Restricting Information Available to Anonymous Logon Users

• Microsoft Windows NT Workstation version 4.0 Service Pack 3
• Microsoft Windows NT Server version 4.0 Service Pack 3
• Microsoft Windows NT Workstation version 3.51
• Microsoft Windows NT Server version 3.51

SUMMARY

Windows NT has a feature where anonymous logon users can list domain user names and enumerate share names. Customers who want enhanced security have requested the ability to optionally restrict this functionality. Windows NT 4.0 Service Pack 3 and a hotfix for Windows NT 3.51 provide a mechanism for administrators to restrict the ability for anonymous logon users (also known as NULL session connections) to list account names and enumerate share names. Listing account names from Domain Controllers is required by the Windows NT ACL editor, for example, to obtain the list of users and groups to select who a user wants to grant access rights. Listing account names is also used by Windows NT Explorer to select from list of users and groups to grant access to a share.

MORE INFORMATION

Windows NT networks based on a single Windows NT domain will always be able to authenticate connections to list domain account information. Windows NT networks that use multiple domains may require anonymous user logon to list account information. A brief example shows how anonymous connections are used. Consider two Windows NT domains, an account domain and a resource domain. The resource domain has a one-way trust relationship with the account domain. That is, the resource domain "trusts" the account domain, but the account domain does not trust the resource domain. Users from the account domain can authenticate and access resources in the resource domain based on the one-way trust. Suppose an administrator in the resource domain wants to grant access to a file to a user from the account domain. They will want to obtain the list of users and groups from the account domain to select a user/group to grant access rights. Since the account domain does not trust the resource domain, the administrator request to obtain the list of users and groups from the resource domain cannot be authenticated. The connection is made using a NULL session to obtain the list of account domain users.

There are similar situations where obtaining account names using an anonymous connection allows the user interface tools, including Windows NT Explorer, User Manager, and ACL editor, to administer and manage access control information across multiple Windows NT domains. Another example is using User Manager in the resource domain to add users from the trusted account domain to a local group. One way to add the account domain user to a local group in the resource domain is to manually enter a known domain\username to add access without getting the complete list of names from the account domain. Another approach is to logon to the system in the resource domain using an account in the trusted account domain.

Windows NT environments that want to restrict anonymous connections from listing account names can control this operation after installing Windows NT 4.0 Service Pack 3 or the Windows NT 3.51 hotfix.

After installation of Windows NT 4.0 Service Pack 3 or the Windows NT 3.51 hotfix, administrators who want to require only authenticated users to list account names, and exclude anonymous connections from doing so, need to make the following change to the registry:

WARNING: Using Registry Editor incorrectly can cause serious, system-wide problems that may require you to reinstall Windows NT to correct them. Microsoft cannot guarantee that any problems resulting from the use of Registry Editor can be solved. Use this tool at your own risk.

1. Run Registry Editor (Regedt32.exe).

2. Go to the following key in the registry:

         HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\LSA
   

3. On the Edit menu, click Add Value and use the following entry:

         Value Name: RestrictAnonymous
         Data Type:  REG_DWORD
         Value:      1
   
   

4. Exit the Registry Editor and restart the computer for the change to take effect.

Windows NT networks using a multiple domain model can restrict anonymous connections without loss of functionality. The initial steps in planning to disable anonymous connections is for administrators in resource domains to add members of trusted account domains to specific local groups as needed before changing the value for the LSA RestrictAnonymous registry entry. Users logged on using accounts from trusted account domains will continue to use authenticated connections to obtain list of account names to manage security access control.

Restricting Anonymous List of Share Names

The Server service that provides remote file access to share resources will also use the LSA registry value, RestrictAnonymous, to control whether anonymous connections can obtain a list of share names. Therefore, administrators can set the value of a single registry configuration entry to define how the system responds to enumeration requests by anonymous logons.

Restricting Anonymous Remote Registry Access

Installation of Windows NT 4.0 Service Pack 3 or the Windows NT 3.51 hotfix removes the ability for anonymous users to connect to the registry remotely. Anonymous users cannot connect to the registry and cannot read or write any registry data. As a reminder, Windows NT 4.0 restricts remote access to the registry by domain users using the access control list on the registry key:

   "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet
   \SecurePipeServers\winreg"

   ARTICLE-ID: Q155363
   TITLE     : How To Regulate Network Access to the Windows NT Registry

Authenticated Users Built-in Group

A new built-in group is created when installing Windows NT 4.0 Service Pack 3 or the Windows NT 3.51 hotfix known as "Authenticated Users." The Authenticated Users group is similar to the "Everyone" group, except for one important difference: anonymous logon users (or NULL session connections) are never members of the Authenticated Users group. The built-in Security Identifier for Authenticated Users is S-1-5-11. Authenticated network connections from any account in the server's Windows NT domain, or any domain trusted by the server's domain, is identified as an Authenticated User. The Authenticated Users group is available for granting access rights to resources in the security ACL editor. Windows NT 4.0 Service Pack 3 and the Windows NT 3.51 hotfix do not modify any access control lists to change access rights granted to Everyone to use Authenticated Users.

Windows NT 3.51 Hotfix

The Windows NT 3.51 hotfix has been posted to the following Internet location:

   ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/NT351/
   hotfixes-postSP5/sec-fix