Implement impersonation in an ASP.NET application

 

This article introduces how to implement impersonation by modifying the Web.config file and running a particular section of code.

It refers to the following Microsoft .NET Framework Class Library namespaces:

• System.Web.Security
• System.Security.Principal
• System.Runtime.InteropServices

You can use the following code to determine what user the thread is executing as:

System.Security.Principal.WindowsIdentity.GetCurrent().Name

Impersonate IIS authenticated account or user

To impersonate the Internet Information Services (IIS) authenticating user on every request for every page in an ASP.NET application, you must include an <identity> tag in the Web.config file of this application and set the impersonate attribute to true. For example:

<identity impersonate="true" />

Impersonate a specific user for all requests of an ASP.NET application

To impersonate a specific user for all the requests on all pages of an ASP.NET application, you can specify the userName and password attributes in the <identity> tag of the Web.config file for that application. For example:

XMLCopy

 

<identity impersonate="true" userName="accountname" password="password" />

 Note

The identity of the process that impersonates a specific user on a thread must have the Act as part of the operating system privilege. By default, the Aspnet_wp.exe process runs under a computer account named ASPNET. However, this account doesn't have the required privileges to impersonate a specific user. You receive an error message if you try to impersonate a specific user. This information applies only to the .NET Framework 1.0. This privilege is not required for the .NET Framework 1.1.

To work around this problem, use one of the following methods:

• Grant the Act as part of the operating system privilege to the ASPNET account (the least privileged account).

   Note

  Although you can use this method to work around the problem, Microsoft doesn't recommend this method.

• Change the account that the Aspnet_wp.exe process runs under to the System account in the <processModel> configuration section of the Machine.config file.

Impersonate the authenticating user in code

To impersonate the authenticating user (User.Identity) only when you run a particular section of code, you can use the code to follow. This method requires that the authenticating user identity is of type WindowsIdentity.

 

• System.Security.Principal.WindowsImpersonationContext impersonationContext;

• impersonationContext = ((System.Security.Principal.WindowsIdentity)User.Identity).Impersonate();
  //Insert your code that runs under the security context of the authenticating user here.
  impersonationContext.Undo();
  

Impersonate a specific user in code

To impersonate a specific user only when you run a particular section of code, use the following code:

<%@ Page Language="C#"%>
<%@ Import Namespace = "System.Web" %>
<%@ Import Namespace = "System.Web.Security" %>
<%@ Import Namespace = "System.Security.Principal" %>
<%@ Import Namespace = "System.Runtime.InteropServices" %>

<script runat=server>
public const int LOGON32_LOGON_INTERACTIVE = 2;
public const int LOGON32_PROVIDER_DEFAULT = 0;

WindowsImpersonationContext impersonationContext;

[DllImport("advapi32.dll")]
public static extern int LogonUserA(String lpszUserName,
String lpszDomain,
String lpszPassword,
int dwLogonType,
int dwLogonProvider,
ref IntPtr phToken);
[DllImport("advapi32.dll", CharSet=CharSet.Auto, SetLastError=true)]
public static extern int DuplicateToken(IntPtr hToken,
int impersonationLevel,
ref IntPtr hNewToken);

[DllImport("advapi32.dll", CharSet=CharSet.Auto, SetLastError=true)]
public static extern bool RevertToSelf();

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

public void Page_Load(Object s, EventArgs e)
{
    if(impersonateValidUser("username", "domain", "password"))
    {
        //Insert your code that runs under the security context of a specific user here.
        undoImpersonation();
    }
    else
    {
        //Your impersonation failed. Therefore, include a fail-safe mechanism here.
    }
}

private bool impersonateValidUser(String userName, String domain, String password)
{
    WindowsIdentity tempWindowsIdentity;
    IntPtr token = IntPtr.Zero;
    IntPtr tokenDuplicate = IntPtr.Zero;

    if(RevertToSelf())
    {
        if(LogonUserA(userName, domain, password, LOGON32_LOGON_INTERACTIVE,
        LOGON32_PROVIDER_DEFAULT, ref token)!= 0)
        {
            if(DuplicateToken(token, 2, ref tokenDuplicate)!= 0) 
            {
                tempWindowsIdentity = new WindowsIdentity(tokenDuplicate);
                impersonationContext = tempWindowsIdentity.Impersonate();
                if (impersonationContext != null)
                {
                    CloseHandle(token);
                    CloseHandle(tokenDuplicate);
                    return true;
                }
            }
        }
    }
    if(token!= IntPtr.Zero)
        CloseHandle(token);
    if(tokenDuplicate!=IntPtr.Zero)
        CloseHandle(tokenDuplicate);
    return false;
}

private void undoImpersonation()
{
    impersonationContext.Undo();
}
</script>

The identity of the process that impersonates a specific user on a thread must have the Act as part of the operating system privilege if the Aspnet_wp.exe process is running on a Windows 2000-based computer. The Act as part of the operating system privilege isn't required if the Aspnet_wp.exe process is running on a Windows XP-based computer or on a Windows Server 2003-based computer. By default, the Aspnet_wp.exe process runs under a computer account named ASPNET. However, this account doesn't have the required privileges to impersonate a specific user. You receive an error message if you try to impersonate a specific user.

To work around this problem, use one of the following methods:

• Grant the Act as part of the operating system privilege to the ASPNET account.

   Note

  We don't recommend this method to work around the problem.

• Change the account that the Aspnet_wp.exe process runs under to the System account in the <processModel> configuration section of the Machine.config file.