Preempt Researchers Find Two Critical Vulnerabilities in Microsoft NTLM Allowing Malicious Remote Code Execution on any Windows Machine

Three flaws in Microsoft’s proprietary authentication protocol give attackers the ability to bypass all NTLM protection mechanisms

SAN FRANCISCO, June 11, 2019 (GLOBE NEWSWIRE) -- Preempt, the leading provider of conditional access for real-time threat prevention, today announced its research team found two critical Microsoft vulnerabilities that consist of three logical flaws in NTLM, the company’s proprietary authentication protocol. These vulnerabilities allow attackers to remotely execute malicious code on any Windows machine or authenticate to any web server that supports Windows Integrated Authentication (WIA) such as Exchange or ADFS. The research shows that all Windows versions are vulnerable.

NTLM is susceptible to relay attacks, which allows actors to capture an authentication and relay it to another server, granting them the ability to perform operations on the remote server using the authenticated user’s privileges. NTLM Relay is one of the most common attack techniques used in Active Directory environments, where the attacker compromises one machine, then moves laterally to other machines by using NTLM authentication directed at the compromised server.

Microsoft previously developed several mitigations for preventing NTLM relay attacks. Preempt researchers discovered those mitigations have the following flaws which can be exploited by attackers:

  • The Message Integrity Code (MIC) field ensures that attackers do not tamper NTLM messages. The bypass discovered by Preempt researchers allows attackers to remove the ‘MIC’ protection and modify various fields in the NTLM authentication flow, such as signing negotiation.
  • SMB Session Signing prevents attackers from relaying NTLM authentication messages to establish SMB and DCE/RPC sessions. The bypass discovered by Preempt researchers enables attackers to relay NTLM authentication requests to any server in the domain, including domain controllers, while establishing a signed session to perform remote code execution. If the relayed authentication is of a privileged user, this means full domain compromise.
  • Enhanced Protection for Authentication (EPA) prevents attackers from relaying NTLM messages to TLS sessions. The bypass discovered by Preempt researchers allows attackers to modify NTLM messages to generate legitimate channel binding information. This allows attackers to connect to various web servers using the attacked user’s privileges and perform operations such as: read the user’s emails (by relaying to OWA servers) or even connect to cloud resources (by relaying to ADFS servers).

To see more details on the reported risks of these flaws, please visit Preempt’s security advisory blog here.

“Even though NTLM Relay is an old technique, enterprises cannot completely eliminate the use of the protocol as it will break many applications. Hence it still poses a significant risk to enterprises, especially with new vulnerabilities discovered constantly,” stated Roman Blachman, Chief Technology Officer and Co-Founder at Preempt, “Companies need to first and foremost ensure all of their Windows systems are patched and securely configured. In addition, organizations can further protect their environments by gaining network NTLM visibility. Preempt works with its customers to ensure they have this visibility and the best protection possible.”

For organizations to protect themselves from these vulnerabilities they must:

  1. Patch - Make sure that workstations and servers are properly patched. However, it is important to note that patching alone is not enough, companies also need to make configuration changes in order to be fully protected.
  2. Configuration:
    1. Enforce SMB Signing - To prevent attackers from launching simpler NTLM relay attacks, turn on SMB Signing on all machines in the network.
    2. Block NTLMv1 - Since NTLMv1 is considered significantly less secure; it is recommended to completely block it by setting the appropriate GPO.
    3. Enforce LDAP/S Signing - To prevent NTLM relay in LDAP, enforce LDAP signing and LDAPS channel binding on domain controllers.
    4. Enforce EPA - To prevent NTLM relay on web servers, harden all web servers (OWA, ADFS) to accept only requests with EPA.
  3. Reduce NTLM usage – Even with fully secured configuration and patched servers, NTLM poses a significantly greater risk than Kerberos. It is recommended that you remove NTLM where it is not needed.

Preempt’s customers already have protections against NTLM vulnerabilities. The Preempt Platform provides full network NTLM visibility, allowing organizations to reduce NTLM traffic and analyze suspicious NTLM activity. In addition, Preempt has an innovative industry-first deterministic NTLM relay detection capabilities and has the ability to inspect all GPO configurations and will alert on insecure configurations. This configuration inspection is also available in Preempt Lite, a free lightweight version of the Preempt Platform. Organizations can download Preempt Lite here and verify which areas of their network are vulnerable.

This vulnerabilities and more will be presented by Preempt researchers Yaron Zinar and Marina Simakov at Black Hat USA 2019.

As of June 11, 2019, Microsoft has issued CVE-2019-1040 and CVE-2019-1019 on Patch Tuesday per Preempt’s responsible disclosure of the NTLM vulnerabilities.  

About Preempt 
Preempt delivers a modern approach to authentication and securing identity in the enterprise. Using patented technology for Conditional Access, Preempt helps enterprises optimize identity hygiene and stop attacks in real-time before they impact business. Preempt continuously detects and preempts threats based on identity, behavior, and risk across all cloud and on-premises authentication & access platforms. This low friction approach empowers security teams more visibility & control over accounts and privileged access, while achieving compliance and auto-resolving incidents. Learn more:

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