Protect Against NTLM Relay Attacks and NTLM Relay Vulnerabilities

Introduction to Basic NTLM Relay Vulnerabilities

Every IT team needs to be aware of and protect against NTLM relay attacks in Microsoft-based networks. While NTLM has long served as a core authentication protocol, its design flaws leave networks open to easy attack. Hackers can intercept and relay authentication requests, gain unauthorized access, move laterally across systems, or even compromise domain controllers.

This article explains how NTLM relay attacks work, which services are most vulnerable, and the concrete steps your organization can take to close these security gaps. If you have questions, refer to my previous post. With clear, actionable advice and the deep expertise of Tanner Security, you’ll learn how to build a stronger defense for your Microsoft environment and reduce your exposure to one of the most persistent threats in enterprise IT.

Background on NTLM Relay Attacks

Overview of NTLM Authentication

NTLM (NT LAN Manager) originated decades ago as Microsoft’s authentication solution for local networks. Although it helped simplify initial network authentication for Windows-based systems, NTLM relies heavily on a challenge-response model. In this structure, a server issues a challenge to the client, and the client responds with a hash of its credentials. Because NTLM lacks more modern security mechanisms, particularly with channel binding and encryption, attackers who intercept that legitimate challenge-response process can steal and use those credentials in real-time.

More modern protocols like Kerberos were introduced to address these vulnerabilities. However, Microsoft’s default settings often retain NTLM as a fallback option, giving attackers an easy entry point if businesses do not proactively disable or restrict it.

Why NTLM is Vulnerable

NTLM’s vulnerability stems from various factors, including its lack of channel-binding tokens, susceptibility to man-in-the-middle interceptions, and default fallbacks. It does not naturally require end-to-end encryption of authentication traffic, leaving even businesses with otherwise robust cyber hygiene at risk. This means that, without proper configuration and protections in place, NTLM authentication can be hijacked in transit and relayed to critical infrastructure components, where privileges can be illicitly escalated.

Understanding NTLM Relay Attacks

A. Core Mechanics of NTLM Relay Attacks

NTLM relay attacks start with an attacker intercepting the authentication data that passes between a user or a machine and a network resource. Often, this is done by forcing a user or service to authenticate to a malicious system masquerading as a legitimate server. The attacker then relays that intercepted authentication token to an unsuspecting target system, perhaps a file server, domain controller, or database server, where they are authenticated as the original user.

The receiving server assumes the authentication data is legitimate because NTLM does not validate that the requesting machine is the correct endpoint. If the credentials are from a user with elevated privileges or the targeted server lacks strong security controls, the attacker may gain significant access to files, active services, or the entire domain.

B. Attackers’ Motivation and Potential Impact

Attackers who employ NTLM relay attacks typically aim to gain undue access to data, deploy malware, or seize full administrative control. The specifics of the impact often depend on the permission levels of the captured credentials. For example, relaying the credentials of a standard user may allow only minimal data theft, while relaying the credentials of a domain administrator could open up the entire network to compromise. Attackers can pivot from a single point of compromise to install additional malicious tools, exfiltrate confidential data, and cover their tracks by creating new accounts with stolen privileges.

Common Attack Vectors

Although NTLM relay attacks can target nearly any service that supports NTLM authentication, certain services are more attractive or vulnerable than others. Below is an overview of four key services where relay attacks commonly occur:

SMB (Server Message Block)

SMB is Microsoft’s file and print sharing protocol. By default, SMB signing is not enforced, and attackers can intercept an NTLM authentication request intended for a file server and then relay it to another server to gain file system access under the impersonated user context. This can be particularly damaging when a relayed user holds administrative privileges. Attackers can execute remote procedures, register malicious services, and gain control of the full system.

To reduce this risk, companies should enable SMB signing via Group Policy. Requiring signed SMB traffic ensures that each client and server validate the authenticity of the messages they exchange, thereby preventing malicious interception or alteration en route.

ADCS (Active Directory Certificate Services)

Active Directory Certificate Services can provide attackers an even more devastating breach path when using NTLM authentication. This service sometimes offers a web enrollment portal to issue certificates, and if NTLM authentication is not disabled or restricted, attackers can steal a user’s authentication request and then request legitimate certificates on that user’s behalf.

