The Most Significant Concern with an Industrial Control System (ICS) that Uses Older Unsupported Technology

D.

The most significant concern for an IS auditor reviewing an industrial control system (ICS) that uses older unsupported technology in the scope of an upcoming audit is D. There is a greater risk of system exploitation.

Explanation: When a system is no longer supported, it means that there are no more updates or patches available to address any security vulnerabilities that may be discovered. This leaves the system open to exploitation by attackers, who can take advantage of the system's weaknesses to gain unauthorized access, disrupt operations, or steal sensitive information.

In the case of an industrial control system (ICS), which is used to control critical infrastructure such as power grids, water treatment plants, and transportation systems, a successful attack could have severe consequences, including physical harm to people and damage to the environment.

Option A, technical specifications not documented, is a concern but not the most significant concern as it doesn't directly increase the risk of system exploitation.

Option B, disaster recovery plans (DRPs) not in place, is also a concern, but it is not the most significant concern in this scenario because it is possible to develop DRPs even if the technology is unsupported.

Option C, attack vectors are evolving for industrial control systems, is a general concern for all ICS regardless of the technology used, and is not the most significant concern when reviewing an ICS with unsupported technology.

Therefore, the auditor should focus on the greater risk of system exploitation associated with unsupported technology as the most significant concern. The auditor should also ensure that the organization has implemented compensating controls and taken other steps to mitigate this risk as much as possible.