Network security: Restrict NTLM: Add server exceptions in this domain

Article
08/31/2016

Applies To: Windows 7, Windows 8.1, Windows Server 2008 R2, Windows Server 2012 R2, Windows Server 2012, Windows 8

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 in this domain policy setting allows you to create an exception list of servers in this domain to which client computers are allowed to use NTLM pass-through authentication if any of the deny options are set in the Network Security: Restrict NTLM: NTLM authentication in this domain policy setting.

If you configure this policy setting, you can define a list of servers in this domain 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: NTLM authentication in this domain is enabled, all NTLM authentication attempts in the domain will fail.

List the NetBIOS server names as the naming format, one per line. A single asterisk (*) can be used anywhere in the string as a wildcard character.

Possible values

User-defined list of servers

When you enter a list of servers in this domain 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 NTLM authentication in this domain policy setting, and then review the Operational log to understand what domain controllers 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 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 different features and tools available to help you manage this policy.

Restart requirement

None. Changes to this policy become effective without a restart when saved locally or distributed through Group Policy.

Group Policy

Setting and deploying this policy via Group Policy takes precedence over the setting on the local computer. If the Group Policy 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 within a domain because you are required to use a more secure protocol such as Kerberos, there might be some NTLM authentication traffic that is still present in the domain. If so, and you set Network Security: Restrict NTLM: NTLM authentication in this domain to any of the deny options, any NTLM authentication request will fail because the pass-through member server will block the NTLM request.

If you define an exception list of servers in this domain to which client computers are allowed to use NTLM pass-through authentication, then NTLM authentication traffic will continue to flow between those servers, which make them vulnerable to any malicious attack that takes advantage of security weaknesses in NTLM.

Countermeasure

When you use Network Security: Restrict NTLM: NTLM authentication in this domain in audit-only mode, you can determine by reviewing which client applications are making NTLM authentication requests to the pass-through authentication servers. When assessed, you will have to determine on a case-by-case basis if NTLM authentication still minimally meets your security requirements.

Potential impact

Defining a list of servers for this policy setting will enable NTLM authentication traffic between those servers might result in a security vulnerability.

If this list is not defined and Network Security: Restrict NTLM: NTLM authentication in this domain is enabled, then NTLM authentication will fail on those pass-through servers in the domain that they have previously used

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