CAPEC-302 TCP FIN Scan

An adversary uses a TCP FIN scan to determine if ports are closed on the target machine. This scan type is accomplished by sending TCP segments with the FIN bit set in the packet header. 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 the adversary 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.

In addition to its relative speed in comparison with other types of scans, the major advantage a TCP FIN Scan 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. FIN packets, like out-of-state ACK packets, tend to pass through such devices undetected. FIN scanning is still relatively stealthy as the packets tend to blend in with the background noise on a network link.

1. [Experiment] An adversary sends TCP packets with the FIN flag but not associated with an existing connection to target ports. 2. [Experiment] 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.

• FIN 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.

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

• FIN scans are detected via heuristic (non-signature) based algorithms, much in the same way as other scan types are detected. An IDS/IPS system with heuristic algorithms is required to detect them.