An adversary uses a SYN scan to determine the status of ports on the remote target. SYN scanning is the most common type of port scanning that is used because of its many advantages and few drawbacks. As a result, novice attackers tend to overly rely on the SYN scan while performing system reconnaissance. As a scanning method, the primary advantages of SYN scanning are its universality and speed. RFC 793 defines the required behavior of any TCP/IP device in that an incoming connection request begins with a SYN packet, which in turn must be followed by a SYN/ACK packet from the receiving service. For this reason, like TCP Connect scanning, SYN scanning works against any TCP stack. Unlike TCP Connect scanning, it is possible to scan thousands of ports per second using this method. This type of scanning is usually referred to as 'half-open' scanning because it does not complete the three-way handshake. The scanning rate is extremely fast because no time is wasted completing the handshake or tearing down the connection. This technique allows an attacker to scan through stateful firewalls due to the common configuration that TCP SYN segments for a new connection will be allowed for almost any port. TCP SYN scanning can also immediately detect 3 of the 4 important types of port status: open, closed, and filtered.
This table shows the other attack patterns and high level categories that are related to this attack pattern. These relationships are defined as ChildOf and ParentOf, and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as CanFollow, PeerOf, and CanAlsoBe are defined to show similar attack patterns that the user may want to explore.
Standard Attack Pattern - A standard level attack pattern in CAPEC is focused on a specific methodology or technique used in an attack. It is often seen as a singular piece of a fully executed attack. A standard attack pattern is meant to provide sufficient details to understand the specific technique and how it attempts to accomplish a desired goal. A standard level attack pattern is a specific type of a more abstract meta level attack pattern.
An adversary sends SYN packets to ports they want to scan and checks the response without completing the TCP handshake.
An adversary uses the response from the target to determine the port's state. The adversary can determine the state of a port based on the following responses. When a SYN is sent to an open port and unfiltered port, a SYN/ACK will be generated. When a SYN packet is sent to a closed port a RST is generated, indicating the port is closed. When SYN scanning to a particular port generates no response, or when the request triggers ICMP Type 3 unreachable errors, the port is filtered.
This scan type is not possible with some operating systems (Windows XP SP 2). On Linux and Unix systems it requires root privileges to use raw sockets.
The ability to send TCP SYN segments to a host during network reconnaissance 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.
This table specifies different individual consequences associated with the attack pattern. The Scope identifies the security property that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in their attack. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a pattern will be used to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
Bypass Protection Mechanism
A Related Weakness relationship associates a weakness with this attack pattern. Each association implies a weakness that must exist for a given attack to be successful. If multiple weaknesses are associated with the attack pattern, then any of the weaknesses (but not necessarily all) may be present for the attack to be successful. Each related weakness is identified by a CWE identifier.
Exposure of Sensitive Information to an Unauthorized Actor
[REF-33] Stuart McClure, Joel Scambray
and George Kurtz. "Hacking Exposed: Network Security Secrets & Solutions". Chapter 2: Scanning, pg. 56. 6th Edition. McGraw Hill. 2009.
[REF-128] 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). 1981-09.
[REF-34] Gordon "Fyodor" Lyon. "Nmap Network Scanning: The Official Nmap Project Guide to Network Discovery and Security Scanning". Section 5.32 TCP SYN (Stealth) Scan, pg. 100. 3rd "Zero Day" Edition,. Insecure.com LLC, ISBN: 978-0-9799587-1-7. 2008.