How do you de-elevate privileges for a child process

Question

I know how to launch a process with Admin privileges from a process using:

proc.StartInfo.UseShellExecute = true;
proc.StartInfo.Verb = "runas";

where proc is a System.Diagnostics.Process. But how does one do the opposite?

If the process you're in is already elevated, how do you launch the new process without admin privileges? More accurately, we need to launch the new process with the same permission level as Windows Explorer, so no change if UAC is disabled, but if UAC is enabled, but our process is running elevated, we need to perform a certain operation un-elevated because we're creating a virtual drive and if it's created with elevated permissions and Windows explorer is running unelevated it won't show up.

Feel free to change the title to something better, I couldn't come up with a good description.

Answer 1

The solution for you is to use EXPLORER.exe process.

The idea is to run the process in UN-ELEVATED mode, using windows's file explorer process explorer.exe (info). Lets say the process that we want to launch is on $TEMP\MyUnElevatedProcess.exe.

So, for NSIS code, I will just write: (but can be run in ANY language)

 Exec '"$WINDIR\explorer.exe" "$TEMP\MyUnElevatedProcess.exe"'

Example code (using NSIS installer)

Exec '"$WINDIR\explorer.exe" "$TEMP\MyUnElevatedProcess.exe"'

***code taken from http://mdb-blog.blogspot.com/2013/01/nsis-lunch-program-as-user-from-uac.html

Answer 2

We ended up using the sample from this Code Project article: High elevation can be bad for your application: How to start a non-elevated process at the end of the installation

It seems to work so far, I gather it injects into RunDll32.exe, my C++/Win32 is fairly weak so I didn't look too much into the actual implementation, just it's use. Confirmed that it works in Vista and Win7 both x86 and x64 (at least for us, x86 and x64 require different dll's which is checked for at install time and the proper one is used).

Answer 3

Raymond Chen addressed this in his blog:

How can I launch an unelevated process from my elevated process and vice versa?

Searching in GitHub for a C# version of this code, I found the following implementation in Microsoft's Node.js tools for Visual Studio repository: SystemUtilities.cs (the ExecuteProcessUnElevated function).

Just in case the file disappears, here's the file's contents:

// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.

using System;
using System.Runtime.InteropServices;

namespace Microsoft.NodejsTools.SharedProject
{
    /// <summary>
    /// Utility for accessing window IShell* interfaces in order to use them to launch a process unelevated
    /// </summary>
    internal class SystemUtility
    {
        /// <summary>
        /// We are elevated and should launch the process unelevated. We can't create the
        /// process directly without it becoming elevated. So to workaround this, we have
        /// explorer do the process creation (explorer is typically running unelevated).
        /// </summary>
        internal static void ExecuteProcessUnElevated(string process, string args, string currentDirectory = "")
        {
            var shellWindows = (IShellWindows)new CShellWindows();

            // Get the desktop window
            object loc = CSIDL_Desktop;
            object unused = new object();
            int hwnd;
            var serviceProvider = (IServiceProvider)shellWindows.FindWindowSW(ref loc, ref unused, SWC_DESKTOP, out hwnd, SWFO_NEEDDISPATCH);

            // Get the shell browser
            var serviceGuid = SID_STopLevelBrowser;
            var interfaceGuid = typeof(IShellBrowser).GUID;
            var shellBrowser = (IShellBrowser)serviceProvider.QueryService(ref serviceGuid, ref interfaceGuid);

            // Get the shell dispatch
            var dispatch = typeof(IDispatch).GUID;
            var folderView = (IShellFolderViewDual)shellBrowser.QueryActiveShellView().GetItemObject(SVGIO_BACKGROUND, ref dispatch);
            var shellDispatch = (IShellDispatch2)folderView.Application;

            // Use the dispatch (which is unelevated) to launch the process for us
            shellDispatch.ShellExecute(process, args, currentDirectory, string.Empty, SW_SHOWNORMAL);
        }

