Any process reading from or writing to the clipboard must have WINSTA_ACCESSCLIPBOARD permission.
In a C2 level system, any process that has access rights to the clipboard can read or write to the clipboard.
In a B level system, objects that go into the clipboard will retain the security of their owner. Clipboard objects can move upwards through levels to higher securities but not downwards. For example if a top secret object is in the clipboard, the clipboard will appear empty to an unclassified process. If an unclassified object is in the clipboard, any top secret application will have access to that object.