As the initial authorizing agency, are we responsible for performing continuous monitoring (ConMon) oversignt on behalf of other leveraging agencies?

No. It is not the initial authorizing agency’s responsibility to conduct ConMon oversight on behalf of all other agencies. OMB Circular A-130 requires federal agencies to implement the Risk Management Framework (RMF) described in NIST SP 800-37 . The RMF process includes a Monitor step. The purpose of this step is to maintain ongoing situational awareness about the security posture of the system in support of risk management decisions. Each agency that issues an ATO or ATU for a cloud offering must review the cloud service provider’s (CSP’s) ConMon activities to ensure the security posture remains sufficient for its own use and supports an ongoing authorization. This includes reviewing the monthly Plan of Action and Milestones (POA&M), approving deviation requests and significant change requests, and reviewing the results of the annual assessment. With the release of the FedRAMP Rev 5 baselines, security control CA-7 requires CSPs with more than one customer agency to implement collaborative ConMon. This approach is intended to streamline the ConMon process and potentially minimize duplicative efforts in a way that helps each agency still perform their due diligence related to ConMon. The PMO developed a recommended Collaborative ConMon approach, which is described in the FedRAMP Collaborative ConMon Quick Guide. Collaborative ConMon benefits agencies by allowing them to share responsibility for ConMon oversight, and it benefits the CSP by creating a central forum for addressing questions and achieving consensus related to deviation requests, significant change requests and the annual assessment - versus having to coordinate with each agency separately.