Microsoft network server: Server SPN target name validation level

Applies To: Windows Server 2003, Windows Vista, Windows XP, Windows Server 2008, 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, and values, policy management and security considerations for this policy setting.

Reference

This policy setting controls the level of validation that a server with shared folders or printers performs on the service principal name (SPN) that is provided by the client computer when the client computer establishes a session by using the Server Message Block (SMB) protocol. The level of validation can help prevent a class of attacks against SMB services (referred to as SMB relay attacks). This setting will affect both SMB1 and SMB2.

Servers that use SMB provide availability to their file systems and other resources, such as printers, to networked client computers. Most servers that use SMB validate user access to resources by using NT Domain authentication (NTLMv1 and NTLMv2) and the Kerberos protocol.

Windows Server 2008 and Windows Vista support SMB protocol (SMB 2.0 or SMB2). Windows Server 2008 R2 and Windows 7 supports SMB 2.0, SMB2 and SMB 2.1.

Possible values

The options for validation levels are:

• Off

  The SPN from a SMB client is not required or validated by the SMB server.

• Accept if provided by client

  The SMB server will accept and validate the SPN provided by the SMB client and allow a session to be established if it matches the SMB server’s list of SPN’s. If the SPN does not match, the session request for that SMB client will be denied.

• Required from client

  The SMB client must send a SPN name in session setup, and the SPN name provided must match the SMB server that is being requested to establish a connection. If no SPN is provided by the client computer, or the SPN provided does not match, the session is denied.

The default setting is Off.

Best practices

This setting affects the server SMB behavior, and its implementation should be carefully evaluated and tested to prevent disruptions to file and print serving capabilities.

Location

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

Default values

The following table lists the actual and effective default values for this policy. Default values are also listed on the policy’s property page.

Operating system version differences

This policy was introduced in Windows Server 2008 and Windows Vista, and it can be applied through Group Policy to all Windows server operating systems beginning with Windows Server 2003 and Windows XP.

Policy management

This section describes 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.

Policy conflict considerations

None.

Group Policy

This policy setting can be configured by using the Group Policy Management Console (GPMC) to be distributed through Group Policy Objects (GPOs). If this policy is not contained in a distributed GPO, this policy can be configured on the local computer by using the Local Security Policy snap-in.

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

This policy setting controls the level of validation that a server with shared folders or printers performs on the service principal name (SPN) that is provided by the client computer when the client computer establishes a session by using the SMB protocol. The level of validation can help prevent a class of attacks against SMB servers (referred to as SMB relay attacks). This setting will affect both SMB1 and SMB2.

Countermeasure

For countermeasures that are appropriate to your environment, see Possible values above.

Potential impact

All Windows operating systems support a client-side SMB component and a server-side SMB component. This setting affects the server SMB behavior, and its implementation should be carefully evaluated and tested to prevent disruptions to file and print serving capabilities.

Because the SMB protocol is widely deployed, setting the options to Accept if provided by client or Required from client will prevent some clients from successfully authenticating to some servers in your environment.