Least privilege means every client, service, and user gets only the minimum rights they need. Nothing more. No unused API methods. No broad scopes. No silent trust. In gRPC, this turns into strict service definitions, locked-down roles, and sharp boundaries between calls.
Start at the proto file. Define separate RPC methods for separate permissions. Do not put different privilege levels behind the same endpoint. Use service-level authorization checks that fire before the handler logic runs. Keep access control outside business logic so it can be audited and maintained.
Apply mutual TLS everywhere. Verify both client and server identities before any request passes through. Use short-lived credentials with strong rotation policies. Integrate with IAM systems that can grant and revoke access quickly. With gRPC’s metadata, enforce token-based checks that map directly to roles.