Some people know Remko Weijnen’s blog about stuff regarding Windows Terminal Server. If you don’t you should read his articles. You’ll get a deep insight how Windows works.
Recently another author published an article about TS internals on Remko’s blog. Danila Galimov exposes how communication is done between Terminal Service and a Terminal Session (like RDP [...]

The following code is really simple. It restricts access to the current process. In this way no other process can open the process handle and for example terminate this process.
uses
  JwaWindows,
  JwsclSecureObjects,
  JwsclDescriptor,
  JwsclToken,
  JwsclTypes,
  JwsclAcl,
  JwsclKnownSid;
var
  SD : TJwSecurityDescriptor;
  hProcess : TJwProcessHandle;
begin
  JwInitWellKnownSIDs;
  hProcess := OpenProcess(READ_CONTROL or WRITE_DAC, false, GetCurrentProcessId());
  if hProcess <> 0 [...]