Which assessment technique is considered the least invasive?

The technique recognized as the least invasive is the Gap Assessment. This assessment method focuses on identifying the differences between the current state of an industrial control system's cybersecurity posture and the desired security standards or frameworks, such as ISA/IEC 62443. It primarily involves reviewing documentation, policies, and procedures, along with interviews and discussions with stakeholders, rather than attempting to exploit vulnerabilities or simulate attacks.

This approach allows organizations to evaluate their security readiness and areas for improvement without disrupting operations or exposing networks to risk, making it a more conservative and non-intrusive method compared to others like penetration testing. In contrast, penetration testing actively seeks to exploit vulnerabilities, which can lead to potential disruptions, while active assessments and cyber risk assessments also involve more direct analysis and testing that may affect system performance or availability.

Thus, a Gap Assessment is a strategic, collaborative, and supportive technique aimed at enhancing security while minimizing impact.