By responding to LLMNR/NBT-NS network traffic, adversaries may spoof an authoritative source for name resolution to force communication with an adversary controlled system. This activity may be used to collect or relay authentication materials.
Link-Local Multicast Name Resolution (LLMNR) and NetBIOS Name Service (NBT-NS) are Microsoft Windows components that serve as alternate methods of host identification. LLMNR is based upon the Domain Name System (DNS) format and allows hosts on the same local link to perform name resolution for other hosts. NBT-NS identifies systems on a local network by their NetBIOS name. 
Adversaries can spoof an authoritative source for name resolution on a victim network by responding to LLMNR (UDP 5355)/NBT-NS (UDP 137) traffic as if they know the identity of the requested host, effectively poisoning the service so that the victims will communicate with the adversary controlled system. If the requested host belongs to a resource that requires identification/authentication, the username and NTLMv2 hash will then be sent to the adversary controlled system. The adversary can then collect the hash information sent over the wire through tools that monitor the ports for traffic or through Network Sniffing and crack the hashes offline through Brute Force to obtain the plaintext passwords.
In some cases where an adversary has access to a system that is in the authentication path between systems or when automated scans that use credentials attempt to authenticate to an adversary controlled system, the NTLMv1/v2 hashes can be intercepted and relayed to access and execute code against a target system. The relay step can happen in conjunction with poisoning but may also be independent of it. Additionally, adversaries may encapsulate the NTLMv1/v2 hashes into various protocols, such as LDAP, SMB, MSSQL and HTTP, to expand and use multiple services with the valid NTLM response.
Impacket modules like ntlmrelayx and smbrelayx can be used in conjunction with Network Sniffing and LLMNR/NBT-NS Poisoning and SMB Relay to gather NetNTLM credentials for Brute Force or relay attacks that can gain code execution.
|M1042||Disable or Remove Feature or Program||
Disable LLMNR and NetBIOS in local computer security settings or by group policy if they are not needed within an environment. 
|M1037||Filter Network Traffic|
|M1031||Network Intrusion Prevention||
Network intrusion detection and prevention systems that can identify traffic patterns indicative of AiTM activity can be used to mitigate activity at the network level.
Network segmentation can be used to isolate infrastructure components that do not require broad network access. This may mitigate, or at least alleviate, the scope of AiTM activity.
|ID||Data Source||Data Component||Detects|
|DS0029||Network Traffic||Network Traffic Content||
Monitor for traffic on ports UDP 5355 and UDP 137 if LLMNR/NetBIOS is disabled by security policy.
|Network Traffic Flow||
Monitor for network traffic originating from unknown/unexpected hardware devices. Local network traffic metadata (such as source MAC addressing) as well as usage of network management protocols such as DHCP may be helpful in identifying hardware.
|DS0024||Windows Registry||Windows Registry Key Modification||
Monitor HKLM\Software\Policies\Microsoft\Windows NT\DNSClient for changes to the "EnableMulticast" DWORD value. A value of "0" indicates LLMNR is disabled.