Win32: How to validate credentials against Active Directory?

Question

It has been asked, and answered for .NET, but now it's time to get an answer for native Win32 code:

How do i validate a Windows username and password?

i asked this question before for managed code. Now it's time for the native solution.

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It needs to be pointed the pitfalls with some of the more commonly proposed solutions:

Invalid Method 1. Query Active Directory with Impersonation

A lot of people suggest querying the Active Directory for something. If an exception is thrown, then you know the credentials are not valid - as is suggested in this stackoverflow question.

There are some serious drawbacks to this approach however:

• You are not only authenticating a domain account, but you are also doing an implicit authorization check. That is, you are reading properties from the AD using an impersonation token. What if the otherwise valid account has no rights to read from the AD? By default all users have read access, but domain policies can be set to disable access permissions for restricted accounts (and or groups).

• Binding against the AD has a serious overhead, the AD schema cache has to be loaded at the client (ADSI cache in the ADSI provider used by DirectoryServices). This is both network, and AD server, resource consuming - and is too expensive for a simple operation like authenticating a user account.

• You're relying on an exception failure for a non-exceptional case, and assuming that means invalid username and password. Other problems (e.g. network failure, AD connectivity failure, memory allocation error, etc) are then mis-intrepreted as authentication failure.

The use of the DirectoryEntry class is .NET is an example of an incorrect way to verify credentials:

Invalid Method 1a - .NET

DirectoryEntry entry = new DirectoryEntry("persuis", "iboyd", "Tr0ub4dor&3");
object nativeObject = entry.NativeObject;

Invalid Method 1b - .NET #2

public static Boolean CheckADUserCredentials(String accountName, String password, String domain)
{
    Boolean result;

    using (DirectoryEntry entry = new DirectoryEntry("LDAP://" + domain, accountName, password))
    {
        using (DirectorySearcher searcher = new DirectorySearcher(entry))
        {
            String filter = String.Format("(&(objectCategory=user)(sAMAccountName={0}))", accountName);
            searcher.Filter = filter;
            try
            {
                SearchResult adsSearchResult = searcher.FindOne();
                result = true;
            }
            catch (DirectoryServicesCOMException ex)
            {
                const int SEC_E_LOGON_DENIED = -2146893044; //0x8009030C;
                if (ex.ExtendedError == SEC_E_LOGON_DENIED)
                {
                    // Failed to authenticate. 
                    result = false;
                }
                else
                {
                    throw;
                }
            }
        }
    }

As well as querying Active Directory through an ADO connection:

Invalid Method 1c - Native Query

connectionString = "Provider=ADsDSOObject;
       User ID=iboyd;Password=Tr0ub4dor&3;
       Encrypt Password=True;Mode=Read;
       Bind Flags=0;ADSI Flag=-2147483648';"

SELECT userAccountControl 
FROM 'LDAP://persuis/DC=stackoverflow,DC=com'
WHERE objectClass='user' and sAMAccountName = 'iboyd'

These both fail even when your credentials are valid, but you do not have permission to view your directory entry:

Invalid Method 2. LogonUser Win32 API

Others have suggested using the LogonUser() API function. This sounds nice, but unfortunatly the calling user sometimes needs a permission ususally only given to the operating system itself:

The process calling LogonUser requires the SE_TCB_NAME privilege. If the calling process does not have this privilege, LogonUser fails and GetLastError returns ERROR_PRIVILEGE_NOT_HELD.

In some cases, the process that calls LogonUser must also have the SE_CHANGE_NOTIFY_NAME privilege enabled; otherwise, LogonUser fails and GetLastError returns ERROR_ACCESS_DENIED. This privilege is not required for the local system account or accounts that are members of the administrators group. By default, SE_CHANGE_NOTIFY_NAME is enabled for all users, but some administrators may disable it for everyone.

Handing out the "Act as a part of the operating system" privelage is not something you want to do willy-nilly - as Microsoft points out in a knowledge base article:

...the process that is calling LogonUser must have the SE_TCB_NAME privilege (in User Manager, this is the "Act as part of the Operating System" right). The SE_TCB_NAME privilege is very powerful and should not be granted to any arbitrary user just so that they can run an application that needs to validate credentials.

Additionally, a call to LogonUser() will fail if a blank password is specified.

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Valid .NET 3.5 Method - PrincipalContext

There is a validation method, only available in .NET 3.5 and newer, that allows authentication by a user without performing an authorization check:

// create a "principal context" - e.g. your domain (could be machine, too)
using(PrincipalContext pc = new PrincipalContext(ContextType.Domain, "stackoverflow.com"))
{
    // validate the credentials
    bool isValid = pc.ValidateCredentials("iboyd", "Tr0ub4dor&3")
}

Unfortunately this code is only available in .NET 3.5 and later.

It's time to find the native equivalent.

Answer 1

Here is Microsoft's recommendation.

As for the other answers, I'm not really sure why you're shooting them down. You are complaining about (relatively edge case) failures while trying to validate credentials, but if you are going to actually do something with those credentials then that operation is just going to fail anyway. If you are not going to actually do something with those credentials, then why do you need to validate them in the first place? It seems like a somewhat contrived situation, but obviously I don't know what you're trying to accomplish.

Answer 2

For the native equivalnt of your valid .NET solution see this MSDN page and ldap_bind

Howerver I think that LogonUser is the right API for the task when use with LOGON32_LOGON_NETWORK. Note that the limitation of SE_CHANGE_NOTIFY_NAME is only for Windows 2000 (so Windows XP and newer do not require this priviledge) and that by default SE_CHANGE_NOTIFY_NAME is enabled for all users. Also the MSDN page says

The SE_TCB_NAME privilege is not required for this function unless you are logging onto a Passport account.

In this case you are logging onto an AD account so SE_TCB_NAME is not required.