        /// <summary>
        /// Interop definitions
        /// </summary>
        private const int CSIDL_Desktop = 0;
        private const int SWC_DESKTOP = 8;
        private const int SWFO_NEEDDISPATCH = 1;
        private const int SW_SHOWNORMAL = 1;
        private const int SVGIO_BACKGROUND = 0;
        private readonly static Guid SID_STopLevelBrowser = new Guid("4C96BE40-915C-11CF-99D3-00AA004AE837");

        [ComImport]
        [Guid("9BA05972-F6A8-11CF-A442-00A0C90A8F39")]
        [ClassInterfaceAttribute(ClassInterfaceType.None)]
        private class CShellWindows
        {
        }

        [ComImport]
        [Guid("85CB6900-4D95-11CF-960C-0080C7F4EE85")]
        [InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
        private interface IShellWindows
        {
            [return: MarshalAs(UnmanagedType.IDispatch)]
            object FindWindowSW([MarshalAs(UnmanagedType.Struct)] ref object pvarloc, [MarshalAs(UnmanagedType.Struct)] ref object pvarlocRoot, int swClass, out int pHWND, int swfwOptions);
        }

        [ComImport]
        [Guid("6d5140c1-7436-11ce-8034-00aa006009fa")]
        [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
        private interface IServiceProvider
        {
            [return: MarshalAs(UnmanagedType.Interface)]
            object QueryService(ref Guid guidService, ref Guid riid);
        }

        [ComImport]
        [Guid("000214E2-0000-0000-C000-000000000046")]
        [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
        private interface IShellBrowser
        {
            void VTableGap01(); // GetWindow
            void VTableGap02(); // ContextSensitiveHelp
            void VTableGap03(); // InsertMenusSB
            void VTableGap04(); // SetMenuSB
            void VTableGap05(); // RemoveMenusSB
            void VTableGap06(); // SetStatusTextSB
            void VTableGap07(); // EnableModelessSB
            void VTableGap08(); // TranslateAcceleratorSB
            void VTableGap09(); // BrowseObject
            void VTableGap10(); // GetViewStateStream
            void VTableGap11(); // GetControlWindow
            void VTableGap12(); // SendControlMsg
            IShellView QueryActiveShellView();
        }

        [ComImport]
        [Guid("000214E3-0000-0000-C000-000000000046")]
        [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
        private interface IShellView
        {
            void VTableGap01(); // GetWindow
            void VTableGap02(); // ContextSensitiveHelp
            void VTableGap03(); // TranslateAcceleratorA
            void VTableGap04(); // EnableModeless
            void VTableGap05(); // UIActivate
            void VTableGap06(); // Refresh
            void VTableGap07(); // CreateViewWindow
            void VTableGap08(); // DestroyViewWindow
            void VTableGap09(); // GetCurrentInfo
            void VTableGap10(); // AddPropertySheetPages
            void VTableGap11(); // SaveViewState
            void VTableGap12(); // SelectItem

            [return: MarshalAs(UnmanagedType.Interface)]
            object GetItemObject(UInt32 aspectOfView, ref Guid riid);
        }

        [ComImport]
        [Guid("00020400-0000-0000-C000-000000000046")]
        [InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
        private interface IDispatch
        {
        }

        [ComImport]
        [Guid("E7A1AF80-4D96-11CF-960C-0080C7F4EE85")]
        [InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
        private interface IShellFolderViewDual
        {
            object Application { [return: MarshalAs(UnmanagedType.IDispatch)] get; }
        }

        [ComImport]
        [Guid("A4C6892C-3BA9-11D2-9DEA-00C04FB16162")]
        [InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
        public interface IShellDispatch2
        {
            void ShellExecute([MarshalAs(UnmanagedType.BStr)] string File, [MarshalAs(UnmanagedType.Struct)] object vArgs, [MarshalAs(UnmanagedType.Struct)] object vDir, [MarshalAs(UnmanagedType.Struct)] object vOperation, [MarshalAs(UnmanagedType.Struct)] object vShow);
        }
    }
}