Introduction to the Purdue Model

Introduction to the Purdue Model

• The Purdue Model is a hierarchical industrial control system (ICS) architecture, defining various levels of control within a facility.
• It provides a framework for understanding and categorizing the different types of systems and devices within an ICS and their interactions.
• The model facilitates better security design by considering the unique needs of each layer and the potential attack vectors.
• It classifies control levels from the low-level field devices to the high-level supervisory control and data acquisition (SCADA) systems.

Layers of the Purdue Model

• Level 0: Field Devices: These are the sensors, actuators, and other equipment directly interacting with physical processes.
  • Examples: Pressure sensors, valves, programmable logic controllers (PLCs).
  • Characteristics: Often located in harsh environments and have limited processing power.
  • Security concerns: Direct physical access, exploits of embedded software.
• Level 1: Control Systems: PLCs, programmable automation controllers (PACs), or other equipment used to collect input data, perform algorithms, and send output signals.
  • Responsible for processing information collected from field devices and executing control actions.
  • Security concerns: Vulnerable to unauthorized access and modification of control logic.
• Level 2: Supervisory Control and Data Acquisition (SCADA): Software applications that monitor and control various processes.
  • Interfaces with Level 1 control systems and often Level 3.
  • Provides overall picture of the plant's operations and can set high-level operational parameters.
  • Security concerns: Potential for denial-of-service attacks, remote exploitation of vulnerabilities.
• Level 3: Process Management: This is where the business logic of the enterprise resides.
  • This is the level where business operations are planned and decisions about the business are made.
  • Security concerns: Potential for disrupting business activities by malicious code, potentially having a much larger effect on the overall corporate network.
• Level 4: Enterprise Control Systems: Integration with enterprise-level systems, like business process management, accounting, and financial systems.
  • Provides interfaces for managing data and reports.
  • This is the level where integration with corporate systems occurs.
  • Security concerns: Potential for unauthorized access to corporate databases and information.
• Level 5: Enterprise Management: Highest level in the model, representing the corporate management and decision-making levels.
  • Typically interacts with Level 4 and possibly even the external world through business-facing applications.
  • Security concerns: Potential for data breaches and sabotage of corporate systems.

Security Implications of the Purdue Model

• Each level of the Purdue Model presents unique security challenges.
• Security measures should be tailored to the specific vulnerabilities and attack surfaces at each level.
• The model highlights the importance of separation and isolation of different levels to limit the impact of security breaches.
• Implementing network segmentation across levels is crucial to limit the risks associated with compromises to a lower level.
• This separation of concerns helps prevent a compromised lower-level system from affecting higher levels and the enterprise-wide network.

Security Principles of the Purdue Model

• Defense in Depth: Implement multiple security mechanisms at each level to increase resilience against breaches.
• Least Privilege: Grant only essential access to each system and user to minimize the potential impact of compromised accounts.
• Network Segmentation: Isolate different levels of the ICS using networks and firewalls to prevent attacks across levels.
• Secure Communication Channels: Employ encryption and other secure protocols between all levels to protect data communication.
• Security Monitoring: Continuously monitor system logs and activities at different levels to detect threats and respond to vulnerabilities.
• Regular Audits and Assessments: Conduct periodic security checks and maintain systems, assess vulnerabilities, and take preventative steps.

Addressing specific security concerns for each level

• Level 0: Physical security, access control, and environmental monitoring are crucial for protecting field devices from tampering.
• Level 1: Access control to controllers, regular software updates, and patches are important to address vulnerabilities.
• Level 2: Encryption, intrusion detection and prevention systems (IDPS), and strong user authentication are essential.
• Levels 3-5: Follow standard enterprise security protocols and policies based on the organization's risk model.

Conclusion

• The Purdue Model provides a structured approach to understanding and addressing security risks in complex industrial control systems.
• A layered security approach, aligned with the model, is vital to safeguard critical infrastructure and operations.
• Implementing appropriate security measures at each level is crucial for protecting the entire system from cyberattacks.