Chapter 7 Firewall for FortiOS 5.0 : Network defense : Blocking external probes : Evasion techniques : HTTP header obfuscation
  
HTTP header obfuscation
The headers of HTTP requests or responses can be modified to make the discovery of patterns and attacks more difficult. To prevent this, the FortiGate unit will:
remove junk header lines
reassemble an HTTP header that’s been folded onto multiple lines
move request parameters to HTTP POST body from the URL
The message is scanned for any enabled HTTP IPS signatures once these problems are corrected.