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Anthropic-Cybersecurity-Skills/skills/detecting-modbus-command-injection-attacks/references/api-reference.md
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API Reference: Detecting Modbus Command Injection Attacks

Modbus Function Codes

Code Function Risk
1 Read Coils Read
3 Read Holding Registers Read
5 Write Single Coil Write (dangerous)
6 Write Single Register Write (dangerous)
15 Write Multiple Coils Write (dangerous)
16 Write Multiple Registers Write (dangerous)
8 Diagnostics Diagnostic

Zeek Modbus Log

#fields ts uid id.orig_h id.orig_p id.resp_h id.resp_p func

Suricata Modbus Rules

alert modbus any any -> any 502 (msg:"Modbus Write Coil"; \
  modbus: function 5; sid:3000001;)
alert modbus any any -> any 502 (msg:"Modbus Write Multiple Registers"; \
  modbus: function 16; sid:3000002;)

pymodbus Library

from pymodbus.client import ModbusTcpClient

client = ModbusTcpClient("192.168.1.100", port=502)
client.connect()
result = client.read_holding_registers(0, 10, slave=1)
print(result.registers)
client.close()

Scapy Modbus Parsing

from scapy.contrib.modbus import ModbusADURequest
from scapy.all import rdpcap

pkts = rdpcap("modbus.pcap")
for pkt in pkts:
    if pkt.haslayer(ModbusADURequest):
        print(f"Function: {pkt.funcCode}")

Detection Thresholds

Anomaly Threshold Severity
Write flood >20 writes/60s CRITICAL
Unknown function code Any HIGH
Unauthorized master Not in allowlist CRITICAL

CLI Usage

python agent.py --zeek-log modbus.log
python agent.py --zeek-log modbus.log --authorized-masters 10.0.0.1 10.0.0.2