Potential Impact: The attacker could manipulate the HMI to display false information, causing the operator to take incorrect actions. They could also potentially send malicious commands to the PLCs at Level 1, disrupting the physical process, damaging equipment, or creating an unsafe condition.
How the Model Helps: Proper segmentation limits the immediate blast radius. The malware is contained within the Level 2 zone. The firewall between Level 2 and Level 3 should prevent it from spreading upwards to the site-level systems. More importantly, a well-configured firewall between Level 2 and Level 1 would restrict the types of commands the HMI can send, potentially blocking unauthorized commands from reaching the PLCs. This containment gives security teams time to respond before the entire system is compromised.

Purdue Model for ICS Cyber Security – Top 30 Question and Answers

Zones: A zone is a grouping of logical or physical assets that share common security requirements. For example, all devices in Level 1 of a specific production line could form a zone. Security policies are applied consistently across all assets within a zone.
Conduits: A conduit is the communication path between two or more zones. Security controls are applied to conduits to protect the data in transit and enforce access control rules, ensuring that only authorized communication is allowed between zones. Firewalls are a common tool for securing conduits.

Part 1: Fundamental Concepts

Part 2: Security Principles & Implementation

Part 3: Advanced & Scenario-Based Questions

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