Industrial Control Systems (ICS) play a crucial role in controlling industrial processes. Unlike conventional IT systems or networks, cyber attacks against ICS can cause destructive physical damage. Zero-day exploits (i.e. unknown exploits) have demonstrated their essential contributions to causing such damage by Stuxnet. In this work, we investigate the possibility of improving the tolerance of a system against zero-day attacks by defending against known weaknesses of the system. We first propose a metric to measure the system tolerance against zero-day attacks, which is the minimum effort required by zero-day exploits to compromise a system. We then apply this metric to evaluate different defensive plans to decide the most effective one in maximising the system tolerance against zero-day attacks. A case study about ICS security management is demonstrated in this paper.
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