Certificates procured in this way allow attackers to impersonate the user, even obtaining Kerberos tokens that grant extensive access to other network resources. If high-privilege accounts are relayed, the attacker can compromise vital domain services. The essential mitigation includes requiring HTTPS for certificate enrollment, implementing Extended Protection for Authentication (EPA), removing NTLM from the list of allowed providers on the enrollment portal, and opting for Kerberos-based authentication.

LDAP (Lightweight Directory Access Protocol)

LDAP underpins much of Active Directory’s identity management, cataloging users, computers, and configurations. By relaying NTLM credentials to LDAP-ready domain controllers, attackers can manipulate object permissions, create new computer accounts, or even modify existing objects depending on the privileges of the compromised user. A common tactic involves creating an account that grants an attacker a foothold in the domain, which can then escalate into more serious intrusions.

LDAP signing and channel binding help ensure that neither the authentication data nor the session can be tampered with. These can be enforced via specific Group Policy settings in the Default Domain Controllers Policy, compelling signed LDAP communications and preventing malicious relaying activities.

MSSQL

Often overlooked as a prime target, Microsoft SQL Server also accepts NTLM authentication. If an attacker manages to relay credentials to an SQL Server, they could read and modify sensitive database contents. In systems that permit privilege escalation via database roles, or if the MSSQL service runs with highly privileged local accounts, the attacker might compromise the underlying server operating system.

Companies should enable TLS-encrypted communications to shield SQL logins from being hijacked in transit and activate Extended Protection for Authentication within SQL Server Configuration Manager. By mandating encryption and leveraging EPA, malicious interception becomes far more difficult, and credential integrity is better protected.

Expert Insights and Recommendations

“Maintaining a secure Microsoft environment must go beyond just patching. It involves rigorous policy configuration, continuous monitoring, and a strong commitment to verifying every step of the authentication chain.” — Jake Otte

While addressing just one or two services at a time might be tempting, a truly effective security strategy requires a company-wide effort, spanning all endpoints that could accept NTLM authentication.

Overview of a Comprehensive Security Approach

First, thorough monitoring of all authentication attempts is the most important step. Implement real-time visibility tools that flag unusual activities, such as repeated authentication requests or attempts to access network resources from unexpected addresses. Second, patch management should be frequent and timely, ensuring the most recent fixes reduce the likelihood of known exploits. Third, endpoint hardening practices, like enforcing complex passwords, controlling local administrator privileges, and tightly monitoring group memberships, complete the basic points of a strong security program.

As you deploy or expand Microsoft infrastructure, consider moving towards Kerberos-only authentication whenever possible. Restricting or fully disabling NTLM across your environment will go a long way toward blocking the east relay attack scenarios.

How Tanner Can Help

Tanner Security has a team of experts with extensive knowledge of Microsoft infrastructures to pinpoint weaknesses, guide the remediation process, and help you reach a stronger defensive posture. Whether you face immediate remediation needs or require large-scale architectural revisions, our consultants can design Group Policy configurations that fortify SMB, LDAP, ADCS, and MSSQL services. We also provide in-depth security assessments and penetration testing tailored to identify potential NTLM relay issues. Our team emphasizes knowledge transfer, training in best practices, and ongoing support to ensure you remain protected as new threats emerge.

NTLM Relay Vulnerabilities Conclusion

NTLM relay vulnerabilities represent a tricky situation for companies heavily invested in Microsoft technologies. Attackers target standard services like SMB, ADCS, LDAP, and MSSQL to intercept and reroute authentication flows, often with devastating results. To protect your business, it is crucial to implement strong security protocols, including SMB signing, extended protection in ADCS and MSSQL, and LDAP channel binding. Even more important is the need for third-party penetration testing to verify that all misconfigurations are remediated. By coupling your Microsoft infrastructure with deliberate policy enforcement and expert guidance from a partner like Tanner, you can close the gaps that enable NTLM relay attacks, build a more resilient network, and maintain confidence in preventing unauthorized access.