Introduction to Network Segmentation in OT/ICS Link to heading

Network segmentation is a fundamental cybersecurity strategy for protecting Operational Technology (OT) and Industrial Control System (ICS) environments. Segmentation helps isolate critical assets, reduces the attack surface, and limits lateral movement by attackers. This approach is especially important in environments where availability and safety are paramount.

Warning
Improper network segmentation can lead to a flat network architecture, where a single compromised device can lead to widespread access, increasing the risk of a successful cyber attack.

The Purdue Model for ICS Security Link to heading

The Purdue Enterprise Reference Architecture (PERA), commonly known as the Purdue Model, is the foundational framework for network segmentation in OT/ICS environments. It defines a multi-layered approach to isolate different parts of an industrial network, enhancing both security and operational integrity.

Levels of the Purdue Model Link to heading

The Purdue Model organizes industrial networks into six levels, ranging from Level 0 to Level 5. Each level has a distinct function and set of devices, making network segmentation easier to implement.

Level Name Description Devices/Components
Level 5 Enterprise Network IT networks where business systems and data centers reside. ERP, MES, email servers, cloud applications
Level 4 Site Business Planning and Logistics Manages plant-wide production planning, scheduling, and logistics. MES, CMMS, business analytics
Level 3 Operations Management Site-wide supervisory control systems that coordinate the production process. SCADA servers, HMIs, data historians
Level 2 Supervisory Control Supervisory control and data acquisition systems that interface with control-level devices. SCADA software, HMIs, PLCs, RTUs
Level 1 Basic Control Controls individual processes and equipment, providing direct control and monitoring. PLCs, DCS controllers, sensors, actuators
Level 0 Process Physical processes that involve direct interaction with equipment and sensors. Field devices: sensors, actuators, pumps, motors

Network Zones and Conduits Link to heading

  • Zones: Logical or physical groupings of assets at the same level of trust, function, or security requirement. For example, all devices in Level 1 can be part of a single “Control Zone.”
  • Conduits: Controlled communication pathways between zones, with strict access controls, data flow monitoring, and filtering to ensure secure data transfer.
Info
The Purdue Model’s segmentation levels help isolate OT networks from IT networks, reducing the risk of cyber attacks moving from IT to OT environments.

Best Practices for Network Segmentation in OT/ICS Link to heading

  1. Establish Network Zones Based on Criticality and Function

    • Use the Purdue Model as a guide to define zones within OT networks. Separate enterprise networks (Levels 4 and 5) from control networks (Levels 0-3) to protect critical ICS assets from external threats.

  2. Implement Secure Conduits Between Zones

    • Define and control all network traffic between zones using firewalls, unidirectional gateways, or DMZs (Demilitarized Zones). This helps in monitoring and filtering traffic to prevent unauthorized access.

  3. Deploy Demilitarized Zones (DMZs) for IT-OT Integration

    • Place a DMZ between the IT (Levels 4-5) and OT networks (Levels 0-3) to act as a buffer zone. The DMZ can host services like data historians, jump servers, and intermediary systems that bridge IT and OT communication without exposing critical OT assets directly to the IT network.

  4. Use Firewalls and Intrusion Detection/Prevention Systems (IDS/IPS)

    • Deploy industrial-grade firewalls and IDS/IPS systems at each zone boundary to control and monitor traffic flow. Firewall rules should be restrictive, and network traffic should be logged and analyzed for anomalies.

  5. Implement Network Access Control (NAC)

    • Use NAC to ensure that only authenticated and authorized devices can access specific zones. This helps to prevent rogue devices or unauthorized users from connecting to sensitive network segments.

  6. Adopt a Zero Trust Architecture

    • Implement a “never trust, always verify” approach, especially for devices and users traversing between zones. Ensure that every device is authenticated, and every network flow is verified before allowing access.

Importance of Network Segmentation in OT/ICS Security Link to heading

  • Reduces Attack Surface: By isolating sensitive segments of the network, it limits the potential impact of a breach.
  • Contains Potential Incidents: Segmentation helps contain malware or attackers within a specific zone, reducing the risk of lateral movement.
  • Improves Incident Response: Segmented networks make it easier to detect, isolate, and respond to security incidents.
Example
In the 2017 Triton attack, the lack of proper network segmentation allowed attackers to move from IT to OT environments, targeting safety instrumented systems (SIS). Proper segmentation could have isolated the attack.

Conclusion Link to heading

Network segmentation is a critical cybersecurity strategy for protecting OT/ICS environments. Following the Purdue Model and implementing best practices like DMZs, secure conduits, and Zero Trust principles can significantly enhance the security posture of industrial networks. As OT and IT networks continue to converge, robust segmentation becomes indispensable for ensuring safe, reliable, and resilient industrial operations.

Tip
Regularly review and update your network segmentation strategy to adapt to new threats, technology changes, and operational requirements. A static segmentation model may not suffice in the dynamic world of OT/ICS cybersecurity.