A key control indicator (KCI) is a metric that measures the effectiveness of a control in mitigating a risk. A good KCI for a vulnerability management program should reflect how well the program is reducing the exposure to high-risk vulnerabilities. The percentage of high-risk vulnerabilities addressed is a KCI that shows the proportion of identified high-risk vulnerabilities that have been remediated or mitigated within a defined time frame. This KCI can help monitor the progress and performance of the vulnerability management program and identify areas for improvement.

The other options are not the best KCI for a vulnerability management program because they do not measure the effectiveness of the control. The percentage of high-risk vulnerabilities missed is a measure of the completeness of the vulnerability scanning process, not the control. The number of high-risk vulnerabilities outstanding is a measure of the current risk exposure, not the control. The defined thresholds for high-risk vulnerabilities are a measure of the risk appetite, not the control. References = Risk and Information Systems Control Study Manual, 7th Edition, Chapter 3: IT Risk Assessment, Section 3.4: Risk Indicators, p. 133-134.