Session 0 Isolation
In the Windows environment, more than one user can be logged on to a computer at one time, for example, when a computer is hosting several simultaneous Microsoft Terminal Server sessions. In these cases, each instance of a user logon is given a session number. In Windows XP and earlier versions of Windows, the first local logon, that is, the console session, was given a session ID number of zero (0). All subsequent users who logged on were given successively higher session ID numbers. All Windows system-level software also operated in session 0, which sometimes caused processes that the console user initiated in session 0 to operate in the same session context as those that the system created.
In Windows Vista, for security reasons, session 0 is reserved for system processes. The first user to log on is given a session ID number of one (1). This can create difficulties when printing, for example, if a print driver user interface (UI) that is operating in the User session, such as session 1, creates a call through the print spooler to show a piece of UI. The print spooler is a system component so that any UI it creates is displayed in session 0, making it invisible and inaccessible to the user. The session 1 user is unable to read, interact, or dismiss such a UI. If this is a critical interface that requires a response, it may appear to the user as though the print driver has hung.
To avoid this situation, Microsoft has provided the asynchronous UI mechanism for driver writers. Some earlier drivers, however, have not implemented an asynchronous UI. To accommodate the cases where a UI was generated in session 0, Windows Vista detects this and presents a dialog box to the user, asking if the user wants to interact with this UI. If the user accepts, the session 0 UI is made available to the user.
This is a temporary solution to aid in the transition to using an asynchronous UI. Although this solution will be supported in Windows Vista, it may not be available in future versions of Windows.
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