Windows Information Disclosure Vulnerability

An information disclosure vulnerability exists when Windows fails to properly validate NT LAN Manager (NTLM) Single Sign-On (SSO) requests during Microsoft Account (MSA) login sessions. An attacker who successfully exploited the vulnerability could attempt to brute force a user’s NTLM password hash. To exploit the vulnerability, an attacker would have to trick a user into browsing to a malicious website, or to an SMB or UNC path destination, or convince a user to load a malicious document that initiates an NTLM SSO validation request without the consent of the user. To validate MSA NTLM SSO authentication requests properly, it is imperative that Windows client operating system firewall profiles and enterprise perimeter firewalls are configured correctly. If users are connected to the “Guest or public networks” firewall profile, it would imply that no enterprise perimeter firewall is present between the user and the Internet. The security update addresses the vulnerability by preventing NTLM SSO authentication to non-private SMB resources when users are signed in to Windows via a Microsoft Account network firewall profile for users who are signed in to Windows via a Microsoft account (https://www.microsoft.com/account) and connected to a “Guest or public networks” firewall profile. VPNs are considered private networks. For more information, see the Mitigations section.