monitoring-scada-modbus-traffic-anomalies

name	monitoring-scada-modbus-traffic-anomalies
description	Use when monitors Modbus TCP traffic on SCADA and ICS networks to detect anomalous function code usage, unauthorized register writes, and suspicious communication patterns. The analyst uses deep packet inspection with pymodbus, Scapy, and Zeek to baseline normal PLC/RTU communication behavior, then applies statistical and rule-based anomaly detection to identify reconnaissance, parameter manipulation, and denial-of-service attacks targeting Modbus devices on port 502.
domain	cybersecurity
tags	["Modbus-TCP","SCADA","ICS-security","deep-packet-inspection","anomaly-detection","OT-monitoring"]
subdomain	ot-security
version	1.0.0
author	oyi77
license	Apache-2.0
nist_csf	["PR.IR-01","DE.CM-01","ID.AM-05"]

Monitoring Scada Modbus Traffic Anomalies

Overview

Cybersecurity skill for monitoring scada modbus traffic anomalies. Follows industry best practices and security standards.

When to Use

Trigger phrases:

• "monitoring scada modbus traffic anomalies"

• "Monitors Modbus TCP traffic on SCADA and ICS networks to detect anomalous functi"

• Monitoring OT/ICS networks for unauthorized Modbus commands targeting PLCs, RTUs, or HMIs

• Detecting reconnaissance activity such as Modbus device enumeration (function code 43, Read Device Identification)

• Identifying unauthorized write operations (function codes 05, 06, 15, 16) to coils and holding registers that could alter physical process parameters

• Baselining normal Modbus communication patterns and alerting on deviations in function code distribution, register access ranges, or timing intervals

• Investigating suspected sabotage or insider threats manipulating SCADA process values through Modbus register writes

Do not use on networks without authorization from the asset owner, for active injection or fuzzing against production SCADA systems, or as a replacement for safety-instrumented systems (SIS) that provide physical process protection.

When NOT to Use

• When you lack proper authorization for testing
• For production systems without change management
• When the task requires legal or compliance expertise beyond technical scope

Prerequisites

• Network tap or SPAN port on the OT network segment carrying Modbus TCP traffic (port 502)
• Python 3.9+ with pymodbus (>=3.6), scapy (>=2.5), and pandas for traffic analysis
• Zeek (formerly Bro) installed with the Modbus protocol analyzer enabled for passive traffic logging
• Wireshark or tshark for initial packet capture and validation of Modbus frame structure
• A baseline period of normal operations (minimum 48-72 hours) to establish communication profiles per device pair
• Network diagram identifying Modbus master-slave relationships, device IP addresses, and expected function code usage

Workflow

# Example: IOC detection
import re

IOC_PATTERNS = {
    "ip": r"\b(?:\d{1,3}\.){3}\d{1,3}\b",
    "domain": r"\b[a-z0-9-]+\.[a-z]{2,}\b",
    : ,
    : ,
}

 () -> :
     {k: re.findall(v, text)  k, v  IOC_PATTERNS.items()}