Microsoft is releasing an optional security enhancement to NT LAN Manager (NTLM), limiting which network resources various clients in the Windows 10 or the Windows Server 2016 operating systems can use NTLM Single Sign On(SSO) as an authentication method. When you deploy the new security enhancement with a Network Isolation Policy defining your organization's resources, attackers can no longer redirect a user to a malicious resource outside your organization to obtain the NTLM authentication messages.