Which threat could be categorized as an external threat for IACS?

The categorization of external threats encompasses any risks that originate from outside the organization or system boundary. In the context of Industrial Automation and Control Systems (IACS), cyber attacks from hackers or malware are quintessential examples of such external threats because they involve malicious actions perpetrated by individuals or entities that do not have authorized access or control over the IACS. These types of threats typically exploit vulnerabilities in the system and can lead to unauthorized access, data breaches, or online disruptions.

While insider misuse of resources pertains to actions taken by individuals within the organization, system configuration errors arise from internal management or operational mistakes, and physical theft of hardware, although potentially facilitated by external elements, typically concerns internal resource management, their threat levels are not classified as external. Thus, cyber attacks from hackers or malware clearly stand out as the only choice that exemplifies an external threat within the cybersecurity framework of IACS.