Service-to-Service Authentication

In a monolith, modules call each other inside one process and one trust boundary. In microservices, every internal call becomes a network request between separate services.

That creates a new security problem: how does one service know the caller is really another trusted service?

Service-to-service authentication solves this by verifying internal callers instead of blindly trusting the network.

This chapter covers zero trust, why internal authentication matters, mTLS, JWTs, OAuth client credentials, and token propagation across service calls.