CAPEC-299 TCP SYN Ping

An adversary uses TCP SYN packets as a means towards host discovery. Typical RFC 793 behavior specifies that when a TCP port is open, a host must respond to an incoming SYN "synchronize" packet by completing stage two of the 'three-way handshake' - by sending an SYN/ACK in response. When a port is closed, RFC 793 behavior is to respond with a RST "reset" packet. This behavior can be used to 'ping' a target to see if it is alive by sending a TCP SYN packet to a port and then looking for a RST or an ACK packet in response.

Due to the different responses from open and closed ports, SYN packets can be used to determine the remote state of the port. A TCP SYN ping is also useful for discovering alive hosts protected by a stateful firewall. In cases where a specific firewall rule does not block access to a port, a SYN packet can pass through the firewall to the host and solicit a response from either an open or closed port. When a stateful firewall is present, SYN pings are preferable to ACK pings because a stateful firewall will typically drop all unsolicited ACK packets as they are not part of an existing or new connection. TCP SYN pings often fail when a stateless ACL or firewall is configured to blanket-filter incoming packets to a port. The firewall device will discard any SYN packets to a blocked port. Often, an adversary will alternate between SYN and ACK pings to discover if a host is alive.

• The ability to send a TCP SYN packet to a remote target. Depending upon the operating system, the ability to craft SYN packets may require elevated privileges.

• [Low] The adversary needs to know how to craft and send protocol commands from the command line or within a tool.

• Confidentiality: Other • Confidentiality, Access Control, Authorization: Bypass Protection Mechanism, Hide Activities