Least privilege in machine-to-machine communication is not optional. It is a control plane for trust. Every API, microservice, job runner, and daemon must talk to others with the smallest possible scope—no more, no less. This removes lateral movement paths. It reduces blast radius. It turns compromise from a system-wide incident into a single denied request.
Start with strict identity. Every machine process needs a unique, verifiable identity—short-lived, scoped, and rotated often. Eliminate static, all-powerful secrets stored in environment variables or configuration files. Use workload identity providers, mutual TLS, or token exchange with encrypted transport.
Then enforce tight authorization. Bind each machine identity to only the endpoints and actions it must access. Define policy centrally but enforce it at every hop. Include contextual checks: IP ranges, job type, time windows. Make access immutable from the calling service.