The Common Vulnerability Scoring System (CVSS) is a standardized framework for assessing the severity of security vulnerabilities. It helps organizations prioritize vulnerability patching by providing a numerical score that reflects the potential impact and exploitability of a vulnerability.
CVSS scores are used to gauge the urgency of patching vulnerabilities within a company's IT environment.

Cryptojacking is the likely cause in this scenario. It involves malware that hijacks the resources of infected computers to mine cryptocurrency, usually without the user's knowledge. This type of attack doesn't typically degrade performance significantly or result in obvious system failures, which matches the situation described, where the machines showed no signs of degraded performance or excessive failed logins.

Monitoring outbound traffic is essential for detecting unauthorized data exfiltration from a system.
A new vulnerability that allows malware to move data unauthorizedly would typically attempt to send this data out of the network. By monitoring outbound traffic, security tools can detect unusual data transfers, trigger alerts, and help prevent the exfiltration of sensitive information.

The first priority is to revoke any compromise certificates. This ensures that those certificates can no longer be used for unauthorized access.

Mitigate is the risk management strategy that involves reducing the likelihood or impact of a risk.
If a legacy application is critical to business operations and there are preventative controls that are not yet implemented, the enterprise should adopt the mitigate strategy first to address the existing vulnerabilities and gaps in the application. This could involve applying patches, updates, or configuration changes to the application, or adding additional layers of security controls around the application. Accept, transfer, and avoid are other risk management strategies, but they are not the best options for this scenario. Accept means acknowledging the risk and accepting the consequences without taking any action. Transfer means shifting the risk to a third party, such as an insurance company or a vendor. Avoid means eliminating the risk by removing the source or changing the process. These strategies may not be feasible or desirable for a legacy application that is critical to business operations and has no preventative controls in place.