Best Practices for Designing a Secure ICS Network

Introduction

Industrial Control Systems (ICS) form the operational core of critical infrastructures such as energy, manufacturing, water, and transportation. The convergence of IT (Information Technology) and OT (Operational Technology) in these environments has increased productivity and process visibility but has also expanded the attack surface for potential adversaries. Crafting a secure ICS network architecture is therefore a technical and operational imperative.

Historical Perspective: Evolution of ICS Networking

Traditional ICS environments were isolated, built on proprietary protocols and hardware (e.g., Modbus, DNP3, PROFIBUS). The shift in the late 1990s and early 2000s toward IP networking, standardization, and remote connectivity introduced both interoperability benefits and security risks. Incidents such as Stuxnet (2010) and BlackEnergy (2014) underscored the potential consequences of inadequate ICS security, prompting frameworks such as NIST SP 800-82 and IEC 62443.

Network Segmentation: The Foundation of ICS Security

The Purdue Model and Beyond

The Purdue Enterprise Reference Architecture (PERA) remains a cornerstone for ICS network segmentation. At a high level, it delineates zones:

• Level 5: Enterprise Zone (corporate IT)

• Level 4: Site Business Planning and Logistics

• Level 3: Site Operations

• Level 2: Area Supervisory Control

• Level 1: Basic Control (PLCs, RTUs, HMIs)

• Level 0: Process (sensors, actuators, I/O)

ITT/OT segmentation is not solely about network topologies but also about enforcing policy at the boundaries. Use demilitarized zones (DMZs) to broker communications between IT and OT domains, applying strict firewall policies and proxying access via dedicated application gateways.

Best Practice:

Implement unidirectional gateways ("data diodes") when possible; otherwise, employ stateful, application-layer firewalls. Avoid direct routing between corporate IT resources and ICS endpoints.

Principle of Least Privilege and Access Control

Authentication & Authorization

Legacy ICS environments often assumed "trust by location," with wide privileges accorded based on network presence. Modern security design must use strong, identity-driven mechanisms:

• Integrate centralized directory services (e.g., Active Directory with Group Policy scoping) while isolating OT credentials from standard IT repositories where feasible.

• Leverage multi-factor authentication (MFA) for remote and privileged access; deploy jump hosts as chokepoints for administrative sessions.

• Implement robust RBAC/ABAC (Role-/Attribute-Based Access Control) to restrict operations at the user and device level.

Managing Third-Party Access

Maintenance contractors and service providers often require remote access. Isolate third-party connections through dedicated VPN appliances or ZTNA (Zero Trust Network Access) systems with rigorous session logging and just-in-time privilege elevation.

Protocol and Traffic Management

Legacy Protocol Risks

ICS protocols (e.g., Modbus/TCP, DNP3, Ethernet/IP) were not designed with confidentiality or integrity in mind. Unencrypted sessions and unauthenticated commands can be intercepted or manipulated.

Mitigation Strategies:

• Where vendor support exists, migrate to secure protocol variants (e.g., Modbus Secure, DNP3 Secure Authentication, OPC UA with TLS).

• Enforce strict network allow-listing: filter by protocol, port, and network segment—disable unused services and unnecessary broadcast traffic.

• Monitor for anomalous protocol commands through deep packet inspection (DPI) or specialized ICS intrusion detection solutions.

Visibility, Monitoring, and Incident Response

Passive Network Monitoring

Active scanning can disrupt fragile ICS endpoints. Instead, deploy passive solutions that mirror network traffic (via SPAN or TAP) to capture protocol usage, baseline behaviors, and detect deviations.

Log Aggregation and Forensics

Correlate logs from network devices, servers, and control applications. Aggregation is optimally performed in a SIEM tuned for industrial environments, with runbooks for escalation and IR tailored to the process context.

Threat Hunting and Continuous Improvement

Encourage periodic threat hunting exercises. Leverage MITRE ATT&CK for ICS to structure detection and response planning. Learn from both historical incidents (e.g., Ukraine power grid attacks 2015-2016) and near-misses to adapt defenses.

OT Asset Management and Lifecycle Considerations

Many OT assets have long service lives (10–30 years) and run outdated operating systems. Network design must account for:

• Rigorous asset inventory and mapping—automated discovery aids resilience.

• Secure baselining of device configurations and timely patch management, accounting for vendor patch cycles and maintenance windows.

• Network-based compensating controls for unpatchable systems (e.g., local firewalls, microsegmentation, virtual patching).

Cooperation Between IT and OT Teams

Historically, IT and OT operated with divergent priorities—confidentiality vs. availability, agility vs. stability. However, converged environments necessitate collaboration:

• Joint risk assessments and incident response exercises.

• Shared playbooks and vocabulary for security technologies and operational constraints.

• Unified governance models: clearly defined roles, accountability, and escalation paths.

Conclusion: Practical Steps Forward

Securing ICS networks is a technical, organizational, and cultural challenge that requires ongoing adaptation. Core best practices include rigorous network segmentation, robust identity and access control, vigilant protocol management, continuous monitoring, detailed asset inventory, and, critically, durable collaboration between IT and OT stakeholders. As the threat landscape evolves, so too must our architectural and operational approaches—transparent, precise, and rooted in both historical lessons learned and pragmatic requirements of the plant floor.

Further Reading and Guidance

• NIST SP 800-82: Guide to Industrial Control Systems (ICS) Security

• IEC 62443 Series: Security for Industrial Automation and Control Systems

• MITRE ATT&CK for ICS Knowledge Base

• Firsthand accounts of high-impact incidents: SANS ICS Security Blog