Top 20 Secure PLC Coding Practices

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Your PLC might be running, but are you sure it’s running right? Small gaps in secure coding can cause real problems. Below are key practices to enhance ICS/OT security, along with practical implementations.

✅ 1. Modularize PLC Code

  • Split PLC code into reusable modules.
  • Use function blocks or subroutines for better maintainability.
  • Test modules independently before integration.

Example (Structured Text – ST):

FUNCTION_BLOCK MotorControl 
VAR_INPUT 
Start : BOOL; 
Stop : BOOL; 
END_VAR 
VAR_OUTPUT 
MotorRunning : BOOL; 
END_VAR 
IF Start AND NOT Stop THEN 
MotorRunning := TRUE; 
ELSIF Stop THEN 
MotorRunning := FALSE; 
END_IF;

✅ 2. Track Operating Modes

  • Ensure PLC stays in RUN mode.
  • Raise an alarm if mode changes unexpectedly.

Example (Ladder Logic):

--[ ]--[bash]--(ALARM_COIL)--

✅ 3. Keep Operational Logic in PLC

  • Use HMI only for visualization, not critical logic.
  • PLCs handle real-time operations better.

    ✅ 4. Use PLC Flags as Integrity Checks

  • Monitor error flags (e.g., math overflow).
  • Log anomalies for troubleshooting.

Example (ST):

IF DIV(Value1, Value2) => ERROR THEN 
Alarm := TRUE; 
END_IF;

✅ 5. Use Cryptographic/Integrity Checks

  • Implement checksums or hashes for code validation.
  • Alert if unauthorized changes occur.

Example (Python checksum check):

import hashlib 
def verify_checksum(file_path, expected_hash): 
with open(file_path, 'rb') as f: 
file_hash = hashlib.sha256(f.read()).hexdigest() 
return file_hash == expected_hash

You Should Know:

Linux & Windows Commands for ICS Security

  • Check Open Ports (Linux):
    sudo netstat -tulnp | grep -E '502|44818'  Common PLC ports
  • Monitor Network Traffic (Windows):
    Get-NetTCPConnection -State Established | Where-Object {$_.RemotePort -eq 502}
  • Log PLC Communications (Linux):
    tcpdump -i eth0 port 502 -w plc_traffic.pcap
  • Verify Firmware Integrity (Linux):
    sha256sum firmware.bin

What Undercode Say:

PLC security is often overlooked but critical in ICS/OT environments. Implementing secure coding practices, monitoring operational states, and validating integrity can prevent catastrophic failures. Automation without security is a ticking time bomb—always verify, log, and restrict access.

Prediction:

As ICS/OT systems become more connected, attacks targeting PLCs will rise. Secure coding and real-time monitoring will be mandatory, not optional.

Expected Output:

  • Secure, modular PLC code.
  • Active monitoring of PLC states.
  • Cryptographic validation of firmware/logic.
  • Restricted network access to critical ports.

Reference: Top 20 Secure PLC Coding Practices

IT/Security Reporter URL:

Reported By: Zakharb Labshock – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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