A technical comparison of mTLS and OAuth 2.0 Client Credentials for service-to-service authentication, covering how each works, their tradeoffs, and when to use them. mTLS operates at the transport layer providing workload identity via X.509 certificates, while OAuth 2.0 Client Credentials operates at the application layer providing scoped, auditable authorization. The post explains Certificate-Bound Access Tokens (RFC 8705) and DPoP (RFC 9449) as mechanisms to prevent bearer token theft by binding tokens to cryptographic keys. A practical decision matrix guides when to use each approach, and step-by-step Istio configuration examples show how to run strict mTLS and OAuth JWT validation simultaneously in a Kubernetes cluster. Operational tradeoffs including certificate management, latency, and secret handling are also addressed.
