Microsoft KB Archive/926642

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Article ID: 926642

Article Last Modified on 10/11/2007



APPLIES TO

  • Microsoft Windows Server 2003 SP1, when used with:
    • Microsoft Windows Server 2003, Standard Edition (32-bit x86)
    • Microsoft Windows Server 2003, Enterprise Edition
    • Microsoft Windows Server 2003, Standard x64 Edition
    • Microsoft Windows Server 2003, Enterprise x64 Edition
    • Microsoft Windows Server 2003, Enterprise Edition for Itanium-based Systems
  • Microsoft Windows XP Service Pack 2



Important This article contains information that shows you how to help lower security settings or how to turn off security features on a computer. You can make these changes to work around a specific problem. Before you make these changes, we recommend that you evaluate the risks that are associated with implementing this workaround in your particular environment. If you implement this workaround, take any appropriate additional steps to help protect your system.

Important This article contains information about how to modify the registry. Make sure to back up the registry before you modify it. Make sure that you know how to restore the registry if a problem occurs. For more information about how to back up, restore, and modify the registry, click the following article number to view the article in the Microsoft Knowledge Base:

256986 Description of the Microsoft Windows registry


SYMPTOMS

Consider the following scenario. You install Microsoft Windows Server 2003 Service Pack 1 (SP1) on a Windows Server 2003-based computer. After you do this, you experience authentication issues when you try to access a server locally by using its fully qualified domain name (FQDN) or its CNAME alias in the following Universal Naming Convention (UNC) path:

\\servername\sharename


In this scenario, you experience one of the following symptoms:

  • You receive repeated logon windows.
  • You receive an "Access denied" error message.
  • You receive a "No network provider accepted the given network path" error message.
  • Event ID 537 is logged in the Security event log.

Note You can access the server by using its FQDN or its CNAME alias from another computer in the network other than this computer on which you installed Windows Server 2003 SP1. Additionally, you can access the server on the local computer by using the following paths:

  • \\IPaddress-of-local-computer
  • \\Netbiosnameor \\ComputerName


CAUSE

This problem occurs because Windows Server 2003 SP1 includes a new security feature named loopback check functionality. By default, loopback check functionality is turned on in Windows Server 2003 SP1, and the value of the DisableLoopbackCheck registry entry is set to 0 (zero).

Note The loopback check functionality is stored in the following registry subkey:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\DisableLoopbackCheck


RESOLUTION

Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall your operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.

Warning This workaround may make your computer or your network more vulnerable to attack by malicious users or by malicious software such as viruses. We do not recommend this workaround but are providing this information so that you can implement this workaround at your own discretion. Use this workaround at your own risk.

To resolve this problem, set the DisableStrictNameChecking registry entry to 1. This procedure is described in the Knowledge Base article 281308 that is mentioned in the "References" section. Then use either of the following methods, as appropriate for your situation.

Method 1: Disable the authentication loopback check

Re-enable the behavior that exists in Windows Server 2003 by setting the DisableLoopbackCheck registry entry in the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa registry subkey to 1. To set the DisableLoopbackCheck registry entry to 1, follow these steps on the client computer:

  1. Click Start, click Run, type regedit, and then click OK.
  2. Locate and then click the following registry subkey: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa
  3. Right-click Lsa, point to New, and then click DWORD Value.
  4. Type DisableLoopbackCheck, and then press ENTER.
  5. Right-click DisableLoopbackCheck, and then click Modify.
  6. In the Value data box, type 1, and then click OK.
  7. Exit Registry Editor.
  8. Restart the computer.

Note You must restart the server for this change to take effect. By default, loopback check functionality is turned on in Windows Server 2003 SP1, and the DisableLoopbackCheck registry entry is set to 0 (zero). The security is reduced when you disable the authentication loopback check, and you open the Windows Server 2003 server for man-in-the-middle (MITM) attacks on NTLM.

Method 2: Create the Local Security Authority host names that can be referenced in an NTLM authentication request

To do this, follow these steps for all the nodes on the client computer:

  1. Click Start, click Run, type regedit, and then click OK.
  2. Locate and then click the following registry subkey: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0
  3. Right-click MSV1_0, point to New, and then click Multi-String Value.
  4. In the Name column, type BackConnectionHostNames, and then press ENTER.
  5. Right-click BackConnectionHostNames, and then click Modify.
  6. In the Value data box, type the CNAME or the DNS alias, that is used for the local shares on the computer, and then click OK.


Note Type each host name on a separate line.

Note If the BackConnectionHostNames registry entry exists as a REG_DWORD type, you have to delete the BackConnectionHostNames registry entry.

  1. Exit Registry Editor, and then restart the computer.


STATUS

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.

For more information, click the following article number to view the article in the Microsoft Knowledge Base:

281308 Connecting to SMB share on a Windows 2000-based computer or a Windows Server 2003-based computer may not work with an alias name



Additional query words: STATUS_DOWNGRADE_DETECTED localhost DisableStrictNameChecking OptionalNames System error 52 has occurred A duplicate name exists on the network System error 1326 has occurred ERROR_LOGON_FAILURE Logon failure unknown user name or bad password Man-in-the-Middle Attack

Keywords: kbtshoot kbprb kbexpertiseinter KB926642