title: Anomaly diagnosis in industrial control systems for digital forensics
Creators: name: Cook, Marco Montaldi
Publisher: name: University of Glasgow
Year: 2023

description: Over several decades, Industrial Control Systems (ICS) have become more interconnected and highly programmable. An increasing number of sophisticated cyber-attacks have targeted ICS with a view to cause tangible damage. Despite the stringent functional safety requirements mandated within ICS environments, critical national infrastructure (CNI) sectors and ICS vendors have been slow to address the growing cyber threat. In contrast with the design of information technology (IT) systems, security of controls systems have not typically been an intrinsic design principle for ICS components, such as Programmable Logic Controllers (PLCs). These factors have motivated substantial research addressing anomaly detection in the context of ICS. However, detecting incidents alone does not assist with the response and recovery activities that are necessary for ICS operators to resume normal service. Understanding the provenance of anomalies has the potential to enable the proactive implementation of security controls, and reduce the risk of future attacks. Digital forensics provides solutions by dissecting and reconstructing evidence from an incident. However, this has typically been positioned from a post-incident perspective, which inhibits rapid triaging, and effective response and recovery, an essential requirement in critical ICS. This thesis focuses on anomaly diagnosis, which involves the analysis of and discrimination between different types of anomalous event, positioned at the intersection between anomaly detection and digital forensics. An anomaly diagnosis framework is proposed that includes mechanisms to aid ICS operators in the context of anomaly triaging and incident response. PLCs have a fundamental focus within this thesis due to their critical role and ubiquitous application in ICS. An examination of generalisable PLC data artefacts produced a taxonomy of artefact data types that focus on the device data generated and stored in PLC memory. Using the artefacts defined in this first stage, an anomaly contextualisation model is presented that differentiates between cyber-attack and system fault anomalies. Subsequently, an attack fingerprinting approach (PLCPrint) generates near real-time compositions of memory fingerprints within 200ms, by correlating the static and dynamic behaviour of PLC registers. This establishes attack type and technique provenance, and maintains the chain-of-evidence for digital forensic investigations. To evaluate the efficacy of the framework, a physical ICS testbed modelled on a water treatment system is implemented. Multiple PLC models are evaluated to demonstrate vendor neutrality of the framework. Furthermore, several generalised attack scenarios are conducted based on techniques identified from real PLC malware. The results indicate that PLC device artefacts are particularly powerful at detecting and contextualising an anomaly. In general, we achieve high F1 scores of at least 0.98 and 0.97 for anomaly detection and contextualisation, respectively, which are highly competitive with existing state-of-the-art literature. The performance of PLCPrint emphasises how PLC memory snapshots can precisely and rapidly provide provenance by classifying cyber-attacks with an accuracy of 0.97 in less than 400ms. The proposed framework offers a much needed novel approach through which ICS components can be rapidly triaged for effective response.