detecting-modbus-protocol-anomalies

What this skill does

# Detecting Modbus Protocol Anomalies

## When to Use

- When deploying Modbus-specific intrusion detection in an OT environment
- When building baseline models for deterministic Modbus polling patterns
- When investigating suspicious Modbus traffic flagged by OT monitoring tools
- When implementing function code allowlisting on industrial firewalls
- When detecting unauthorized Modbus write commands that could manipulate process setpoints

**Do not use** for securing Modbus communications end-to-end (Modbus has no native security; see implementing-network-segmentation-for-ot for firewall-based controls), for non-Modbus protocol monitoring (see detecting-anomalies-in-industrial-control-systems for multi-protocol), or for active fuzzing of Modbus implementations (see performing-plc-firmware-security-analysis).

## Prerequisites

- Network SPAN/TAP access to monitor Modbus/TCP traffic (port 502)
- Zeek (formerly Bro) with Modbus protocol analyzer or Suricata with OT rulesets
- Python 3.9+ with scapy and pymodbus for custom analysis
- Baseline capture of normal Modbus traffic (minimum 1-2 weeks)
- Documentation of authorized Modbus clients, function codes, and register maps

## Workflow

### Step 1: Capture and Parse Modbus Traffic

Deploy passive monitoring to capture all Modbus/TCP traffic and parse it into structured records for analysis.

```python
#!/usr/bin/env python3
"""Modbus Protocol Anomaly Detection System.

Monitors Modbus/TCP traffic for anomalies including unauthorized
function codes, unusual register access, timing deviations,
and rogue client devices.
"""

import json
import struct
import sys
import time
from collections import defaultdict, deque
from dataclasses import dataclass, field
from datetime import datetime
from statistics import mean, stdev

try:
    from scapy.all import sniff, rdpcap, IP, TCP
except ImportError:
    print("Install scapy: pip install scapy")
    sys.exit(1)


MODBUS_FUNCTION_CODES = {
    1: ("Read Coils", "read"),
    2: ("Read Discrete Inputs", "read"),
    3: ("Read Holding Registers", "read"),
    4: ("Read Input Registers", "read"),
    5: ("Write Single Coil", "write"),
    6: ("Write Single Register", "write"),
    7: ("Read Exception Status", "diagnostic"),
    8: ("Diagnostics", "diagnostic"),
    11: ("Get Comm Event Counter", "diagnostic"),
    12: ("Get Comm Event Log", "diagnostic"),
    15: ("Write Multiple Coils", "write"),
    16: ("Write Multiple Registers", "write"),
    17: ("Report Slave ID", "diagnostic"),
    22: ("Mask Write Register", "write"),
    23: ("Read/Write Multiple Registers", "read_write"),
    43: ("Encapsulated Interface Transport", "diagnostic"),
}


@dataclass
class ModbusAnomaly:
    timestamp: str
    anomaly_type: str
    severity: str
    src_ip: str
    dst_ip: str
    unit_id: int
    func_code: int
    detail: str
    mitre_technique: str = ""


@dataclass
class ModbusSession:
    """Tracks state for a Modbus master-slave session."""
    src_ip: str
    dst_ip: str
    func_codes_seen: dict = field(default_factory=lambda: defaultdict(int))
    register_ranges: set = field(default_factory=set)
    intervals: list = field(default_factory=lambda: deque(maxlen=500))
    last_timestamp: float = 0
    request_count: int = 0
    write_count: int = 0


class ModbusAnomalyDetector:
    """Detects anomalies in Modbus/TCP traffic."""

    def __init__(self):
        self.sessions = {}
        self.baseline_sessions = {}
        self.anomalies = []
        self.authorized_clients = set()
        self.authorized_func_codes = {}  # per-session allowed FCs
        self.packet_count = 0

    def set_authorized_clients(self, clients):
        """Set list of authorized Modbus client IPs."""
        self.authorized_clients = set(clients)

    def set_authorized_func_codes(self, session_key, func_codes):
        """Set allowed function codes for a specific session."""
        self.authorized_func_codes[session_key] = set(func_codes)

