Impersonation class

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Impersonation class

Aug

2006

Impersonation class

Een klant liep tegen een rechten probleem aan. De ingelogde gebruiker waaronder de applicatie werd uitgevoerd had niet voldoende rechten. Afdeling systeem beheer wou hier liever geen uitzondering voor maken en gaf de voorkeur aan een apparte account waaronder deze code uitgevoerd kon worden zonder dat de gebruiker onder deze account ingelogd diende te zijn. Kort gezegt, het is dus de bedoeling dat sommige code uitgevoerd word onder een anderen user account.

Als impersonation het eerste is wat in je opkomt zit je goed. Impersonation, oftewel het uitvoeren van code onder een andere gebruiker, is enorm handig als je iets wilt doen waar de huidige gebruiker niet genoeg rechten heeft. Dit kan bijvoorbeeld zijn dat een bepaalde directory uit te lezen of een bepaalde registery key value te kunnen wijzigen.

Het FCL is enorm groot en bevat veel classes die taken opzich kunnen nemen. Helaas bestaat er nog geen class die de volledige impersonation taak op zich kan nemen en we zullen daarom gebruik moeten maken van P/Invoke om Win32 API calls te kunnen doen. De WindowsIdentity class uit de System.Security.Principal namespace kan een impersonation uitvoeren als jij hem de usertoken leverd. Dit is al een probleem, want deze is zonder een Win32 API niet te verkrijgen. In iedergeval niet vanuit een Domain\Username en password.

Dan maar zelf aan de slag! Zoals ik al aangaf kunnen we als we de usertoken weten een heel eind komen met de WindowsIdentity class of met de ImpersonateLoggedOnUser method uit advapi32.dll.

Aan de slag

Het eerste en het meest belangrijkste wat we nodig hebben is the usertoken van de gebruiker waaronder we tijdelijk code willen uitvoeren. Hiervoor gebruiken we de LogonUser method uit advapi32.dll en zullen de method signature eerst moeten specificeren zodat de compiler weet wat we willen callen en waar dit te vinden is. Voor de LogonUser method gebruiken we het volgende (in de volledige code file vind je ook de enum declaraties):

[DllImport(“advapi32.dll”, SetLastError=true)]
private static extern bool LogonUser( string username, string domain,
                                      string password, LogonType logonType,
                                      LogonProvider logonProvider,
                                      out IntPtr userToken );

Via deze method krijgen we de usertoken, de pointer naar de handle van de gebruiker, waarmee we de impersonation kunnen uitvoeren.
De impersonation is daarna vrij simpel. Door een instantie van het WindowsIdentity te creeëren en de Impersonate method aan te roepen:

// Create windows identity from the token and impersonate the user.
WindowsIdentity identity = new WindowsIdentity( userTokenDuplication );
WindowsImpersonationContext impersonationContext = identity.Impersonate();
 
try
{
    // Run code withing the impersonation.
}
finally
{
    // Revert to previous user.
    impersonationContext.Undo();
}

Omdat ik een herbruikbare class wil maken en ook een class die makelijk te gebruiken is, heb ik er voor gekozen om de impersonation direct in the constructor van de class uitgevoerd en het terug te draaien in de Dispose method. Door de class te implementeren met de IDisposeable interface is hij te gebruiken in een using statemen. Dit resulteerd in een easy-to-use constructie:

using( new Impersonation( “pjvandesande”, “born2code.net”, “password” ) )
{
    // Voer code uit onder anderen account.
}

De volledige code

De volledige source van de Impersonation class:

using System;
using System.ComponentModel;
using System.Runtime.InteropServices;
using System.Security.Principal;
 
namespace Born2Code.Net
{
    public enum LogonType : int
    {
        /// <summary>
        /// This logon type is intended for users who will be interactively using the computer, such as a user being logged on  
        /// by a terminal server, remote shell, or similar process.
        /// This logon type has the additional expense of caching logon information for disconnected operations;
        /// therefore, it is inappropriate for some client/server applications,
        /// such as a mail server.
        /// </summary>
        Interactive = 2,
 
        /// <summary>
        /// This logon type is intended for high performance servers to authenticate plaintext passwords.
        /// The LogonUser function does not cache credentials for this logon type.
        /// </summary>
        Network = 3,
 
        /// <summary>
        /// This logon type is intended for batch servers, where processes may be executing on behalf of a user without
        /// their direct intervention. This type is also for higher performance servers that process many plaintext
        /// authentication attempts at a time, such as mail or Web servers.
        /// The LogonUser function does not cache credentials for this logon type.
        /// </summary>
        Batch = 4,
 
