This policy setting specifies whether to disable the administrator rights to customize security permissions for the Remote Desktop Session Host server. You can use this setting to prevent administrators from making changes to the user groups allowed to connect remotely to the RD Session Host server. By default administrators are able to make such changes.If you enable this policy setting the default security descriptors for existing groups on the RD Session Host server cannot be changed. All the security descriptors are read-only.If you disable or do not configure this policy setting server administrators have full read/write permissions to the user security descriptors by using the Remote Desktop Session WMI Provider.Note: The preferred method of managing user access is by adding a user to the Remote Desktop Users group.
Windows Components\Remote Desktop Services\Remote Desktop Session Host\Security
Machine
At least Windows Server 2003
HKLM\SOFTWARE\Policies\Microsoft\Windows NT\Terminal Services!fWritableTSCCPermTab
TerminalServer.admx