Typical severity: Low. An adversary sends a TCP segment with the ACK flag set to a remote host for the purpose of determining if the host is alive. This is one of several TCP 'ping' types. The RFC 793 expected behavior for a service is to respond with a RST 'reset' packet to any unsolicited ACK segment that is not part of an existing connection. So by sending an ACK segment to a port, the adversary can identify that the host is alive by looking for a RST packet. Typically, a remote server will respond with a RST regardless of whether a port is open or closed. In this way, TCP ACK pings cannot discover the state of a remote port because the behavior is the same in either case. The firewall will look up the ACK packet in its state-table and discard the segment because it does not correspond to any active connection. A TCP ACK Ping can be used to discover if a host is alive via RST response packets sent from the host.
Leverage stateful firewalls that allow for the rejection of a packet that is not part of an existing connection.