Possible to launch a process in a user's session from a service?

Question

In Windows Vista/7/2008/2008R2, is it at all possible to launch a process in a user's session from a service? Specifically, the local session would be most useful.

Everything I've been reading seems to say this isn't possible, but I figured I'd ask here before giving up completely.

My service signs on as Local System. I'm coding in VB.NET, but will take suggestions in anything. Thanks for your time!

Answer 1

Yes, using CreateProcessAsUser you can do this. MSDN has sample articles, and there are some caveats.

Answer 2

It can be done but its not considered good practice for services to interact directly with users' sessions as it can create serious security holes.

http://support.microsoft.com/kb/327618

A better approach is to create 2 programs, a backend service and a front end client UI program. The service backend runs all the time and exposes its operations using WCF (for instance). The client program could be run at startup of a user's session.

Answer 3

It is really possible. The main problem which you have is that Windows should be seen as a terminal server and a users session as a remote session. Your service should be able to start a process which run in the remote session belongs to the user.

By the way, if you write a service which run under Windows XP which is not added to a domain and the fast user switching is activated, you can have the same problems to start a process on running on the second (third and so on) logged users desktop.

I hope you have a user token, which you receive for example with respect of impersonation or you have a dwSessionId of session. If you don't have it you can try use some WTS-function (Remote Desktop Services API http://msdn.microsoft.com/en-us/library/aa383464.aspx, for example WTSEnumerateProcesses or WTSGetActiveConsoleSessionId) or LSA-API to find out the corresponding users session (LsaEnumerateLogonSessions see http://msdn.microsoft.com/en-us/library/aa378275.aspx and LsaGetLogonSessionData see http://msdn.microsoft.com/en-us/library/aa378290.aspx) or ProcessIdToSessionId (see http://msdn.microsoft.com/en-us/library/aa382990.aspx).

You can use GetTokenInformation function with the parameter TokenSessionId (see http://msdn.microsoft.com/en-us/library/aa446671.aspx) to receive the session id dwSessionId of the users session if you knows the users token hClient.

BOOL bSuccess;
HANDLE hProcessToken = NULL, hNewProcessToken = NULL;
DWORD dwSessionId, cbReturnLength;

bSuccess = GetTokenInformation (hClient, TokenSessionId, &dwSessionId,
                                sizeof(DWORD), &cbReturnLength);
bSuccess = OpenProcessToken (GetCurrentProcess(), MAXIMUM_ALLOWED, &hProcessToken);
bSuccess = DuplicateTokenEx (hProcessToken, MAXIMUM_ALLOWED, NULL,
                             SecurityImpersonation,
                             TokenPrimary, &hNewProcessToken);
EnablePrivilege (SE_TCB_NAME);
bSuccess = SetTokenInformation (hNewProcessToken, TokenSessionId, dwSessionId,
                                sizeof(DWORD));
bSuccess = CreateProcessAsUser (hNewProcessToken, NULL, szCommandToExecute, ...);

This code only a schema. EnablePrivilege is a simple function used AdjustTokenPrivileges to enable SE_TCB_NAME privilege (see http://msdn.microsoft.com/en-us/library/aa446619.aspx as a template). It is important that the process from which you are start a process you have TCB privilege, but if your service run under the Local System you have enough permissions. By the way, following code fragment work with not only Local System account, but the account must have SE_TCB_NAME privilege to be able to switch current terminal server session.

One more remark. In the code above we start new process with the same account as the current process have (for example Local System). You change change a code to use another account for example the users token hClient. It is only important to have a primary token. If you have an impersonation token you can convert it to the primary token exactly like in the code above.

In the STARTUPINFO structure used in CreateProcessAsUser you should use lpDesktop = WinSta0\Default".

Depend on your requirements it could be also needed to use CreateEnvironmentBlock to create an new environment block that you will be passing to the new process.

I recommend you also to read http://stackoverflow.com/questions/3100934/how-to-ensure-process-window-launched-by-process-startprocessstartinfo-has-focu/3101061#3101061 where I describe how to force that the process will be started in foreground on the users desktop.