Calling CreateProcessAsUser from C#

Question

I've been attempting to create a new process under the context of a specific user using the CreateProcessAsUser function of the Windows API, but seem to be running into a rather nasty security issue...

Before I explain any further, here's the code I'm currently using to start the new process (a console process - PowerShell to be specific, though it shouldn't matter).

    private void StartProcess()
    {
        bool retValue;

        // Create startup info for new console process.
        var startupInfo = new STARTUPINFO();
        startupInfo.cb = Marshal.SizeOf(startupInfo);
        startupInfo.dwFlags = StartFlags.STARTF_USESHOWWINDOW;
        startupInfo.wShowWindow = _consoleVisible ? WindowShowStyle.Show : WindowShowStyle.Hide;
        startupInfo.lpTitle = this.ConsoleTitle ?? "Console";

        var procAttrs = new SECURITY_ATTRIBUTES();
        var threadAttrs = new SECURITY_ATTRIBUTES();
        procAttrs.nLength = Marshal.SizeOf(procAttrs);
        threadAttrs.nLength = Marshal.SizeOf(threadAttrs);

        // Log on user temporarily in order to start console process in its security context.
        var hUserToken = IntPtr.Zero;
        var hUserTokenDuplicate = IntPtr.Zero;
        var pEnvironmentBlock = IntPtr.Zero;
        var pNewEnvironmentBlock = IntPtr.Zero;

        if (!WinApi.LogonUser("UserName", null, "Password",
            LogonType.Interactive, LogonProvider.Default, out hUserToken))
            throw new Win32Exception(Marshal.GetLastWin32Error(), "Error logging on user.");

        var duplicateTokenAttrs = new SECURITY_ATTRIBUTES();
        duplicateTokenAttrs.nLength = Marshal.SizeOf(duplicateTokenAttrs);
        if (!WinApi.DuplicateTokenEx(hUserToken, 0, ref duplicateTokenAttrs,
            SECURITY_IMPERSONATION_LEVEL.SecurityImpersonation, TOKEN_TYPE.TokenPrimary,
            out hUserTokenDuplicate))
            throw new Win32Exception(Marshal.GetLastWin32Error(), "Error duplicating user token.");

        try
        {
            // Get block of environment vars for logged on user.
            if (!WinApi.CreateEnvironmentBlock(out pEnvironmentBlock, hUserToken, false))
                throw new Win32Exception(Marshal.GetLastWin32Error(),
                    "Error getting block of environment variables for user.");

            // Read block as array of strings, one per variable.
            var envVars = ReadEnvironmentVariables(pEnvironmentBlock);

            // Append custom environment variables to list.
            foreach (var var in this.EnvironmentVariables)
                envVars.Add(var.Key + "=" + var.Value);

            // Recreate environment block from array of variables.
            var newEnvironmentBlock = string.Join("\0", envVars.ToArray()) + "\0";
            pNewEnvironmentBlock = Marshal.StringToHGlobalUni(newEnvironmentBlock);

            // Start new console process.
            retValue = WinApi.CreateProcessAsUser(hUserTokenDuplicate, null, this.CommandLine,
                ref procAttrs, ref threadAttrs, false, CreationFlags.CREATE_NEW_CONSOLE |
                CreationFlags.CREATE_SUSPENDED | CreationFlags.CREATE_UNICODE_ENVIRONMENT,
                pNewEnvironmentBlock, null, ref startupInfo, out _processInfo);
            if (!retValue) throw new Win32Exception(Marshal.GetLastWin32Error(),
                "Unable to create new console process.");
        }
        catch
        {
            // Catch any exception thrown here so as to prevent any malicious program operating
            // within the security context of the logged in user.

            // Clean up.
            if (hUserToken != IntPtr.Zero)
            {
                WinApi.CloseHandle(hUserToken);
                hUserToken = IntPtr.Zero;
            }

            if (hUserTokenDuplicate != IntPtr.Zero)
            {
                WinApi.CloseHandle(hUserTokenDuplicate);
                hUserTokenDuplicate = IntPtr.Zero;
            }

            if (pEnvironmentBlock != IntPtr.Zero)
            {
                WinApi.DestroyEnvironmentBlock(pEnvironmentBlock);
                pEnvironmentBlock = IntPtr.Zero;
            }

            if (pNewEnvironmentBlock != IntPtr.Zero)
            {
                Marshal.FreeHGlobal(pNewEnvironmentBlock);
                pNewEnvironmentBlock = IntPtr.Zero;
            }

            throw;
        }
        finally
        {
            // Clean up.
            if (hUserToken != IntPtr.Zero)
                WinApi.CloseHandle(hUserToken);

            if (hUserTokenDuplicate != IntPtr.Zero)
                WinApi.CloseHandle(hUserTokenDuplicate);

            if (pEnvironmentBlock != IntPtr.Zero)
                WinApi.DestroyEnvironmentBlock(pEnvironmentBlock);

            if (pNewEnvironmentBlock != IntPtr.Zero)
                Marshal.FreeHGlobal(pNewEnvironmentBlock);
        }

        _process = Process.GetProcessById(_processInfo.dwProcessId);
    }

For the sake of the issue here, ignore the code dealing with the environment variables (I've tested that section independently and it seems to work.)

