Chapter 1 What’s New for FortiOS 5.2.1 : SSL VPN : Host Security Check Error Replacement Message
  
Host Security Check Error Replacement Message
The replacement message that now appears when an SSL VPN host security check fails can now be customized using the CLI.
Syntax
config system replacemsg sslvpn hostcheck-error
set buffer <string>
set header {none | http | 8bit}
set format {none | text | html}
end