Why do Microsoft 365 environments need access reviews as well as technical controls?

Why This Matters for Security Teams

Microsoft 365 access is not static. Shared mailboxes, guest users, delegated admin rights, app consent, and OneDrive or SharePoint links can outlive the business reason they were granted. Technical controls help prevent unsafe access paths, but they do not prove that access is still required. Access reviews close that governance gap by confirming that entitlements still map to a current business need, which is why they complement least-privilege and conditional access rather than replace them.

For identity teams, the issue is not just overprovisioning. It is also drift: a permission that was valid during onboarding, a project, or an incident response window may become stale and invisible unless someone revalidates it. That problem is familiar across NHI governance too, where long-lived access and unattended secrets create hidden exposure. NHIMG’s Ultimate Guide to NHIs notes that 97% of NHIs carry excessive privileges, and the same pattern appears in collaboration platforms when access is granted faster than it is reviewed.

Technical enforcement and periodic review serve different purposes: one blocks or limits bad states, the other detects whether the state is still justified. In practice, many security teams discover stale Microsoft 365 permissions only after a tenant audit, a legal hold, or an account compromise rather than through intentional governance.

How It Works in Practice

Access reviews in Microsoft 365 are the evidence layer. They ask owners, managers, or reviewers to confirm whether users, guests, groups, and app assignments should remain active. Technical controls are the enforcement layer. They include MFA, conditional access, privileged identity management, sensitivity labels, app consent restrictions, and sharing policies. Together, they reduce both the likelihood of misuse and the chance that dormant access survives indefinitely.

This separation aligns with current identity guidance. The OWASP Non-Human Identity Top 10 emphasizes that identity state must be continuously governed, while NIST’s identity guidance treats assurance, lifecycle, and access decisions as distinct controls. In Microsoft 365, that means a reviewer can remove a user from a group even if the technical policy already limits what that group can do. Policy blocks abuse; review removes unnecessary entitlement.

That model works best when it is operationalized around specific objects:

• Guest accounts that remain active after a project ends.
• Privileged roles assigned through Entra ID PIM but never revalidated.
• OAuth app permissions that persist after the app is no longer used.
• Shared links and mailbox delegation that continue to expose data paths.
• Groups with nested or inherited membership that obscures who truly has access.

NHIMG’s NHI Lifecycle Management Guide is useful here because the operational problem is similar: access must be granted, reviewed, rotated, and revoked on a lifecycle, not assumed safe because it once passed approval. For Microsoft 365, reviewers should be able to answer three questions: who still needs access, what business process justifies it, and what technical control would fail if the entitlement stayed in place. These controls tend to break down in large tenants with fragmented ownership because nobody can reliably attest to entitlement purpose at review time.

Common Variations and Edge Cases

Tighter access review processes often increase administrative overhead, so organisations have to balance assurance against review fatigue and owner burnout. That tradeoff is real, especially in Microsoft 365 environments with thousands of guests, apps, and group memberships.

Best practice is evolving, but current guidance suggests using risk-based scoping rather than reviewing everything equally. High-risk areas such as privileged roles, external guests, and sensitive SharePoint sites should be reviewed more often than low-risk collaboration spaces. Reviews should also be paired with technical guardrails that reduce review volume, such as auto-expiration for guest access, restricted app consent, and approval workflows for elevated roles.

The hard cases are usually not the obvious ones. Shared links can bypass group-based controls, delegated mailbox permissions can remain after a staffing change, and app-only permissions can persist even when no human user notices them. Where Microsoft 365 environments span legal, compliance, and IT ownership, access reviews also need a clear decision rule: remove by default unless a business owner actively reattests. That principle is consistent with Ultimate Guide to NHIs — Key Challenges and Risks and the emerging consensus in Zero Trust programs. It is not enough to lock down the tenant; organisations still need a durable way to prove access is justified.

Related resources from NHI Mgmt Group

• Why do Microsoft 365 environments create access governance risk?
• Why do role-based access controls still leave governance gaps in cloud environments?
• What breaks when access reviews stay manual in SaaS environments?
• How should pre-IPO companies govern access reviews for SOX controls?