Network security: Restrict NTLM: Add remote server exceptions for NTLM authentication
Updated: November 15, 2012
Applies To: Windows 7, Windows Server 2003, Windows Server 2008, Windows Server 2008 R2, Windows Vista, Windows XP
This security policy reference topic for the IT professional describes the best practices, location, values, management aspects, and security considerations for this policy setting.
Reference
The Network security: Restrict NTLM: Add server exceptions for NTLM authentication policy setting allows you to create an exception list of remote servers to which client computers are allowed to use NTLM authentication if the Network Security: Restrict NTLM: Outgoing NTLM traffic to remote servers policy setting is configured.
If you configure this policy setting, you can define a list of remote servers to which client computers are allowed to use NTLM authentication.
If you do not configure this policy setting, no exceptions will be applied, and if Network Security: Restrict NTLM: Outgoing NTLM traffic to remote servers is enabled, NTLM authentication attempts from the client computers will fail.
List the NetBIOS server names that are used by the applications as the naming format, one per line. To ensure exceptions, the names that are used by all applications need to be in the list. A single asterisk (*) can be used anywhere in the string as a wildcard character.
Possible values
User-defined list of remote servers
When you enter a list of remote servers to which clients are allowed to use NTLM authentication, the policy is defined and enabled.
Not defined
If you do not configure this policy setting by defining a list of servers, the policy is undefined and no exceptions will be applied.
Best practices
First enforce the Network Security: Restrict NTLM: Audit Incoming NTLM Traffic or Network Security: Restrict NTLM: Audit NTLM authentication in this domain policy setting and then review the Operational log to understand which servers are involved in these authentication attempts so you can decide which servers to exempt.
After you have set the server exception list, enforce the Network Security: Restrict NTLM: Audit Incoming NTLM Traffic or Network Security: Restrict NTLM: Audit NTLM authentication in this domain policy setting and then review the Operational log again before setting the policies to block NTLM traffic.
Location
GPO_name**\Computer Configuration\Windows Settings\Security Settings\Local Policies\Security Options**
Default values
| Server type or GPO | Default value |
|---|---|
Default domain policy |
Not defined |
Default domain controller policy |
Not defined |
Stand-alone server default settings |
Not defined |
Domain controller effective default settings |
Not defined |
Member server effective default settings |
Not defined |
Client computer effective default settings |
Not defined |
Operating system version differences
This policy setting was introduced in Windows Server 2008 R2 and Windows 7.
Policy management
This section describes the features and tools that are available to help you manage this policy.
Restart requirement
None. Changes to this policy become effective without a computer restart when they are saved locally or distributed through Group Policy.
Group Policy
Setting and deploying this policy through Group Policy takes precedence over the setting on the local computer. If the Group Policy setting is set to Not Configured, local settings will apply.
Auditing
View the Operational log to see if your server exception list is functioning as intended. Audit and block events are recorded on this computer in the Operational log located in Applications and Services Log\Microsoft\Windows\NTLM.
There are no Security Audit policies that can be configured to view output from this policy.
Security considerations
This section describes how an attacker might exploit a feature or its configuration, how to implement the countermeasure, and the possible negative consequences of countermeasure implementation.
Vulnerability
When it has been determined that the NTLM authentication protocol should not be used from a client computer to any remote servers because you are required to use a more secure protocol such as Kerberos, there might be some client applications that still use NTLM. If so, and you set Network Security: Restrict NTLM: Outgoing NTLM traffic to remote servers to any of the deny options, those applications will fail because the outbound NTLM authentication traffic from the client computer will be blocked.
If you define an exception list of servers to which client computers are allowed to use NTLM authentication, then NTLM authentication traffic will continue to flow between those client applications and servers. The servers then are vulnerable to any malicious attack that takes advantage of security weaknesses in NTLM.
Countermeasure
When you use Network Security: Restrict NTLM: Outgoing NTLM traffic to remote servers in audit-only mode, you can determine by reviewing which client applications are making NTLM authentication requests to the remote servers in your environment. When assessed, you will have to determine on a case-by-case basis if NTLM authentication still minimally meets your security requirements. If not, the client application has to be upgraded to use something other than NTLM authentication.
Potential impact
Defining a list of servers for this policy setting will enable NTLM authentication traffic from the client application that uses those servers, and this might result in a security vulnerability.
If this list is not defined and Network Security: Restrict NTLM: Outgoing NTLM traffic to remote servers is enabled, then client applications that use NTLM will fail to authenticate to those servers that they have previously used.