Microsoft network server: Server SPN target name validation level

The server message block (SMB) protocol provides the basis for file and printer sharing and many other networking operations, such as remote Windows administration. The SMB protocol supports validating the SMB server service principal name (SPN) within the authentication blob provided by a SMB client to prevent a class of attacks against SMB servers referred to as SMB relay attacks. This setting will affect both SMB1 and SMB2.

This security setting determines the level of validation a SMB server performs on the service principal name (SPN) provided by the SMB client when trying to establish a session to an SMB server.

The options are:

No validation - validation of the SPN will not be performed by the SMB server.

Validate if provided by client - the SMB server will 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 for itself. If the SPN does NOT match, the session request for that SMB client will be denied.

Require match 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 client, or the SPN provided does not match, the session is denied.

Default: No validation

All Windows operating systems support both 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. Additional information on implementing and using this to secure your SMB servers can be found at: KB# - Security options for SMB servers".