In every PaaS offering, which responsibility does Microsoft retain?

In a Platform as a Service (PaaS) model, Microsoft manages the underlying infrastructure, including the operating systems, network, and servers. This also extends to middleware and runtime environments. Customers are responsible for their application code, data, and configuration. Therefore, Microsoft retains responsibility for middleware and runtime management. Encrypting customer data and application code integrity are generally customer responsibilities, though Azure provides tools to assist with these. Client endpoint security is primarily the customer's responsibility, as it involves securing the devices used to access the PaaS service.