARPA)
REF-34	reference_from_CAPEC		Gordon "Fyodor" Lyon, Nmap Network Scanning: The Official Nmap Project Guide to Network Discovery and Security Scanning (3rd "Zero Day" Edition,), 2008, Insecure.com LLC, ISBN: 978-0-9799587-1-7
REF-130	reference_from_CAPEC	http://phrack.org/issues/51/11.html	Gordon "Fyodor" Lyon, The Art of Port Scanning (Volume: 7, Issue. 51), Phrack Magazine, 1997

Execution Flow

Experiment

An adversary sends TCP packets with all flags set but not associated with an existing connection to target ports.
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.

Prerequisites

The adversary needs logical access to the target network. XMAS 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.

Resource Required

This attack can be carried out with a network mapper or 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.

Skills Required

Not present

Consequences

Availability	Authorization	Access Control	Confidentiality
Unreliable Execution	Bypass Protection Mechanism	Bypass Protection Mechanism	Other
	Hide Activities	Hide Activities	Bypass Protection Mechanism
			Hide Activities

Example Instances