What should an IS auditor do when noticing that security patches for a mission-critical system have not been installed for two months?

Choosing to review the patch management policy and assess the associated risks is the most appropriate action in this scenario. By starting with an evaluation of the existing policy, the auditor can determine whether there are established guidelines for timely patch installation and what the implications are of falling behind on this critical maintenance task.

Understanding the risks involved with unpatched security vulnerabilities is crucial, especially for a mission-critical system. The auditor can analyze factors such as the potential for exploitation, the impact of a breach, and the organization's risk tolerance. This thorough review can guide the next steps, whether that involves reinforcing the policy, recommending immediate patch application, or providing broader strategic advice to enhance the security posture of the organization. It emphasizes the importance of a systematic approach to risk management, rather than acting impulsively without considering the context or underlying policy.

The other options present more reactive or prescriptive actions without first establishing a comprehensive understanding of these risks. They may lead to immediate solutions, but without addressing potential deficiencies in the overall patch management strategy, these actions might not fully resolve the underlying issue.