Cisco PIX 6.3 and 7.0 allows remote attackers to cause a denial of service (blocked new connections) via spoofed TCP packets that cause the PIX to create embryonic connections that that would not produce a valid connection with the end system, including (1) SYN packets with invalid checksums, which do not result in a RST; or, from an external interface, (2) one byte of "meaningless data," or (3) a TTL that is one less than needed to reach the internal destination.