Common Attack Pattern Enumeration and Classification
A Community Resource for Identifying and Understanding Attacks
An attacker 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. he 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. Many operating systems, however, 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 an attacker from distinguishing between open and closed ports.
NULL scans are limited by the range of platforms against which they work. 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.
Target Attack Surface Description
Targeted OSI Layers: Transport Layer
Target Attack Surface Localities
Target Attack Surface Types: Host Service
The ability to send TCP segments with no flags set to a host during network reconnaissance. This can be achieved via the use of 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.
[R.304.1] [REF-20] Stuart McClure, Joel Scambray and George Kurtz. "Hacking Exposed: Network Security Secrets & Solutions". Chapter 2: Scanning, pg. 56. 6th Edition. McGraw Hill. 2009.
[R.304.2] [REF-21] Defense Advanced Research Projects Agency Information Processing Techniques Office and Information Sciences Institute University of Southern California. "RFC793 - Transmission Control Protocol". Defense Advanced Research Projects Agency (DARPA). September 1981. <http://www.faqs.org/rfcs/rfc793.html>.
[R.304.3] [REF-22] Gordon "Fyodor" Lyon. "Nmap Network Scanning: The Official Nmap Project Guide to Network Discovery and Security Scanning". Section 5.5 TCP FIN, NULL, XMAS Scans, pg. 107. 3rd "Zero Day" Edition,. Insecure.com LLC, ISBN: 978-0-9799587-1-7. 2008.
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