User Impersonation in .NET

• Download source files - 11.32 KB
• Download DLL - 2.43 KB

Description

This sample shows how to switch between security contexts within the same process. It also demostrates how this can be done on a single or multiple threads.

Why It's Here

I was looking through the Message Board on The Code Project site and came across a very interesting question. One of the members was asking how to access a file on another computer in a Workgroup without being prompted to specify user credentials.

I started researching the topic and found the solution in the .NET class library reference. I was impressed by the elegance of the solution, so I decided to have it published here for the benefit of other fellow-developers.

Explanation

The main class within the project is the ImpersonateUser that uses Win32 API function LogonUser and the .NET class WindowsImpersonationContext that represents the Windows user before impersonation.

To impersonate a different user, the class follows three distincts steps as listed below.

Step 1

Using LogonUser, it creates a new security token and obtains a handle to it.

bool returnValue = LogonUser(
    userName,
    domainName,
    password,
    LOGON32_LOGON_INTERACTIVE,
    LOGON32_PROVIDER_DEFAULT,
    ref tokenHandle);           // tokenHandle is a new security token

Step 2

Using this handle, it creates an instance of the WindowsIdentity class.

WindowsIdentity newId = new WindowsIdentity(tokenHandle);

Step 3

It then uses the WindowsIdentity.Impersonate() method that returns WindowsImpersonationContext to assign the new security token to the current WindowsImpersonationContext.

// Declared in the ImpersonateUser class
private static WindowsImpersonationContext impersonatedUser;

// used within the class Impersonate method
impersonatedUser = newId.Impersonate();

Main Class

Here's the code for the ImpersonateUser class:

public class ImpersonateUser
{
    [DllImport("advapi32.dll", SetLastError = true)]
    public static extern bool LogonUser(
            String lpszUsername,
            String lpszDomain,
            String lpszPassword,
            int dwLogonType,
            int dwLogonProvider,
            ref IntPtr phToken);

    [DllImport("kernel32.dll", CharSet = CharSet.Auto)]
    public extern static bool CloseHandle(IntPtr handle);

    private static IntPtr tokenHandle = new IntPtr(0);
    private static WindowsImpersonationContext impersonatedUser;

    // If you incorporate this code into a DLL, be sure to demand that it
    // runs with FullTrust.
    [PermissionSetAttribute(SecurityAction.Demand, Name = "FullTrust")]
    public void Impersonate(string domainName, string userName, string password) {
    try {
        // Use the unmanaged LogonUser function to get the user token for
        // the specified user, domain, and password.
        const int LOGON32_PROVIDER_DEFAULT = 0;
        // Passing this parameter causes LogonUser to create a primary token.
        const int LOGON32_LOGON_INTERACTIVE = 2;
        tokenHandle = IntPtr.Zero;
            
        // ---- Step - 1 
        // Call LogonUser to obtain a handle to an access token.
        bool returnValue = LogonUser(
                            userName,
                            domainName,
                            password,
                            LOGON32_LOGON_INTERACTIVE,
                            LOGON32_PROVIDER_DEFAULT,
                            ref tokenHandle);         // tokenHandle - new security token

        if (false == returnValue) {
            int ret = Marshal.GetLastWin32Error();
            Console.WriteLine("LogonUser call failed with error code : " + ret);
            throw new System.ComponentModel.Win32Exception(ret);
        }

        // ---- Step - 2 
        WindowsIdentity newId = new WindowsIdentity(tokenHandle);
                
        // ---- Step - 3 
        impersonatedUser = newId.Impersonate();
    }
    catch (Exception ex) {
    Console.WriteLine("Exception occurred. " + ex.Message);
    }
}

    // Stops impersonation
    public void Undo() {
        impersonatedUser.Undo();
        // Free the tokens.
        if (tokenHandle != IntPtr.Zero)
        CloseHandle(tokenHandle);
    }
}

Example

ImpersonateUser iU = new ImpersonateUser();
// TODO: Replace credentials
iU.Impersonate("remoteMachine", "userID", "password");
   
// Next lines of code will run in the security context of the user specified above
// ....
   
// Revert back to the previous user identity
iU.Undo();

Please download the source code and run the demo project - it will give you a better idea how easy it is to use the class.

How To Use ImpersUsr.dll

1. Download and extract ImpersUsr.dll
2. Create a Console type project
3. Add a reference to ImpersUsr.dll
4. Instantiate the ImpersonateUser class
5. Call ImpersonateUser.Impersonate(..) method
6. Code whatever you need to run in the new security context
7. Call ImpersonateUser.Undo() method to stop impersonating

Multithreading

Please download the source to see how to start a separate thread in a different security context.

History

• 6^th June, 2006: Initial post