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Introduction:
Many organizations invest heavily in cybersecurity, deploying complex tools and stringent policies. However, a critical oversight in a fundamental protocol often renders these efforts ineffective, leaving backdoors wide open to persistent attackers.
Learning Objectives:
- Understand the critical vulnerability associated with NTLM Relay attacks.
- Learn to identify and mitigate weak SMB signing configurations across a network.
- Master the use of offensive security tools to audit and defend your environment.
You Should Know:
1. The Achilles’ Heel: SMB Signing Disabled
The core weakness exploited in NTLM Relay attacks is the lack of SMB (Server Message Block) packet signing on many Windows systems. This allows an attacker to intercept and “relay” authentication attempts to other machines, impersonating the user.
Command (PowerShell): Check SMB Signing Requirements
Get-SmbServerConfiguration | Select-Object RequireSecuritySignature, EnableSecuritySignature
Step-by-Step Guide:
This PowerShell command queries the local machine’s SMB server configuration. A system is vulnerable to NTLM relay if `RequireSecuritySignature` is set to False. This means the client decides whether to sign packets, and many legacy systems do not, creating the opportunity for attack. Run this on critical servers and workstations to audit the current policy.
2. Network Reconnaissance: Finding the Vulnerable Targets
Before an attacker can relay, they must map the network to find systems that do not require SMB signing. This can be done efficiently from a Linux-based attack system.
Command (Impacket’s nmap): Scan for SMB Signing Status
nmap --script smb2-security-mode -p 445 192.168.1.0/24
Step-by-Step Guide:
This Nmap command uses the NSE script `smb2-security-mode` to scan the entire `192.168.1.0/24` subnet on port 445 (SMB). The script’s output will classify each host. Look for `Message signing: disabled` or `Message signing: enabled but not required` in the results. These are your primary targets for remediation.
3. The Attacker’s Toolbox: Intercepting with Responder
To capture authentication attempts, attackers poison network protocols like LLMNR, NBT-NS, and WPAD, tricking systems into sending their credentials to a malicious host.
Command (Responder): Start Poisoning
sudo python3 Responder.py -I eth0 -dwv
Step-by-Step Guide:
This command launches Responder on interface eth0. The `-d` enables DHCP poison, `-w` enables WPAD poison, and `-v` gives verbose output. As soon as a user on the network mistypes a share name (e.g., `\\filsrv` instead of \\filesrv), their system will broadcast a request. Responder will answer, forcing the victim to authenticate to the attacker’s machine and capturing the NTLMv2 hash.
4. The Relay Attack: Using ntlmrelayx.py
Simply capturing a hash is one thing; relaying it is far more powerful as it provides instant access without the need to crack hashes.
Command (Impacket’s ntlmrelayx): Relay Captured Credentials
sudo python3 ntlmrelayx.py -tf targets.txt -smb2support -i
Step-by-Step Guide:
This command initializes the NTLM relay attack. The `-tf targets.txt` option specifies a text file containing IP addresses of systems with SMB signing disabled (found in step 2). The `-smb2support` enables support for modern SMB2, and `-i` spawns an interactive SMB client upon a successful relay. When a hash is captured by Responder, ntlmrelayx will relay it to a target. If the user has administrative privileges on that target, you will gain a shell.
5. Weaponizing Access: Dumping Secrets with Impacket
Once you have relayed credentials and gained access to a system, the next step is to harvest more credentials from memory and the SAM database to pivot through the network.
Command (Impacket’s secretsdump): Dump Hashes Remotely
python3 secretsdump.py 'DOMAIN/USER@TARGET_IP' -hashes aad3b435b51404eeaad3b435b51404ee:31D6CFE0D16AE931B73C59D7E0C089C0
Step-by-Step Guide:
This command uses a technique called DCSync to impersonate a domain controller and pull the entire NTDS.dit database (all user hashes). Replace DOMAIN/USER, TARGET_IP, and the NTLM hash after -hashes. The first part of the hash (aad3b…) is the LM hash, often blank. The second part is the NT hash. This provides the attacker with the keys to the entire kingdom.
- The Ultimate Defense: Enforcing SMB Signing via GPO
The definitive mitigation is to enforce SMB signing, preventing the relay attack from working. This is best deployed through Group Policy in an Active Directory environment.
GPO Path:
Computer Configuration -> Policies -> Windows Settings -> Security Settings -> Local Policies -> Security Options -> Microsoft network server: Digitally sign communications (always)
Step-by-Step Guide:
1. Open the Group Policy Management Console (GPMC).
- Create a new GPO (e.g., “Enforce SMB Signing”) and edit it.
3. Navigate to the path shown above.
4. Set the policy to Enabled.
- Link this GPO to the appropriate Organizational Units (OUs) containing your servers and workstations. Enforcing it on all clients is the most secure posture.
7. Advanced Mitigation: Disabling NTLM and Protocol Poisoning
While enforcing SMB signing stops the relay, eliminating the source of the authentication attempts and the poisoning mechanism provides defense-in-depth.
Command (GPO): Disable LLMNR
Computer Configuration -> Policies -> Administrative Templates -> Network -> DNS Client -> Turn off multicast name resolution -> Enabled
Command (PowerShell): Disable NBT-NS
Get-WmiObject -Class Win32_NetworkAdapterConfiguration -Filter TcpipNetbiosOptions!=2 | Where-Object {$<em>.TcpipNetbiosOptions -ne 2} | ForEach-Object { $</em>.SetTcpipNetbios(2) }
Step-by-Step Guide:
Disabling LLMNR is done via Group Policy. Disabling NBT-NS (NetBIOS over TCP/IP) can be done with this PowerShell script, which checks all network adapters and ensures their NBT setting is set to `2` (Disabled). This prevents Windows from using these broadcast-based name resolution protocols, drastically reducing the attack surface for tools like Responder.
What Undercode Say:
- The Illusion of Security: A strong perimeter and endpoint protection are meaningless if internal protocol weaknesses are left unaddressed. This attack bypasses most traditional security controls.
- The Power of Relaying: The real danger isn’t just hash capture; it’s the ability to relay that hash for instant, authenticated access without cracking, making the attack fast and silent.
- Analysis: The persistence of this attack vector, years after its discovery, highlights a critical gap in enterprise security postures: the focus on advanced external threats over foundational internal hygiene. Organizations pour resources into next-gen solutions while a decades-old protocol flaw provides attackers with a direct path to domain admin. This underscores the non-negotiable necessity of continuous internal vulnerability assessments and adherence to hardening baselines like the CIS Benchmarks. The most sophisticated firewall cannot stop an attack that rides on allowed, essential internal traffic.
Prediction:
The fundamental nature of this vulnerability ensures it will remain a primary attack vector for years to come. As organizations continue to migrate to the cloud, we will see these relay techniques adapted to target cloud-specific protocols and identity services, such as relaying Azure AD logons or manipulating SCIM provisioning. The principle of abusing trust in internal network protocols will persist, making continuous auditing and hardening of authentication mechanisms one of the most critical ongoing tasks for defensive security teams.
🎯Let’s Practice For Free:
IT/Security Reporter URL:
Reported By: Alvin Huang – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
