Internal APIs are often overlooked when it comes to security, but they can be just as risky as public-facing ones. As organisations scale, internal APIs connecting Microservices, teams, and internal tools become just as vulnerable. Hardcoded tokens, over-permissive access, and network-level trust can expose sensitive data even within your own perimeter.
A zero-trust approach flips the model by assuming no user, device, or service is inherently trusted, not even inside your network. Every API call must be explicitly verified, authenticated, and authorised based on identity, context, and policy. Here I break down how to implement it without breaking your systems.
Step-by-step guide to enforce a zero-trust API system
1. Discover and inventory all internal APIs: You cannot secure what you cannot see. Begin by identifying all your internal APIs, including those running in staging, CI/CD pipelines, and older legacy systems. Use automated discovery tools or network scanning to map endpoints, dependencies, and consumers.
2. Classify APIs by sensitivity and function: Not all APIs need the same level of protection. Group them by the type of data they expose, the business function they support, and who consumes them. This helps prioritise enforcement and tailor access controls without blocking workflows.
3. Remove implicit trust between services: Internal traffic is often assumed to be safe. That assumption must be removed. All services calling an API should be treated as external actors and must be authenticated explicitly. Network location or firewall rules alone should not imply trust.
4. Enforce strong authentication at every hop: Use identity-aware methods like OAuth 2.0, mutual TLS, or SPIFFE to verify who or what is calling your APIs. Avoid long-lived shared secrets. Use short-lived tokens that rotate automatically and bind access to a verified identity.
5. Implement fine-grained authorisation policies: Authentication proves who you are, but authorisation controls what you can do. Define access policies based on roles, attributes, and context, like IP range, time of day, or device status. Use policy engines like OPA or custom RBAC/ABAC models for dynamic enforcement.
6. Encrypt all internal API traffic: Even within your internal network, use TLS for all traffic. This prevents sniffing or tampering from compromised devices or misconfigured proxies. Enforce TLS at both ingress and service-to-service communication layers.
7. Monitor all activity and build observability: Enable detailed logging for each request, including source, target, action, and result. Feed these logs into your SIEM or monitoring stack. Use this data to detect anomalies, audit access, and refine your trust policies over time.
8. Limit access based on environment and identity: Develop policies that separate access by environment. For instance, development services should not call production APIs. Identity-based rules make it easier to enforce the separation of duties and protect against accidental cross-environment access.
9. Automate enforcement through gateways or service mesh: Use an API gateway, service mesh, or sidecar proxies to enforce authentication, routing, and policy centrally. This prevents teams from hardcoding security logic and makes zero trust easier to scale across environments.
10. Review, rotate, and refine policies regularly: Zero trust is not a set-and-forget system. Rotate credentials frequently, decommission unused APIs, and update policies based on usage trends or detected threats. Make regular reviews part of your DevSecOps process.