The risk owner is accountable for managing risk, including overseeing controls addressing identified risk scenarios, regardless of who operates them.
Reference:CRISC Manual - Domain 1: Governance, Slide 82

When a new security vulnerability is made public, the first step is to determine whether the affected technology is used within the organization. This will help to assess the impact and exposure of the vulnerability on the organization's assets, processes, and objectives. If the affected technology is not used within the organization, then the vulnerability does not pose a direct threat and no further action is required.
However, if the affected technology is used within the organization, then the next steps are to identify the systems and components that are vulnerable, evaluate the severity and likelihood of the vulnerability being exploited, and implement appropriate mitigating controls or remediation actions. References = Risk and Information Systems Control Study Manual, Chapter 5, Section 5.3.2.1, p. 240-241

The best course of action when a new application is added to the environment after testing of the SSO control has been completed is to initiate a retest of the full control, as it may reveal any new issues or gaps that the new application may introduce to the SSO control, and ensure that the control remains effective and adequate.
Retesting the control using the new application as the only sample, reviewing the corresponding change control documentation, and re-evaluating the control during the next assessment are not the best courses of action, as they may not provide sufficient assurance, evidence, or timeliness of the control testing, respectively. References = CRISC Review Manual, 7th Edition, page 154.

Risk appetite is the broad-based amount of risk that an organization is willing to accept in pursuit of its objectives. Risk appetite reflects the level of risk that the organization is prepared to take to achieve its strategic goals, and provides guidance and boundaries for the risk management activities and decisions. To define the risk management strategy, which is the plan and approach for managing the risks that may affect the achievement of the organization's objectives, the factor that must be set by the board of directors is the risk appetite. The board of directors is the highest governing body of the organization, and has the ultimate responsibility and authority for setting the direction and oversight of the organization. By setting the risk appetite, the board of directors can communicate its expectations and preferences for the risk exposure and performance of the organization, and ensure alignment with the business objectives and strategies. References = 3

The best recommendation to reduce the risk associated with potential system compromise when a vendor stops releasing security patches and updates for a business-critical legacy system is to segment the system on its own network. Network segmentation is the process of dividing a network into smaller subnetworks or segments, based on different criteria, such as function, location, or security level. Network segmentation helps to isolate the system from the rest of the network, and limit the exposure and access to the system. Network segmentation also helps to improve the performance and security of the network, by reducing the network traffic and congestion, and enhancing the monitoring and control capabilities. The other options are not as effective as segmenting the system on its own network, although they may provide some additional protection or recovery options. Ensuring regular backups take place, virtualizing the system in the cloud, and installing antivirus software on the system are all measures that can help to reduce the risk of data loss or system damage, but they do not address the root cause of the risk, which is the lack of security patches and updates for the system. References = Risk and Information Systems Control Study Manual, Chapter 3, Section 3.2.1, page 3-11.