Network Security: Restrict NTLM: Outgoing NTLM traffic to remote servers

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: Outgoing NTLM traffic to remote servers policy setting allows you to deny or audit outgoing NTLM traffic from a computer running Windows 7 or Windows Server 2008 R2 to any remote server running the Windows operating system.

Possible values

• Allow all

  The computer can authenticate identities to a remote server by using NTLM authentication because no restrictions exist.

• Audit all

  The computer that sends the NTLM authentication request to a remote server logs an event for each request. This allows you to identify those servers that receive NTLM authentication requests from the client computer.

• Deny all

  The computer cannot authenticate any identities to a remote server by using NTLM authentication. You can use the Network security: Restrict NTLM: Add remote server exceptions for NTLM authentication policy setting to define a list of remote servers to which client computers are allowed to use NTLM authentication while denying others. This setting will also log an event on the computer that is making the authentication request.

• Not defined

  This is the same as Allow all, and the computer will allow all NTLM authentication requests when the policy is deployed. See the Group Policy section for additional information.

Best practices

If you select Deny all, the client computer cannot authenticate identities to a remote server by using NTLM authentication. First, select Audit all and then review the Operational log to understand which servers are involved in these authentication attempts. You can then add those server names to a server exception list by using the Network security: Restrict NTLM: Add remote server exceptions for NTLM authentication policy setting.

Location

GPO_name**\Computer Configuration\Windows Settings\Security Settings\Local Policies\Security Options**

Default values

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 using 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 this policy 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 Event policies that can be configured to view event 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.

NTLM and NTLMv2 authentication is vulnerable to a variety of malicious attacks, including SMB replay, man-in-the-middle attacks, and brute force attacks. Reducing and eliminating NTLM authentication from your environment forces the Windows operating system to use more secure protocols, such as the Kerberos version 5 protocol, or different authentication mechanisms, such as smart cards.

Vulnerability

Malicious attacks on NTLM authentication traffic that result in a compromised server or domain controller can occur only if the server or domain controller handles NTLM requests. If those requests are denied, this attack vector is eliminated.

Countermeasure

When it has been determined that the NTLM authentication protocol should not be used within a network because you are required to use a more secure protocol such as Kerberos, then you can select from several options to restrict NTLM usage to servers.

Potential impact

If you configure this policy setting to deny all requests, numerous NTLM authentication requests to remote servers could fail, which could degrade productivity. Before implementing this restriction through this policy setting, select Audit all so that you can review the log for the potential impact, perform an analysis of servers, and create an exception list of servers to exclude from this policy setting by using Network security: Restrict NTLM: Add remote server exceptions for NTLM authentication.