An NTLM Relay attack is a specific type of Man-in-the-Middle (MitM) exploit targeting the Microsoft NTLM authentication protocol. In the context of CompTIA PenTest+, this attack is a critical concept for lateral movement because it allows an attacker to gain unauthorized access to servers without e…An NTLM Relay attack is a specific type of Man-in-the-Middle (MitM) exploit targeting the Microsoft NTLM authentication protocol. In the context of CompTIA PenTest+, this attack is a critical concept for lateral movement because it allows an attacker to gain unauthorized access to servers without ever knowing or cracking the victim's plaintext password.
The attack exploits the NTLM challenge-response mechanism. It typically begins with the attacker poisoning the local network using tools like **Responder** (via LLMNR/NBT-NS spoofing). When a victim client attempts to access a network resource, the attacker intercepts the request and forwards (relays) the authentication negotiation to a different target server on the network. The target server replies with a cryptographic 'challenge,' which the attacker relays back to the victim. The victim's machine encrypts this challenge using its password hash and sends the valid 'response' to the attacker. Finally, the attacker forwards this valid response to the target server. The server verifies the response is correct and authenticates the session, effectively granting the attacker access under the victim's identity.
A crucial prerequisite for NTLM relaying over SMB is that **SMB Signing** must be disabled or set to 'not required' on the target machine. If SMB signing is enforced, the server requires packet signatures derived from the user's session key (which the attacker does not possess), causing the attack to fail. Attackers often use tools like **ntlmrelayx** (part of Impacket) to automate this process, leading to Remote Code Execution (RCE), dumping local SAM hashes, or adding administrative users. Remediation involves enforcing SMB signing across the domain and disabling NTLM in favor of Kerberos.
Comprehensive Guide to NTLM Relay Attacks for CompTIA PenTest+
What is an NTLM Relay Attack? An NTLM Relay attack is a specific type of Man-in-the-Middle (MitM) attack targeting Microsoft's NTLM authentication protocol. Instead of capturing a password hash to crack it offline (as seen in standard credential harvesting), the attacker intercepts an incoming authentication attempt from a victim and relays (forwards) that authentication to a different target server. If successful, the attacker gains access to the target server with the privileges of the victim.
Why is it Important? This attack is critical in penetration testing because it allows for lateral movement without ever knowing the user's actual plaintext password. It exploits the trust relationship between Windows clients and servers. It is highly effective in environments where SMB Signing is disabled or set to optional.
How it Works: The Step-by-Step Process 1. The Trigger: The attacker sits on the local network. A victim machine attempts to authenticate to a resource. This is often triggered by the attacker using LLMNR/NBT-NS poisoning (using tools like Responder) to tell the victim, "I am the server you are looking for." 2. The Interception: The victim sends their NTLM authentication handshake (Challenge/Response) to the attacker. 3. The Relay: Instead of responding, the attacker forwards this handshake to a specific target server (e.g., a file server or Domain Controller). 4. The Access: The target server validates the credentials. Since the credentials are valid, the target server grants a session. The attacker now has a session on the target server, impersonating the victim.
Tools Involved Common tools used to execute this attack include Responder (to poison traffic and capture the handshake) and ntlmrelayx (from the Impacket suite) to perform the actual relaying.
Exam Tips: Answering Questions on NTLM Relay Attacks When facing questions about this topic on the CompTIA PenTest+ exam, look for these specific indicators and keywords:
1. Identifying the Vulnerability If a scenario mentions that "SMB Signing is disabled" or "SMB Signing is not enforced," the network is vulnerable to NTLM Relay attacks. SMB signing digitally signs packets to confirm their origin; if it is off, relaying is possible.
2. Differentiating from Pass-the-Hash Do not confuse Relay with Pass-the-Hash (PtH). Pass-the-Hash: You already have the hash, and you re-inject it to authenticate. NTLM Relay: You do not know the hash; you are catching a live handshake and forwarding it in real-time.
3. Remediation/Mitigation If a question asks how to fix or prevent this attack, the correct answer is almost always: Enable/Enforce SMB Signing on all devices. Disabling NTLM entirely in favor of Kerberos is also a valid, though more complex, solution.
4. Attack Pre-requisites Remember that for an NTLM relay attack to work, the attacker and victim must usually be on the same local subnet, and the user accounts must have administrative privileges on the target machine for the attacker to execute code (though simply accessing files requires lower privileges).