Using the Information Harm Triangle to Mitigate Sequences of Unsafe Control Actions with Defense-in-Depth Cybersecurity Measures

The Information Harm Triangle (IHT) is an approach that seeks to simplify the defense-in-depth design of digital instrumentation and control (I&C) systems.The IHT provides a novel framework for understanding how cyber-attacks targeting digital I&C systems can harm the physical process.The utility of the IHT arises from the decomposition of cybersecurity analysis into two orthogonal vectors: data harm and physical information harm.Cyber-attacks on I&C systems can only directly cause data harm.Data harm is then transformed into physical information harm by unsafe control actions (UCAs) identified using Systems-Theoretic Process Analysis (STPA).Because data harm and physical information harm are orthogonal, defense-in-depth can be achieved by identifying control measures that independently limit data harm and physical information harm.This paper furthers the development of the IHT by investigating the defense-in-depth design of cybersecurity measures for sequences of UCAs.The effects of the order and timing of UCAs are examined for several case studies to determine how to represent these sequences using the IHT.These considerations are important for the identification of data harm and physical information harm security measures, and they influence the selection of efficient measures to achieve defensein-depth.This research enables the benefits of the IHT's simple approach to be realized for increasingly complex cyber-attack scenarios.