Why does perimeter security fail in the cloud?

Perimeter security fails in the cloud because cloud platforms such as Microsoft 365 are accessed over the public internet and do not sit behind a corporate network boundary in the traditional sense. That means a firewall or VPN can no longer act as the main control point for trust. In practice, access decisions need to happen closer to identity, device posture, session risk, and data sensitivity, which is why cloud security moves toward Conditional Access, compliance signals, and policy-driven access instead of network location alone.