Authorization and Programmatic Impersonation

If your application uses Forms or Passport authentication, you can impersonate the anonymous Internet user account associated by IIS with your application's virtual directory. You can impersonate the original caller, which might be the anonymous Internet user account or a fixed identity.  

To impersonate the original caller (the IIS authenticated identity), use the following configuration:

<identity impersonate="true" />

To impersonate a fixed identity, use additional userName and password attributes on the <identity> element, but make sure you use Aspnet_setreg.exe to store encrypted credentials in the registry.  

Using Programmatic Impersonation 

If you do not want to impersonate an account for the entire request, you can use programmatic impersonation to impersonate for a portion of the request. For example, you want to use the ASP.NET process account to access you application's primary resources and downstream database, but you need to access an alternate resource, such as another remote database or a remote file share, using an alternate identity.

To do this, use IIS to configure the anonymous user account as the trusted alternate identity. Then use the following code to create an impersonation token using the anonymous account only while you execute your remote resource access code

HttpContext context = HttpContext.Current;
// Get the service provider from the context
IServiceProvider iServiceProvider = context as IServiceProvider;
//Get a Type which represents an HttpContext
Type httpWorkerRequestType = typeof(HttpWorkerRequest);
// Get the HttpWorkerRequest service from the service provider
// NOTE: When trying to get a HttpWorkerRequest type from the HttpContext
// unmanaged code permission is demanded.
HttpWorkerRequest httpWorkerRequest =
iServiceProvider.GetService(httpWorkerRequestType)
as HttpWorkerRequest;
// Get the token passed by IIS
IntPtr ptrUserToken = httpWorkerRequest.GetUserToken();
// Create a WindowsIdentity from the token
WindowsIdentity winIdentity = new WindowsIdentity(ptrUserToken);
// Impersonate the user
Response.Write("Before impersonation: " +WindowsIdentity.GetCurrent().Name + "<br>");
WindowsImpersonationContext impContext = winIdentity.Impersonate();
Response.Write("Impersonating: " + WindowsIdentity.GetCurrent().Name + "<br>");
// Place resource access code here
// Stop impersonating
impContext.Undo();
Response.Write( "After Impersonating: " +WindowsIdentity.GetCurrent().Name + "<br>");

Note: This approach assumes Forms or Passport authentication where your application's virtual directory is configured in IIS to support anonymous access.

If you use this code, use the following <identity> configuration:

<identity impersonate="false" />

Note: The code demands the unmanaged code permission SecurityPermission (SecurityPermissionFlag.UnmanagedCode), which is granted only to fully trusted Web applications.