        /// <summary>
        /// Indicates a service-type logon. The account provided must have the service privilege enabled.
        /// </summary>
        Service = 5,
 
        /// <summary>
        /// This logon type is for GINA DLLs that log on users who will be interactively using the computer.
        /// This logon type can generate a unique audit record that shows when the workstation was unlocked.
        /// </summary>
        Unlock = 7,
 
        /// <summary>
        /// This logon type preserves the name and password in the authentication package, which allows the server to make
        /// connections to other network servers while impersonating the client. A server can accept plaintext credentials
        /// from a client, call LogonUser, verify that the user can access the system across the network, and still
        /// communicate with other servers.
        /// NOTE: Windows NT:  This value is not supported.
        /// </summary>
        NetworkCleartText = 8,
 
        /// <summary>
        /// This logon type allows the caller to clone its current token and specify new credentials for outbound connections.
        /// The new logon session has the same local identifier but uses different credentials for other network connections.
        /// NOTE: This logon type is supported only by the LOGON32_PROVIDER_WINNT50 logon provider.
        /// NOTE: Windows NT:  This value is not supported.
        /// </summary>
        NewCredentials = 9,
    }
 
    public enum LogonProvider : int
    {
        /// <summary>
        /// Use the standard logon provider for the system.
        /// The default security provider is negotiate, unless you pass NULL for the domain name and the user name
        /// is not in UPN format. In this case, the default provider is NTLM.
        /// NOTE: Windows 2000/NT:   The default security provider is NTLM.
        /// </summary>
        Default = 0,
    }
 
    public class Impersonation : IDisposable
    {
        #region Dll Imports
        /// <summary>
        /// Closes an open object handle.
        /// </summary>
        /// <param name="&rdquo;hObject&rdquo;">A handle to an open object.</param>
        /// <returns><c>True</c> when succeeded; otherwise <c>false</c>.</returns>
        [DllImport(“kernel32.dll”)]
        private static extern Boolean CloseHandle(IntPtr hObject);
 
        /// <summary>
        /// Attempts to log a user on to the local computer.
        /// </summary>
        /// <param name="&rdquo;username&rdquo;">This is the name of the user account to log on to. 
        /// If you use the user principal name (UPN) format, user@DNSdomainname, the 
        /// domain parameter must be <c>null</c>.</param>
        /// <param name="&rdquo;domain&rdquo;">Specifies the name of the domain or server whose 
        /// account database contains the lpszUsername account. If this parameter 
        /// is <c>null</c>, the user name must be specified in UPN format. If this 
        /// parameter is “.”, the function validates the account by using only the 
        /// local account database.</param>
        /// <param name="&rdquo;password&rdquo;">The password</param>
        /// <param name="&rdquo;logonType&rdquo;">The logon type</param>
        /// <param name="&rdquo;logonProvider&rdquo;">The logon provides</param>
        /// <param name="&rdquo;userToken&rdquo;">The out parameter that will contain the user 
        /// token when method succeeds.</param>
        /// <returns><c>True</c> when succeeded; otherwise <c>false</c>.</returns>
        [DllImport(“advapi32.dll”, CharSet=CharSet.Auto, SetLastError=true)]
        private static extern bool LogonUser( string username, string domain,
                                              string password, LogonType logonType,
                                              LogonProvider logonProvider,
                                              out IntPtr userToken );
 
        /// <summary>
        /// Creates a new access token that duplicates one already in existence.
        /// </summary>
        /// <param name="&rdquo;token&rdquo;">Handle to an access token.</param>
        /// <param name="&rdquo;impersonationLevel&rdquo;">The impersonation level.</param>
        /// <param name="&rdquo;duplication&rdquo;">Reference to the token to duplicate.</param>
        /// <returns></returns>
        [DllImport(“advapi32.dll”, CharSet=CharSet.Auto, SetLastError=true)]
        private static extern bool DuplicateToken( IntPtr token, int impersonationLevel,
            ref IntPtr duplication );
 
        /// <summary>
        /// The ImpersonateLoggedOnUser function lets the calling thread impersonate the 
        /// security context of a logged-on user. The user is represented by a token handle.
        /// </summary>
        /// <param name="&rdquo;userToken&rdquo;">Handle to a primary or impersonation access token that represents a logged-on user.</param>
        /// <returns>If the function succeeds, the return value is nonzero.</returns>
        [DllImport(“advapi32.dll”, SetLastError=true)]
        static extern bool ImpersonateLoggedOnUser( IntPtr userToken );
        #endregion
 
