Impersonation .Net

1) What is Imperonation?

- Impersonation is the process of assigning a user account to an unknown user.By default, the anonymous access account is named IUSER_machinename. This account can be use to control anonymous users' access to resources on the server. This mechanism allows a server process to run using the security credentials of the client. When the server is impersonating the client, any operations performed by the server are performed using the client's credentials. Impersonation does not allow the server to access remote resources on behalf of the client.

2) How to implement impersonation in an ASP.NET application:

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

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

If you want to impersonate a user on a thread in ASP.NET, you can use one of the following methods, based on your requirments:

• Impersonate the IIS authenticated account or user
• Impersonate a specific user for all the requests of an ASP.NET application
• Impersonate the authenticating user in code
• Impersonate a specific user in code

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

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

2.1) 

Impersonate the IIS Authenticated Account or User

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

2.2) Impersonate a Specific User for All the 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 tag of the Web.config file for that application. For example:

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 does not 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 does not recommend this method.
• Change the account that the Aspnet_wp.exe process runs under to the System account in the configuration section of the Machine.config file.

2.3) 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.

For example: 

VS C#:  

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();

2.4) Impersonate a Specific User in Code

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

using System.Web
using System.Web.Security 
using System.Security.Principal
using System.Runtime.InteropServices


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();
}

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 if the Aspnet_wp.exe process is running on a Microsoft Windows 2000-based computer. The "Act as part of the operating system" privilege is not 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 does not 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 do not 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 configuration section of the Machine.config file.