I have an IPS4270 appliance (IPS4270-20-K9) in IDS mode. I am getting continuous triggers on the TCP Hijack signature and the attacker is always the proxy server. with the target of various external addresses and 1 specific internal address hosting web services (ports 80 & 443).
I suggest to you please do somethings for your network security (Configure IPS in Inline mode), Attacker attack to your web server from external site. Also make sure the webserver Antivirus working properly.