Detecting Adversary-in-the-Middle (T1557) with Data Science

Adversary-in-the-Middle (T1557) is how attackers get between hosts to capture credentials and relay authentication. On internal networks the usual tools are Responder for LLMNR and NBT-NS poisoning, mitm6 for IPv6 DNS takeover, and classic ARP cache poisoning. None of these throw a malware signature. They abuse name resolution and Layer 2 mappings that are supposed to be trusted, so the durable signal is structural: one host suddenly claiming to be many others.

That structure is exactly what a few lines of pandas and scapy surface well. Here is how to hunt the three most common T1557 variants.

Where T1557 Shows Up

Zeek dns.log captures LLMNR (UDP 5355) and NBT-NS (UDP 137) name resolution, including who answered
A packet capture (or a SPAN/TAP feed) gives you ARP replies and DHCP offers that Zeek does not log by default
DHCP server logs confirm which host is actually handing out leases

You do not need a SIEM. Parse the Zeek log as a DataFrame and the PCAP with scapy.

LLMNR and NBT-NS Poisoning (T1557.001)

LLMNR and NBT-NS are fallback name resolution. A host that cannot resolve a name over DNS shouts the query to the local segment, and whoever answers wins. Normally almost nobody answers, because the name does not exist. Responder answers everything, claiming that every queried name lives at the attacker's IP.

That is the tell. A legitimate host answers name queries for exactly one name: itself. A poisoner answers for dozens of distinct names. Load dns.log, keep the LLMNR and NBT-NS traffic, and count distinct names each answering IP claims:

import pandas as pd

def load_zeek(path):
    cols = None
    with open(path) as f:
        for line in f:
            if line.startswith("#fields"):
                cols = line.strip().split("\t")[1:]
                break
    return pd.read_csv(path, sep="\t", comment="#", names=cols,
                       na_values=["-", "(empty)"])

dns = load_zeek("dns.log")  # id.orig_h, id.resp_h, id.resp_p, query, answers

name_svc = dns[dns["id.resp_p"].isin([5355, 137])]          # LLMNR + NBT-NS
answered = name_svc[name_svc["answers"].notna()]

# The answer value is the IP the name supposedly resolves to.
# An IP that "owns" many distinct queried names is a Responder-style poisoner.
claims = answered.groupby("answers")["query"].nunique().sort_values(ascending=False)
poisoners = claims[claims > 5]

Set the threshold to your environment. In a quiet segment, any IP answering for more than a handful of distinct names is worth a look. The false positive to expect is a busy print or file server, which you allowlist once and move on.

ARP Cache Poisoning (T1557.002)

ARP poisoning works by lying about the IP-to-MAC mapping, usually telling victims that the gateway's IP is at the attacker's MAC. The structural anomaly is a one-to-many mapping: a single MAC claiming many IPs, or one IP whose MAC flaps. Parse ARP replies (op == 2) from a capture and build the mapping:

from scapy.all import rdpcap, ARP

pkts = rdpcap("capture.pcap")
replies = [(p[ARP].psrc, p[ARP].hwsrc) for p in pkts
           if p.haslayer(ARP) and p[ARP].op == 2]            # is-at replies

arp = pd.DataFrame(replies, columns=["ip", "mac"])

# One IP mapped to multiple MACs over time = spoofing in progress
ip_flap = arp.groupby("ip")["mac"].nunique().sort_values(ascending=False)
conflicts = ip_flap[ip_flap > 1]

# One MAC claiming many IPs = a poisoner flooding the segment
mac_spread = arp.groupby("mac")["ip"].nunique().sort_values(ascending=False)

A gateway IP that suddenly resolves to two MACs is the canonical sign of an in-progress attack. Cross-reference the attacker MAC's vendor prefix (OUI) against your asset inventory: a poisoner is often a host that has no business speaking for the gateway.

Rogue DHCP and Relay Setup (T1557.003)

The same primitive shows up as rogue DHCP: an attacker offers leases that point victims at a malicious gateway or DNS server. The detection is a cardinality check. There should be exactly one DHCP server per scope. Count distinct sources of DHCP OFFER messages (message type 2):

from scapy.all import DHCP, IP

offer_sources = {
    p[IP].src for p in pkts
    if p.haslayer(DHCP)
    and ("message-type", 2) in [o for o in p[DHCP].options if isinstance(o, tuple)]
}

if len(offer_sources) > 1:
    print("Multiple DHCP servers offering leases:", offer_sources)

More than one offering source, outside a known redundant setup, means either a misconfiguration or an attacker staging a relay. Both are worth a page.

Why the Structural View Wins

Signature tools catch the Responder and mitm6 binaries when they match a known hash. The behavior outlives the binary: rename the tool, recompile it, write your own, and it still has to answer for names it does not own or claim a MAC it should not. Counting distinct claims per host catches the technique, not the tool, which is the whole point of hunting with data instead of waiting for a signature.

This is the approach we teach in GTK Cyber's Threat Hunting with Data Science course: turning log and packet data into detections with pandas and statistics. The T1557 reference page has the ATT&CK detail and sub-techniques, and the threat hunting pipeline post shows how to run these checks on a schedule rather than by hand.

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