Inside Hash-Based Relay Attacks: How NTLM Authentication Is Exploited

 Hash-Based Relay Attack

A hash-based relay attack, often referred to as an NTLM relay attack, is a technique used by attackers to exploit authentication mechanisms in Windows environments—particularly those using the NTLM protocol. Here's a detailed explanation:

What Is a Hash-Based Relay?

In a hash-based relay attack, an attacker captures authentication hashes (typically NTLM hashes) from a legitimate user and relays them to another service that accepts them, effectively impersonating the user without needing their password.

How It Works – Step by Step

1. Intercepting the Hash

• The attacker sets up a rogue server (e.g., using tools like Responder) that listens for authentication attempts.
• When a user tries to access a network resource (e.g., a shared folder), their system sends NTLM authentication data (hashes) to the rogue server.

2. Relaying the Hash

• Instead of cracking the hash, the attacker relays it to a legitimate service (e.g., SMB on port 445) that accepts NTLM authentication.
• If the target service does not enforce protections like SMB signing, it will accept the hash and grant access.

3. Gaining Access

• The attacker now has access to the target system or service as the user whose hash was relayed.
• This can lead to privilege escalation, lateral movement, or data exfiltration.

Tools Commonly Used

• Responder: Captures NTLM hashes from network traffic.
• ntlmrelayx (Impacket): Relays captured hashes to target services.
• Metasploit: Includes modules for NTLM relay and SMB exploitation.

Common Targets

• SMB (port 445): Most common and vulnerable to NTLM relay.
• LDAP, HTTP, RDP: Can also be targeted depending on configuration.
• Exchange, SQL Server, and other internal services.

Defenses Against Hash-Based Relay Attacks

• Technical Controls
• Enforce SMB signing: Prevents unauthorized message tampering.
• Disable NTLM where possible: Use Kerberos instead.
• Segment networks: Limit exposure of sensitive services.
• Use strong firewall rules: Block unnecessary ports and services.
• Monitoring & Detection
• Monitor for unusual authentication patterns.
• Use endpoint detection and response (EDR) tools.
• Log and alert on NTLM authentication attempts.