Impersonation the current user using WindowsImpersonationContext to access network drive

Question

I need to access a remote drive from a Web App. The drive isn't accessible to the ASP.NET process, so I want to impersonate the current user for the request.

I saw some basic examples using WindowsImpersonationContext, and have tried the following.

WindowsImpersonationContext impersonationContext = null;

try
{
    impersonationContext = ((WindowsIdentity)User.Identity).Impersonate();

    file.SaveAs(filePath);
}
finally
{
    if (impersonationContext != null)
    {
        impersonationContext.Undo();
    }
}

I'm still getting an access denied exception.

I've read a bit of stuff about LogonUser, but as far as I can tell you need a user name and password to get credentials from that. I'm not looking to log in as a different specific user, just the current user (who should have permissions to access the file store), so I don't think I actually need the LogonUser API.

Does anyone have any idea what I'm missing? Should the above code work?

I'd also note that including

<identity impersonate="true" />

doesn't work, but including

<identity impersonate="true" userName="myName" password="myPassword" />

in the web.config lets me in. I've seen a few people ask questions about why this is, but I've seen no explanation. I'm wondering if it is connected to my problem though.

Answer 1

I wrote a helper class that does it:

public static class ImpersonationUtils
{
    private const int SW_SHOW = 5;
    private const int TOKEN_QUERY = 0x0008;
    private const int TOKEN_DUPLICATE = 0x0002;
    private const int TOKEN_ASSIGN_PRIMARY = 0x0001;
    private const int STARTF_USESHOWWINDOW = 0x00000001;
    private const int STARTF_FORCEONFEEDBACK = 0x00000040;
    private const int CREATE_UNICODE_ENVIRONMENT = 0x00000400;
    private const int TOKEN_IMPERSONATE = 0x0004;
    private const int TOKEN_QUERY_SOURCE = 0x0010;
    private const int TOKEN_ADJUST_PRIVILEGES = 0x0020;
    private const int TOKEN_ADJUST_GROUPS = 0x0040;
    private const int TOKEN_ADJUST_DEFAULT = 0x0080;
    private const int TOKEN_ADJUST_SESSIONID = 0x0100;
    private const int STANDARD_RIGHTS_REQUIRED = 0x000F0000;
    private const int TOKEN_ALL_ACCESS =
        STANDARD_RIGHTS_REQUIRED |
        TOKEN_ASSIGN_PRIMARY |
        TOKEN_DUPLICATE |
        TOKEN_IMPERSONATE |
        TOKEN_QUERY |
        TOKEN_QUERY_SOURCE |
        TOKEN_ADJUST_PRIVILEGES |
        TOKEN_ADJUST_GROUPS |
        TOKEN_ADJUST_DEFAULT |
        TOKEN_ADJUST_SESSIONID;

    [StructLayout(LayoutKind.Sequential)]
    private struct PROCESS_INFORMATION
    {
        public IntPtr hProcess;
        public IntPtr hThread;
        public int dwProcessId;
        public int dwThreadId;
    }

    [StructLayout(LayoutKind.Sequential)]
    private struct SECURITY_ATTRIBUTES
    {
        public int nLength;
        public IntPtr lpSecurityDescriptor;
        public bool bInheritHandle;
    }

    [StructLayout(LayoutKind.Sequential)]
    private struct STARTUPINFO
    {
        public int cb;
        public string lpReserved;
        public string lpDesktop;
        public string lpTitle;
        public int dwX;
        public int dwY;
        public int dwXSize;
        public int dwYSize;
        public int dwXCountChars;
        public int dwYCountChars;
        public int dwFillAttribute;
        public int dwFlags;
        public short wShowWindow;
        public short cbReserved2;
        public IntPtr lpReserved2;
        public IntPtr hStdInput;
        public IntPtr hStdOutput;
        public IntPtr hStdError;
    }

    private enum SECURITY_IMPERSONATION_LEVEL
    {
        SecurityAnonymous,
        SecurityIdentification,
        SecurityImpersonation,
        SecurityDelegation
    }

    private enum TOKEN_TYPE
    {
        TokenPrimary = 1,
        TokenImpersonation
    }

    [DllImport("advapi32.dll", SetLastError = true)]
    private static extern bool CreateProcessAsUser(
        IntPtr hToken,
        string lpApplicationName,
        string lpCommandLine,
        ref SECURITY_ATTRIBUTES lpProcessAttributes,
        ref SECURITY_ATTRIBUTES lpThreadAttributes,
        bool bInheritHandles,
        int dwCreationFlags,
        IntPtr lpEnvironment,
        string lpCurrentDirectory,
        ref STARTUPINFO lpStartupInfo,
        out PROCESS_INFORMATION lpProcessInformation);

    [DllImport("advapi32.dll", SetLastError = true)]
    private static extern bool DuplicateTokenEx(
        IntPtr hExistingToken,
        int dwDesiredAccess,
        ref SECURITY_ATTRIBUTES lpThreadAttributes,
        int ImpersonationLevel,
        int dwTokenType,
        ref IntPtr phNewToken);

