Microsoft Launches Phase 2 Mitigation for Zerologon Flaw

Microsoft has released phase two mitigation for the critical Zerologon vulnerability disclosed in August 2020. CVE-2020-1472 is an elevation of privilege flaw affecting the Windows Netlogon Remote Protocol (MS-NRPC).

MS-NRPC is a core authentication component of Active Directory. This vulnerability exists when an attacker creates a vulnerable Netlogon secure channel connection to a domain controller using MS-NRPC. With that connection, an attacker doesn't need to authenticate in order to elevate their privileges and become an administrator. An unauthenticated attacker could use this access to run arbitrary code on affected Windows domain controllers. 

Concerns about Zerologon quickly escalated after Microsoft issued the first part of its mitigation in August. In mid-September, publicly available exploit code was discovered. Shortly after, the DHS' Cybersecurity and Infrastructure Security Agency (CISA) issued an emergency directive requiring federal agencies to mitigate the flaw by midnight on Sept. 21.

It didn't take long for advanced attackers to add Zerologon to their toolkits. In October 2020, Iranian APT group Mercury was seen using the vulnerability in a series of attacks that Microsoft detected.

Now Microsoft has released the second of its two-part mitigation.

Starting with the Feb. 9, 2021, security update, Windows Domain Controllers will be placed in enforcement mode. This requires all Windows and non-Windows devices to use secure Remote Procedure Call (RPC) with Netlogon secure channel. This update will block vulnerable connections from noncompliant devices, unless those devices are manually given an exception to allow vulnerable Netlogon secure channel connections. 

Read the CISA alert and Microsoft security update for more details.