Episode 11 — Shared Responsibility and Inheritance
A shared responsibility model is the description of how obligations are divided between a provider and a customer across layers of service. It names what the provider delivers by default, what the customer must configure or operate, and what both sides will monitor over time. The model turns assumptions into explicit commitments so that testing is possible rather than theoretical. A strong model is written, not implied, and it maps to controls that are meaningful to assurance, such as identity, data protection, logging, and recovery. The value is not only fairness but traceability: a reviewer can follow a control from requirement to owner to proof. When teams treat the model as a living reference, changes in architecture or scope are absorbed without creating blind spots. That practice preserves trust as systems evolve.
Cloud service models shape the split before any contract is signed. At a high level, infrastructure as a service places more operational work with the customer, platform as a service shifts more baseline controls to the provider, and software as a service moves the most into the provider’s domain. Regardless of model, customers still own identity choices, data classification, role design, and how privileges are granted. Providers operate the layers they expose, but customers must align usage with business risk and compliance obligations. Understanding these patterns prevents unrealistic expectations, such as assuming a software provider manages your internal access reviews. The lesson is not to memorize every difference but to ask, for each control, which layer can actually influence it. That question keeps discussions grounded in what the service model truly offers.
Control boundaries and ownership are practical, not philosophical. A boundary is the point at which one party’s configuration authority ends and the other’s begins. Ownership means the named party ensures the control is designed, operated, and evidenced at the required maturity. For example, a platform may own disk encryption at the infrastructure layer, while the customer owns key assignment, rotation schedules, and user role restrictions. Boundaries should be drawn where tools and permissions naturally sit, not where an org chart suggests. Documenting these edges with system names and interfaces helps prevent drift when teams change or when new services are added. Clear boundaries simplify testing because the assessor knows exactly where to look for proof and whom to ask.
Inheritance is the practice of accepting a provider’s control as meeting a requirement for the customer when the scope, coverage, and rigor align. It is allowed when the provider can show objective evidence that the control exists and operates for the assets in question. Inheritance is not a shortcut; it is a disciplined recognition that duplicating the same work adds no value. The customer still carries obligations around configuration, monitoring, or usage that sit above the inherited layer. Each inherited control should include a short rationale that states what is covered, what is not, and how applicability to in scope systems is proven. Used correctly, inheritance reduces cost and speeds audits while keeping risk visible.
Provider attestations and documentation enable inheritance to be trusted. Attestations might include independent reports, certification letters, or detailed control descriptions with testing results. Documentation should identify the exact services, regions, and features covered, along with dates, versions, and known limitations. Customers should verify that the attestation period overlaps the assessment window and that the described controls match the services actually in use. Where a provider offers multiple levels or options, the customer must confirm which ones are enabled in their tenant. The aim is to ensure that the promised control is the one protecting the in scope assets, not a generic capability described in marketing. Attestations that are current and precise remove doubt and streamline assessment conversations.
Every inheritance decision has limits, gaps, and residual risk that must be acknowledged. A provider may encrypt storage by default, but the customer may still need to manage keys, rotations, and access policies. A provider may log platform events, while the customer must centralize and monitor application logs. Some controls cannot be inherited at all, such as end user training or business process approvals. Writing these limits down prevents a false sense of coverage and keeps improvement work visible. Residual risk should be stated plainly, with mitigation steps and owners, so leadership understands what remains and why it is acceptable. Transparency turns inheritance from a leap of faith into a measured design choice.
Monitoring provider changes and incidents is part of owning the shared model over time. Providers update services, deprecate features, and publish advisories that can alter the control picture. Customers should subscribe to change feeds, review impact on inherited controls, and record any actions taken. Incident notifications from providers should trigger internal checks, such as confirming exposure, adjusting detections, or communicating with stakeholders. Time synchronization and version notes help correlate provider events with local logs if investigation is needed. Treat these activities as routine governance, not emergency tasks, so the model stays accurate without last minute scrambles. Continuous awareness preserves the integrity of inheritance across the service lifecycle.
Assessor expectations and verification center on clarity, consistency, and proof that matches scope. Assessors will ask for the responsibility matrix, the contracts or statements of work that set obligations, the provider attestations, and the evidence that links those attestations to your tenant or accounts. They will test spot controls on both sides of the boundary to confirm that inheritance claims are accurate and that customer responsibilities are truly performed. Expect questions about timing windows, versions, and any exclusions. The strongest answer is a short, traceable chain from requirement to owner to artifact. Meeting this expectation signals a mature program that understands assurance, not just technology.
Integrating inheritance in MyCSF turns design into daily practice. Use scoping records to name providers and services, mark applicable controls as inherited, and attach provider attestations directly to those references. Add short narratives that explain boundaries and customer obligations that remain. Link customer artifacts—like role settings, key policies, or log integrations—to the same control so the package is complete. Assign tasks to owners for periodic verification of inherited coverage and for monitoring provider changes. In this way, inheritance becomes part of workflows, milestones, and quality gates rather than an afterthought. The platform then produces reports that reflect the true split, which saves time during review.