    def load_baseline(self, baseline_file):
        """Load baseline profiles from previous capture analysis."""
        with open(baseline_file) as f:
            baseline = json.load(f)
        for key, data in baseline.get("modbus_baselines", {}).items():
            self.baseline_sessions[key] = data
            self.authorized_func_codes[key] = set(data.get("allowed_function_codes", []))
        print(f"[*] Loaded {len(self.baseline_sessions)} Modbus baselines")

    def process_packet(self, pkt):
        """Process a single packet for Modbus anomaly detection."""
        if not pkt.haslayer(TCP) or not pkt.haslayer(IP):
            return

        # Check for Modbus/TCP (port 502)
        if pkt[TCP].dport != 502 and pkt[TCP].sport != 502:
            return

        payload = bytes(pkt[TCP].payload)
        if len(payload) < 8:
            return

        self.packet_count += 1
        timestamp = float(pkt.time)
        ts_str = datetime.fromtimestamp(timestamp).isoformat()

        # Parse MBAP header
        try:
            trans_id = struct.unpack(">H", payload[0:2])[0]
            proto_id = struct.unpack(">H", payload[2:4])[0]
            length = struct.unpack(">H", payload[4:6])[0]
            unit_id = payload[6]
            func_code = payload[7]
        except (IndexError, struct.error):
            return

        # Determine direction
        if pkt[TCP].dport == 502:
            src_ip = pkt[IP].src
            dst_ip = pkt[IP].dst
            is_request = True
        else:
            src_ip = pkt[IP].dst
            dst_ip = pkt[IP].src
            is_request = False

        if not is_request:
            return  # Only analyze requests

        session_key = f"{src_ip}->{dst_ip}"

        # Get or create session
        if session_key not in self.sessions:
            self.sessions[session_key] = ModbusSession(src_ip=src_ip, dst_ip=dst_ip)

        session = self.sessions[session_key]
        session.request_count += 1
        session.func_codes_seen[func_code] += 1

        # ── Anomaly Detection Rules ──

        # Rule 1: Unauthorized Modbus client
        if self.authorized_clients and src_ip not in self.authorized_clients:
            self.anomalies.append(ModbusAnomaly(
                timestamp=ts_str,
                anomaly_type="UNAUTHORIZED_CLIENT",
                severity="critical",
                src_ip=src_ip, dst_ip=dst_ip,
                unit_id=unit_id, func_code=func_code,
                detail=f"Modbus request from unauthorized client {src_ip}",
                mitre_technique="T0886 - Remote Services",
            ))

        # Rule 2: Unauthorized function code
        allowed_fcs = self.authorized_func_codes.get(session_key)
        if allowed_fcs and func_code not in allowed_fcs:
            fc_info = MODBUS_FUNCTION_CODES.get(func_code, (f"Unknown FC{func_code}", "unknown"))
            severity = "critical" if fc_info[1] == "write" else "high"
            self.anomalies.append(ModbusAnomaly(
                timestamp=ts_str,
                anomaly_type="UNAUTHORIZED_FUNCTION_CODE",
                severity=severity,
                src_ip=src_ip, dst_ip=dst_ip,
                unit_id=unit_id, func_code=func_code,
                detail=f"FC {func_code} ({fc_info[0]}) not in allowlist {sorted(allowed_fcs)}",
                mitre_technique="T0855 - Unauthorized Command Message",
            ))

        # Rule 3: Write operation detection
        if func_code in (5, 6, 15, 16, 22, 23):
            session.write_count += 1
            fc_name = MODBUS_FUNCTION_CODES.get(func_code, ("Unknown", ""))[0]

            # Extract register address
            if len(payload) >= 10:
                register_addr = struct.unpack(">H", payload[8:10])[0]
                session.register_ranges.add((func_code, register_addr))

                self.anomalies.append(ModbusAnomaly(
                    timestamp=ts_str,
                    anomaly_type="WRITE_OPERATION",
                    severity="high",