        #region Private members
        /// <summary>
        /// <c>true</c> if disposed; otherwise, <c>false</c>.
        /// </summary>
        private bool _disposed;
 
        /// <summary>
        /// Holds the created impersonation context and will be used
        /// for reverting to previous user.
        /// </summary>
        private WindowsImpersonationContext _impersonationContext;
        #endregion
 
        #region Ctor &amp; Dtor
        /// <summary>
        /// Initializes a new instance of the <see cref="&rdquo;Impersonation&rdquo;/"> class and
        /// impersonates with the specified credentials.
        /// </summary>
        /// <param name="&rdquo;username&rdquo;">his is the name of the user account to log on 
        /// to. If you use the user principal name (UPN) format, 
        /// user@DNS_domain_name, the lpszDomain parameter must be <c>null</c>.</param>
        /// <param name="&rdquo;domain&rdquo;">The name of the domain or server whose account 
        /// database contains the lpszUsername account. If this parameter is 
        /// <c>null</c>, the user name must be specified in UPN format. If this 
        /// parameter is “.”, the function validates the account by using only the 
        /// local account database.</param>
        /// <param name="&rdquo;password&rdquo;">The plaintext password for the user account.</param>
        public Impersonation( String username, String domain, String password )
        {
            IntPtr userToken = IntPtr.Zero;
            IntPtr userTokenDuplication = IntPtr.Zero;
 
            // Logon with user and get token.
            bool loggedOn = LogonUser( username, domain, password,
                LogonType.Interactive, LogonProvider.Default,
                out userToken );
 
            if( loggedOn )
            {
                try
                {
                    // Create a duplication of the usertoken, this is a solution
                    // for the known bug that is published under KB article Q319615.
                    if( DuplicateToken( userToken, 2, ref userTokenDuplication ) )
                    {
                        // Create windows identity from the token and impersonate the user.
                        WindowsIdentity identity = new WindowsIdentity( userTokenDuplication );
                        _impersonationContext = identity.Impersonate();
                    }
                    else
                    {
                        // Token duplication failed!
                        // Use the default ctor overload
                        // that will use Mashal.GetLastWin32Error();
                        // to create the exceptions details.
                        throw new Win32Exception();
                    }
                }
                finally
                {
                    // Close usertoken handle duplication when created.
                    if( !userTokenDuplication.Equals( IntPtr.Zero ) )
                    {
                        // Closes the handle of the user.
                        CloseHandle( userTokenDuplication );
                        userTokenDuplication = IntPtr.Zero;
                    }
 
                    // Close usertoken handle when created.
                    if( !userToken.Equals( IntPtr.Zero ) )
                    {
                        // Closes the handle of the user.
                        CloseHandle( userToken );
                        userToken = IntPtr.Zero;
                    }
                }
            }
            else
            {
                // Logon failed!
                // Use the default ctor overload that 
                // will use Mashal.GetLastWin32Error();
                // to create the exceptions details.
                throw new Win32Exception();
            }
        }
 
        /// <summary>
        /// Releases unmanaged resources and performs other cleanup operations before the
        /// <see cref="&rdquo;Born2Code.Net.Impersonation&rdquo;/"> is reclaimed by garbage collection.
        /// </summary>
        ~Impersonation()
        {
            Dispose( false );
        }
        #endregion
 
        #region Public methods
        /// <summary>
        /// Reverts to the previous user.
        /// </summary>
        public void Revert()
        {
            if( _impersonationContext != null )
            {
                // Revert to previour user.
                _impersonationContext.Undo();
                _impersonationContext = null;
            }
        }
        #endregion
 
        #region IDisposable implementation.
        /// <summary>
        /// Performs application-defined tasks associated with freeing, releasing, or
        /// resetting unmanaged resources and will revent to the previous user when
        /// the impersonation still exists.
        /// </summary>
        public void Dispose()
        {
            Dispose( true );
            GC.SuppressFinalize(this);
        }
 
        /// <summary>
        /// Performs application-defined tasks associated with freeing, releasing, or
        /// resetting unmanaged resources and will revent to the previous user when
        /// the impersonation still exists.
        /// </summary>
        /// <param name="&rdquo;disposing&rdquo;">Specify <c>true</c> when calling the method directly
        /// or indirectly by a user’s code; Otherwise <c>false</c>.
        protected virtual void Dispose( bool disposing )
        {
            if( !_disposed )
            {
                Revert();
 
                _disposed = true;
            }
        }
        #endregion
    }
}