Over the last decade, Programmable Logic Controllers (PLCs) have been increasingly targeted by attackers to obtain control over industrial processes that support critical services. Such targeted attacks typically require detailed knowledge of system-specific attributes, including hardware configurations, adopted protocols, and PLC control-logic, i.e. process comprehension. The consensus from both academics and practitioners suggests stealthy process comprehension obtained from a PLC alone, to conduct targeted attacks, is impractical. In contrast, we assert that current PLC programming practices open the door to a new vulnerability class based on control-logic constructs. To support this, we propose the concept of Process Comprehension at a Distance (PCaaD), as a novel methodological and automatable approach for system-agnostic exploitation of PLC library functions, leading to the targeted exfiltration of operational data, manipulation of control-logic behavior, and establishment of covert command and control channels through unused memory. We validate PCaaD on widely used PLCs, by identification of practical attacks.
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