Yes, It looks like an attack. here is the explanation of this emssage.
Explanation This is a connection-related message. This message occurs when an attempt to connect to an inside address is denied by the security policy that is defined for the specified traffic type. Possible tcp_flags values correspond to the flags in the TCP header that were present when the connection was denied. For example, a TCP packet arrived for which no connection state exists in the security appliance, and it was dropped. The tcp_flags in this packet are FIN and ACK.
The tcp_flags are as follows:
•
ACK—The acknowledgment number was received.
•FIN—Data was sent.
•PSH—The receiver passed data to the application.
•RST—The connection was reset.
•SYN—Sequence numbers were synchronized to start a connection.
•URG—The urgent pointer was declared valid.
Since ASA has dropped those packet per policy, it should be OK for now.
If your inside host is providing a public service (outside can initiate traffic to that Host_IP/port), you might need to pay attention to it. They can initiate SYN by using spoofing IP to launch a dos attack. Please refer to the "Preventing Network Attacks" in confguration guide.
http://www.cisco.com/en/US/docs/security/asa/asa80/configuration/guide/protect.html
