Penetration Testing for Operational Technology (OT): A Practical Guide

Operational Technology (OT) environments like Supervisory Control and Data Acquisition (SCADA), Distributed Control Systems (DCS), and Programmable Logic Controller (PLC) based networks are the backbone of critical infrastructure and industrial processes. Unlike IT systems, their primary goal is availability and safety, not just confidentiality. As cyber threats increasingly target industrial systems, penetration testing for OT has become a vital activity in validating defences and uncovering weaknesses before adversaries do.

But OT penetration testing isn’t the same as IT pen testing. It requires different methodologies, tools, and mindsets. Done poorly, it can disrupt production, damage equipment, or even create safety incidents. Done well, it can provide unmatched visibility into real-world risks.

In this blog, we’ll explore how to approach OT penetration testing safely and effectively.

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Why Penetration Testing in OT Is Different

Penetration testing in IT typically aims to exploit vulnerabilities to demonstrate potential business impact. In OT environments, the stakes are higher:

• Safety first: Disrupting a PLC or process controller could put human lives at risk.
• Availability trumps confidentiality: Stopping production or affecting uptime has major economic consequences.
• Legacy and fragile systems: Many Industrial Control System (ICS) devices run outdated firmware and can crash under normal scanning tools.
• Proprietary protocols: Standard IT pen test tools may not understand OT-specific protocols like Modbus, DNP3, or Profinet.

Because of these differences, OT penetration testing must be carefully scoped and controlled.

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Goals of OT Penetration Testing

An OT penetration test typically aims to:

• Assess exposure of OT assets to external and internal threats.
• Validate segmentation controls between IT and OT networks.
• Evaluate access control and identity management in engineering workstations and control networks.
• Test resilience of monitoring and detection systems (e.g., OT IDS/IPS).
• Identify exploitable weaknesses without disrupting operations.

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Methodology for OT Penetration Testing

A safe and effective OT pen test usually follows a phased approach:

1. Scoping and Planning

• Define objectives aligned with business risk (not just technical curiosity).
• Identify critical assets and processes.
• Agree on rules of engagement (what’s in-scope, what’s off-limits).
• Determine testing windows to minimise operational risk.

2. Passive Reconnaissance

• Collect asset and network data without active scanning.
• Use network traffic monitoring to map protocols, devices, and communication patterns.
• Review architecture diagrams, firewall rules, and configurations.

3. Vulnerability Assessment

• Perform non-intrusive vulnerability analysis of OT devices.
• Correlate findings with vendor advisories and NCSC guidance.
• Use configuration reviews instead of active exploits.

4. Segmentation and Access Testing

• Test firewalls, jump hosts, and remote access controls between IT and OT.
• Attempt to pivot from corporate IT into OT (in a controlled environment or simulation).

5. Exploitation (Controlled)

• Exploitation in live OT is rare and risky.
• Prefer lab environments, digital twins, or testbeds to safely validate exploits.
• Where on production networks, limit to “proof of concept” (e.g., credential reuse, weak access paths).

6. Reporting and Remediation

• Focus findings on business and safety impact, not just technical flaws.
• Provide prioritised recommendations that balance risk reduction with operational feasibility.
• Deliver a roadmap aligned with OT security frameworks (ISA/IEC 62443, NIST 800-82).

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Best Practices

• Engage operations teams early. Their input is critical for safe testing.
• Prefer passive over active methods. Avoid aggressive scanning and fuzzing.
• Use OT-aware tools. Examples: GRASSMARLIN (asset discovery), Wireshark with ICS dissectors, OT-specific intrusion detection systems.
• Test in lab environments first. Use a mirrored or simulated ICS environment for high-risk testing.
• Integrate findings into ongoing risk management. Pen tests should feed into vulnerability management, incident response, and security awareness programs.

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Common Findings in OT Penetration Tests

• Flat network architectures with no segmentation between IT and OT.
• Use of default credentials or weak authentication on PLCs and Human Machine Interfaces.
• Legacy systems with unpatched vulnerabilities.
• Insecure remote access paths (VPNs without MFA, exposed RDP).
• Lack of monitoring or anomaly detection for OT protocols.

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Conclusion

Penetration testing in OT environments is both challenging and essential. While it shares many concepts with IT security testing, it requires a safety-first mindset, passive-first techniques, and collaboration with engineering teams. Done responsibly, OT penetration testing helps asset owners understand their real risk exposure and prioritise investments in resilient, safe, and secure industrial operations.

For more information on OT penetration testing, see Operational Technology (OT) Penetration Testing or contact us using the form below to arrange a call with one of our OT experts.

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