Final answer:
The best strategy to track the reduction of an ICS's attack surface and validate the security engineer's plan to the CISO is through performing pre- and post-implementation penetration tests, as they offer concrete, comparative insight into security improvements.
Step-by-step explanation:
The question is about a manufacturing company's security engineer proposing techniques to reduce the attack surface of an Industrial Control System (ICS). The goal is to find the best strategy to track reductions and show that the engineer's plan is successful during each phase to the Chief Information Security Officer (CISO).
To accurately track and demonstrate the effectiveness of security enhancements to the CISO, performing pre- and post-implementation penetration tests would be the most comprehensive approach. This technique allows for a clear before-and-after comparison, demonstrating how vulnerabilities have been closed and security controls strengthened. Tabletop exercises are valuable for assessing theoretical risks, but they do not provide tangible evidence of improvements. Contracting a third-party auditor post-project is useful for an unbiased evaluation, but it may not track progressive improvements over time. Regular vulnerability scans are also important, but penetration testing is a more effective means to uncover deeper security issues that scans may overlook.