This message is logged by the attack-detection feature because the ProxySG appliance detected and blocked client communication from an IP address host.
If that message is found in the event log, the appliance has received a large number of packets from another host intentionally or accidentally. You can be relatively certain that the appliance has been attacked.
For details of the client-side attack detection, refer to the following KB articles: KB3401 or FAQ315.
The attack detection feature is only configurable using the CLI. Check the software configuration in the sysinfo file or enter:
#(config attack-detection) view configuration
This command displays how the appliance handles such traffic from outside of the appliance.