    [DllImport("advapi32.dll", SetLastError = true)]
    private static extern bool OpenProcessToken(
        IntPtr ProcessHandle,
        int DesiredAccess,
        ref IntPtr TokenHandle);

    [DllImport("userenv.dll", SetLastError = true)]
    private static extern bool CreateEnvironmentBlock(
            ref IntPtr lpEnvironment,
            IntPtr hToken,
            bool bInherit);

    [DllImport("userenv.dll", SetLastError = true)]
    private static extern bool DestroyEnvironmentBlock(
            IntPtr lpEnvironment);

    [DllImport("kernel32.dll", SetLastError = true)]
    private static extern bool CloseHandle(
        IntPtr hObject);

    private static void LaunchProcessAsUser(string cmdLine, IntPtr token, IntPtr envBlock, int sessionId)
    {
        var pi = new PROCESS_INFORMATION();
        var saProcess = new SECURITY_ATTRIBUTES();
        var saThread = new SECURITY_ATTRIBUTES();
        saProcess.nLength = Marshal.SizeOf(saProcess);
        saThread.nLength = Marshal.SizeOf(saThread);

        var si = new STARTUPINFO();
        si.cb = Marshal.SizeOf(si);
        si.lpDesktop = @"WinSta0\Default";
        si.dwFlags = STARTF_USESHOWWINDOW | STARTF_FORCEONFEEDBACK;
        si.wShowWindow = SW_SHOW;

        if (!CreateProcessAsUser(
            token,
            null,
            cmdLine,
            ref saProcess,
            ref saThread,
            false,
            CREATE_UNICODE_ENVIRONMENT,
            envBlock,
            null,
            ref si,
            out pi))
        {
            throw new Win32Exception(Marshal.GetLastWin32Error(), "CreateProcessAsUser failed");
        }
    }

    private static IDisposable Impersonate(IntPtr token)
    {
        var identity = new WindowsIdentity(token);
        return identity.Impersonate();
    }

    private static IntPtr GetPrimaryToken(Process process)
    {
        var token = IntPtr.Zero;
        var primaryToken = IntPtr.Zero;

        if (OpenProcessToken(process.Handle, TOKEN_DUPLICATE, ref token))
        {
            var sa = new SECURITY_ATTRIBUTES();
            sa.nLength = Marshal.SizeOf(sa);

            if (!DuplicateTokenEx(
                token,
                TOKEN_ALL_ACCESS,
                ref sa,
                (int)SECURITY_IMPERSONATION_LEVEL.SecurityImpersonation,
                (int)TOKEN_TYPE.TokenPrimary,
                ref primaryToken))
            {
                throw new Win32Exception(Marshal.GetLastWin32Error(), "DuplicateTokenEx failed");
            }

            CloseHandle(token);
        }
        else
        {
            throw new Win32Exception(Marshal.GetLastWin32Error(), "OpenProcessToken failed");
        }

        return primaryToken;
    }

    private static IntPtr GetEnvironmentBlock(IntPtr token)
    {
        var envBlock = IntPtr.Zero;
        if (!CreateEnvironmentBlock(ref envBlock, token, false))
        {
            throw new Win32Exception(Marshal.GetLastWin32Error(), "CreateEnvironmentBlock failed");
        }
        return envBlock;
    }

    public static void LaunchAsCurrentUser(string cmdLine)
    {
        var process = Process.GetProcessesByName("explorer").FirstOrDefault();
        if (process != null)
        {
            var token = GetPrimaryToken(process);
            if (token != IntPtr.Zero)
            {
                var envBlock = GetEnvironmentBlock(token);
                if (envBlock != IntPtr.Zero)
                {
                    LaunchProcessAsUser(cmdLine, token, envBlock, process.SessionId);
                    if (!DestroyEnvironmentBlock(envBlock))
                    {
                        throw new Win32Exception(Marshal.GetLastWin32Error(), "DestroyEnvironmentBlock failed");
                    }
                }

                CloseHandle(token);
            }
        }
    }

    public static IDisposable ImpersonateCurrentUser()
    {
        var process = Process.GetProcessesByName("explorer").FirstOrDefault();
        if (process != null)
        {
            var token = GetPrimaryToken(process);
            if (token != IntPtr.Zero)
            {
                return Impersonate(token);
            }
        }

        throw new Exception("Could not find explorer.exe");
    }
}

You can use it like that:

ImpersonationUtils.LaunchAsCurrentUser("notepad");

using (ImpersonationUtils.ImpersonateCurrentUser())
{

}

More explanations and examples you can find here :

Impersonating CurrentUser from SYSTEM

Answer 2

You're probably running into an Impersonation vs. Delegation issue. When you Impersonate a user, you can access local resources as that user, but not remote resources. With Delegation, you can access remote resources too. Try the following:

1. Ensure that Windows authentication is enabled in IIS, and that Anonymous authentication is disabled (Anonymous authentication takes precedence of other authentication mechanisms)
2. Enable <authentication mode="Windows" /> in your web.config to ensure ASP.NET processes Windows authentication (this should only apply to Classic pipeline mode).
3. <identity impersonate="true" /> should be enough.

The other thing you may need to do, is ensure that the the account running the AppPool is permitted to act like a delegate.