How to Disable LM Authentication on Windows NT

[D. Eller]

Probably this information affects Samba,


ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/NT40/hotfixes-postSP3/lm-fix

DOCUMENT:Q147706
TITLE   :How to Disable LM Authentication on Windows NT
PRODUCT :Microsoft Windows NT, Windows 95, Windows for Workgroups 3.11 and LAN Manager 2.2c
PROD/VER:2.2 3.11 4.0 95
OPER/SYS:WINDOWS
KEYWORD :kberrmsg kbfile ntsecurity NTSrvWkst ntstop

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The information in this article applies to:

 - Microsoft Windows NT Workstation version 4.0
 - Microsoft Windows NT Server version 4.0
 - Microsoft LAN Manager version 2.2c
 - Microsoft Windows for Workgroups version 3.11
 - Microsoft Windows 95
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SUMMARY
=======

Windows NT supports the following two types of challenge/response
authentication:

 - LanManager (LM) challenge/response
 - Windows NT challenge/response

To allow access to servers that only support LM authentication, Windows NT
clients currently send both authentication types. Microsoft developed a
patch that supports a new registry key that allows clients to be configured
to send only Windows NT authentication.

MORE INFORMATION
================

Microsoft has posted the patch at the following Internet location:

NOTE: Service Pack 3 must be applied to Windows NT 4.0 prior to applying
this fix.

   ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/NT40/
   hotfixes-postSP3/lm-fix

The new registry parameter was added to the following registry key:

   HKEY_LOCAL_MACHINE\System\CurrentControlSet\control\LSA

   Value: LMCompatibilityLevel
   Value Type: REG_DWORD - Number
   Valid Range: 0,1,2
   Default: 0

   Description: This parameter specifies the type of authentication to be
   used.

   Level 0 Send LM and Windows NT authentication (default).
   Level 1 Send Windows NT authentication and LM authentication only if the
           server requests it.
   Level 2 Never send LM authentication.

If a Windows NT client selects level 2, it cannot connect to servers that
support only LM authentication, such as Windows 95 and Windows for
Workgroups.

NOTE: If the last password change came from a Windows for Workgroups or MS-
DOS LanManager 2.x or earlier client, the data needed for Windows NT
authentication will not be available on the domain controller and a client
selecting level 2 will not be able to connect to Windows NT-based servers.

To eliminate LM authentication with protocols other than remote file
sharing (for example, Microsoft RPC, RAS, Internet Information Server
(IIS), or Internet Explorer -- anything that uses the NTLMSSP), both the
client and the server need to have the hotfix installed.

After installing the hotfix, perform the following steps to configure the
LM compatibility level on a computer running Windows NT Workstation or
Server:

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. From the HKEY_LOCAL_MACHINE subtree, go to the following key:

       HKEY_LOCAL_MACHINE\System\CurrentControlSet\control\LSA

3. Click Add Value on the Edit menu.

4. Add the following two values:

      Value Name: LMCompatibilityLevel
      Data Type: REG_DWORD
      Data:  0 (default), 1, or 2

5. Click OK and then quit Registry Editor.

6. Shut down and restart Windows NT.

STATUS
======

Microsoft has confirmed this to be a problem in Windows NT version 4.0.
A supported fix is now available, but has not been fully regression-tested
and should be applied only to systems experiencing this specific problem.
Unless you are severely impacted by this specific problem, Microsoft
recommends that you wait for the next Service Pack that contains this fix.
Contact Microsoft Technical Support for more information.

Additional query words: 4.00 prodnt win95 wfwg

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