January 21, 2020


Security research shows most successful enumeration and brute force attacks use either NTLM or Kerberos authentication protocols for entry. In fact, they’re the most popular discovery-phase attacks Azure ATP observed in the past 12 months.



Why does account enumeration matter?


In an account enumeration attack, the malicious actor attempts to use different usernames to access a server, with the goal of discovering which users exist within the organization. Once an attacker identifies these users, a brute force attacks begins to get their credentials and move laterally within the organization toward higher-profile assets.



What is NTLM and how does it work?


The NTLM protocol authenticates users and computers, using a challenge/response mechanism designed to prove to a server or domain controller that the user knows the password associated with the account they’re trying to access. Whenever a new access token is needed for domain accounts, a resource server must contact the domain controller to verify the identity of a computer or user.


Standard NTLM authentication flow includes 2 major steps:


  1. The user tries to access a resource server.
  2. The resource server validates the user with a domain controller.




How does Azure ATP provide visibility into NTLM authentications?


Network traffic and Windows Events 4776 and 8004 capture NTLM data. In turn, Azure ATP parses the NTLM traffic and events from your domain controllers.


NTLM 8004 events provide full information on your NTLM authentications, including:


  • Source account
  • Source device
  • Accessed resource server
  • Domain controller that is doing the account validation



Why do some devices show up as “unknown” in Azure ATP alerts?


The source device in Azure ATP account enumeration and brute force detection alerts can be marked as coming from “unknown” devices, such as Workstation, MSTSC, or Unknown. This happens because the source device name field is occasionally overwritten when the attacker is already inside your organization, or when they try to enumerate accounts from the internet. This is common when the accessed server is opened to the internet and used by adversaries to enumerate users from outside the organization.


With enhanced support for Windows event 8004, Azure ATP now determines which servers were attacked and how the attacks happened.



How can Azure ATP detect the actual server accessed inside the network?


Azure ATP sensors parse Windows event 8004 for NTLM authentications. When NTLM auditing is enabled and Windows event 8004 is logged, Azure ATP sensors automatically read the event and enrich your NTLM authentications with the accessed server data.


Account enumeration.png


In addition, Azure ATP now provides Resource Access over NTLM activity, showing the source user, source device, and accessed resource server:

resource access NTLM.png

Example of enhanced NTLM activity details


Use the following links to learn more about enabling NTLM auditing when working with Azure ATP to detect, protect, and remediate NTLM and brute force attacks:


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