How can a Windows service execute a GUI application?

Question

I have written a Windows service that allows me to remotely run and stop applications. These applications are run using CreateProcess, and this works for me because most of them only perform backend processing. Recently, I need to run applications that present GUI to the current log in user. How do I code in C++ to allow my service to locate the currently active desktop and run the GUI on it?

Answer 1

WTSEnumerateSessions and CreateProcessAsUser.

Answer 2

I think as long as you have only one user logged in, it will automatically display on that user's desktop.

Anyway, be very careful when having a service start an exe.

If the write access to the folder with the exe is not restricted, any user can replace that exe with any other program, which will then be run with sytem rights. Take for example cmd.exe (available on all windows sytems). The next time the service tries to start your exe, you get a command shell with system rights...

Answer 3

If you launch a GUI from your service it will show up on the currently active desktop.

But only if you adjusted the service permissions: You need to allow it to interact with the desktop.

Answer 4

Roger Lipscombe's answer, to use WTSEnumerateSessions to find the right desktop, then CreateProcessAsUser to start the application on that desktop (you pass it the handle of the desktop as part of the STARTUPINFO structure) is correct.

However, I would strongly recommend against doing this. In some environments, such as Terminal Server hosts with many active users, determining which desktop is the 'active' one isn't easy, and may not even be possible.

But most importantly, if an application will suddenly appear on a user's desktop, this may very well occur at a bad time (either because the user simply isn't expecting it, or because you're trying to launch the app when the session isn't quite initialized yet, in the process of shutting down, or whatever).

A more conventional approach would be to put a shortcut to a small client app for your service in the global startup group. This app will then launch along with every user session, and can be used start other apps (if so desired) without any juggling of user credentials, sessions and/or desktops.

Also, this shortcut can be moved/disabled by administrators as desired, which will make deployment of your application much easier, since it doesn't deviate from the standards used by other Windows apps...

Answer 5

On Win2K, XP and Win2K3 the console user is logged on in Session 0, the same session the services live in. If a service is configured as interactive, it'll be able to show the UI on the user's desktop.

However, on Vista, no user can be logged on in Session 0. Showing UI from a service there is a bit trickier. You need to enumerate the active sessions using WTSEnumerateSessions API, find the console session and create the process as that user. Of course, you need also a token or user credentials to be able to do that. You can read more details about this process here.

Answer 6

Several people suggested WTSEnumerateSessions and CreateProcessAsUser. I wonder why no one suggested WTSGetActiveConsoleSessionId, since you said you only want to target one logged in user.

Several people sure are right to suggest CreateProcessAsUser though. If you call plain old CreateProcess the way you said, then the application's GUI will run with your service's privileges instead of the user's privileges.

Answer 7

The short answer is "You don't", as opening a GUI program running under another user context is a security vulnerability commonly known as a Shatter Attack.

Take a look at this MSDN article: Interactive Services. It gives some options for a service to interact with a user.

In short you have these options:

• Display a dialog box in the user's session using the WTSSendMessage function.

• Create a separate hidden GUI application and use the CreateProcessAsUser function to run the application within the context of the interactive user. Design the GUI application to communicate with the service through some method of interprocess communication (IPC), for example, named pipes. The service communicates with the GUI application to tell it when to display the GUI. The application communicates the results of the user interaction back to the service so that the service can take the appropriate action. Note that IPC can expose your service interfaces over the network unless you use an appropriate access control list (ACL).

  If this service runs on a multiuser system, add the application to the following key so that it is run in each session: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run. If the application uses named pipes for IPC, the server can distinguish between multiple user processes by giving each pipe a unique name based on the session ID.

Answer 8

Important Services cannot directly interact with a user as of Windows Vista. Therefore, the techniques mentioned in the section titled Using an Interactive Service should not be used in new code.

This is taken from : http://msdn.microsoft.com/en-us/library/ms683502(VS.85).aspx

Answer 9

That problems Session 0 , Interactive Services , Windows Service Allow Service To Interact With Desktop on Windows 7 or Windows Vista

You can read this article http://www.codeproject.com/KB/vista-security/SubvertingVistaUAC.aspx

I try explained here it's working on Windows 7