Detecting NTLM Relay Attacks Against Samba File Servers Through Secure SMB Architecture and Authentication Boundary Enforcement

Enterprise organizations commonly use file-sharing services to provide employees and applications with access to shared documents and internal resources. In Windows-compatible environments, SMB can support authentication through mechanisms such as NTLM.

If NTLM authentication is allowed without appropriate security controls, an attacker positioned within the network may attempt to relay authentication traffic to another service instead of directly compromising the user's credentials.

In this use case, a Samba File Server is deployed on Ubuntu Linux as the controlled enterprise file-sharing infrastructure inside an isolated VirtualBox laboratory.

A controlled NTLM Relay Attack is simulated from Kali Linux against the isolated Samba environment using authorized security-testing techniques.

The assessment focuses on whether the enterprise security architecture properly protects SMB authentication traffic and prevents authentication requests from being improperly relayed to protected services.

Samba, tcpdump, Wireshark, and nftables are used to establish the SMB environment, monitor authentication traffic, and implement network-level security controls.

The proposed defensive architecture introduces SMB signing, restricted SMB exposure, network segmentation, controlled authentication paths, firewall enforcement, and security monitoring.

After implementing the security controls, the NTLM Relay assessment is repeated to verify that authentication-relay attempts are prevented or rejected while legitimate SMB file-sharing continues to function.

Samba provides SMB-compatible file-sharing services within the controlled enterprise environment. The laboratory represents an enterprise network containing: User Network → Authentication / Access Request → SMB File Server → Protected Shared Resources. The SMB server is intended to accept authentication requests only through the approved enterprise communication path.

NTLM authentication can become vulnerable to relay attacks when authentication traffic can be captured and forwarded to another service that accepts the relayed authentication.

A NTLM Relay Attack does not necessarily require the attacker to recover the user's password. Instead, the attacker attempts to forward a valid authentication exchange to another service. The security problem can occur when NTLM authentication is unnecessarily exposed, SMB signing is not enforced, SMB services are reachable from untrusted network segments, authentication paths are not properly segmented, internal services accept authentication from unexpected sources, network-level access controls are weak, or SMB security configuration is not regularly validated. If these weaknesses exist, an attacker may attempt to relay authentication traffic toward a protected SMB service or another authentication-enabled service.

The security problem is therefore:

The proposed solution introduces SMB signing enforcement, restricted SMB network exposure, authentication-path segmentation, network access control, controlled SMB service exposure, security traffic monitoring, and continuous configuration validation.

A NTLM Relay Attack attempts to forward an authentication exchange received from a legitimate client toward another service. The attacker does not need to directly obtain the user's password to attempt the relay. In the controlled laboratory, the attack simulation is performed only against authorized Samba infrastructure and test accounts.

The assessment focuses on identifying unexpected NTLM authentication flows, authentication requests from unauthorized network segments, SMB connections originating from unexpected systems, SMB signing configuration weaknesses, unexpected authentication destinations, and network paths that permit authentication relay conditions.

Enterprise SMB security should ensure that authentication traffic is exchanged only through approved network paths and protected by appropriate SMB security mechanisms.

The security architecture follows: User Endpoint → Controlled Network Segment → Security Boundary → SMB Server → Signed SMB Session → Protected Resource.

The secure processing flow is:

SMB signing provides integrity protection for SMB communications and reduces the feasibility of authentication-relay attacks against SMB. Network segmentation separates user, server, and management networks to prevent unrestricted communication between enterprise security zones. SMB access restriction exposes SMB services only to authorized network segments, reducing the attack surface available to authentication-relay attempts. Authentication boundary enforcement allows authentication requests only through approved communication paths. Firewall enforcement restricts unauthorized SMB connections and reduces unnecessary SMB exposure between network zones. Security monitoring identifies abnormal authentication paths and unexpected SMB activity.