Now, the error I get is the following (thrown at the line following the call to CreateProcessAsUSer):

"A required privilege is not held by the client" (error code 1314)

(The error message was discovered by removing the message parameter from the Win32Exception constructor. Admittedly, my error handling code here may not be the best, but that's a somewhat irrelevant matter. You're welcome to comment on it if you wish, however.) I'm really quite confused as to the cause of this vague error in this situation. MSDN documentation and various forum threads have only given me so much advice, and especially given that the causes for such errors appear to be widely varied, I have no idea which section of code I need to modify. Perhaps it is simply a single parameter I need to change, but I could be making the wrong/not enough WinAPI calls for all I know. What confuses me greatly is that the previous version of the code that uses the plain CreateProcess function (equivalent except for the user token parameter) worked perfectly fine. As I understand, it is only necessary to call the Logon user function to receive the appropriate token handle and then duplicate it so that it can be passed to CreateProcessAsUser.

Any suggestions for modifications to the code as well as explanations would be very welcome.

Notes

I've been primarily referring to the MSDN docs (as well as PInvoke.net for the C# function/strut/enum declarations). The following pages in particular seem to have a lot of information in the Remarks sections, some of which may be important and eluding me:

• CreateProcessAsUser function
• LogonUser function
• DuplicateTokenEx function

Edit

I've just tried out Mitch's suggestion, but unfortunately the old error has just been replaced by a new one: "The system cannot find the file specified." (error code 2)

The previous call to CreateProcessAsUser was replaced with the following:

retValue = WinApi.CreateProcessWithTokenW(hUserToken, LogonFlags.WithProfile, null,
    this.CommandLine, CreationFlags.CREATE_NEW_CONSOLE |
    CreationFlags.CREATE_SUSPENDED | CreationFlags.CREATE_UNICODE_ENVIRONMENT,
    pNewEnvironmentBlock, null, ref startupInfo, out _processInfo);

Note that this code no longer uses the duplicate token but rather the original, as the MSDN docs appear to suggest.

And here's another attempt using CreateProcessWithLogonW. The error this time is "Logon failure: unknown user name or bad password" (error code 1326)

retValue = WinApi.CreateProcessWithLogonW("Alex", null, "password",
    LogonFlags.WithProfile, null, this.CommandLine,
    CreationFlags.CREATE_NEW_CONSOLE | CreationFlags.CREATE_SUSPENDED |
    CreationFlags.CREATE_UNICODE_ENVIRONMENT, pNewEnvironmentBlock,
    null, ref startupInfo, out _processInfo);

I've also tried specifying the username in UPN format ("Alex@Alex-PC") and passing the domain independently as the second argument, all to no avail (identical error).

Answer 1

Ahh... seems liker I've been caught out by one of the biggest gotchas in WinAPI interop programming. Also, Posting the code for my function declarations would have been a wise idea in this case.

Anyway, all that I needed to do was add an argument to the DllImport attribute of the function specifying CharSet = CharSet.Unicode. This did the trick for both the CreateProcessWithLogonW and CreateProcessWithTokenW functions. I guess it finally just hit me that the W suffix of the function names referred to Unicode and that I needed to explicitly specify this in C#! Here are the correct function declarations in case anyone is interested:

[DllImport("advapi32", CharSet = CharSet.Unicode, SetLastError = true)]
public static extern bool CreateProcessWithLogonW(string principal, string authority,
    string password, LogonFlags logonFlags, string appName, string cmdLine,
    CreationFlags creationFlags, IntPtr environmentBlock, string currentDirectory,
    ref STARTUPINFO startupInfo, out PROCESS_INFORMATION processInfo);

[DllImport("advapi32", CharSet = CharSet.Unicode, SetLastError = true)]
public static extern bool CreateProcessWithTokenW(IntPtr hToken, LogonFlags dwLogonFlags,
    string lpApplicationName, string lpCommandLine, CreationFlags dwCreationFlags,
    IntPtr lpEnvironment, string lpCurrentDirectory, [In] ref STARTUPINFO lpStartupInfo,
    out PROCESS_INFORMATION lpProcessInformation);

Answer 2

From here:

Typically, the process that calls the CreateProcessAsUser function must have the SE_ASSIGNPRIMARYTOKEN_NAME and SE_INCREASE_QUOTA_NAME privileges. If this function fails with ERROR_PRIVILEGE_NOT_HELD (1314), use the CreateProcessWithLogonW function instead. CreateProcessWithLogonW requires no special privileges, but the specified user account must be allowed to log on interactively. Generally, it is best to use CreateProcessWithLogonW to create a process with alternate credentials.

See this blog post How to call CreateProcessWithLogonW & CreateProcessAsUser in .NET

Answer 3

Jonathan Peppers provided this great piece of code that fixed my issues

http://social.msdn.microsoft.com/Forums/en-US/csharpgeneral/thread/0c0ca087-5e7b-4046-93cb-c7b3e48d0dfb?ppud=4