CAPEC Details
Name TCP SYN Scan
Likelyhood of attack Typical severity
Medium Low
Summary 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.
Prerequisites 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.
Execution Flow
Step Phase Description Techniques
1 Experiment An adversary sends SYN packets to ports they want to scan and checks the response without completing the TCP handshake.
2 Experiment 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.
Related Weaknesses
CWE ID Description
CWE-200 Exposure of Sensitive Information to an Unauthorized Actor
Related CAPECS
CAPEC ID Description
CAPEC-300 An adversary uses a combination of techniques to determine the state of the ports on a remote target. Any service or application available for TCP or UDP networking will have a port open for communications over the network. Although common services have assigned port numbers, services and applications can run on arbitrary ports. Additionally, port scanning is complicated by the potential for any machine to have up to 65535 possible UDP or TCP services. The goal of port scanning is often broader than identifying open ports, but also give the adversary information concerning the firewall configuration. Depending upon the method of scanning that is used, the process can be stealthy or more obtrusive, the latter being more easily detectable due to the volume of packets involved, anomalous packet traits, or system logging. Typical port scanning activity involves sending probes to a range of ports and observing the responses. There are four port statuses that this type of attack aims to identify: open, closed, filtered, and unfiltered. For strategic purposes it is useful for an adversary to distinguish between an open port that is protected by a filter vs. a closed port that is not protected by a filter. Making these fine grained distinctions is requires certain scan types. Collecting this type of information tells the adversary which ports can be attacked directly, which must be attacked with filter evasion techniques like fragmentation, source port scans, and which ports are unprotected (i.e. not firewalled) but aren't hosting a network service. An adversary often combines various techniques in order to gain a more complete picture of the firewall filtering mechanisms in place for a host.