How do you design an authentication and authorization strategy for a microservices-based application?

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Authentication verifies who you are; authorization determines what you can do. In a microservices architecture, both must be designed carefully because the distributed nature creates unique challenges around token propagation, service-to-service trust, and consistent policy enforcement.

For user authentication, I implement OAuth 2.0 with OpenID Connect through a centralized identity provider such as Auth0, Okta, AWS Cognito, or Keycloak. The identity provider handles user registration, login, multi-factor authentication, and social login integrations. Upon successful authentication, it issues a JWT access token containing the user's identity and claims. This centralized approach ensures consistent authentication policies across all services and eliminates the need for each service to manage user credentials.

The API gateway validates the JWT token on every incoming request, checking the signature, expiration, and issuer. If the token is valid, the gateway forwards the request to the appropriate service with the token or extracted claims in the request headers. This gateway-level validation prevents unauthenticated requests from reaching any service.

For authorization, I use a layered approach. Coarse-grained authorization at the API gateway level checks whether the user's role has access to the requested endpoint. Fine-grained authorization within each service checks whether the user has permission for the specific resource and action. A user might have permission to access the orders endpoint but should only see their own orders, not other customers' orders.

Role-based access control assigns permissions to roles, and roles to users. This works well for broad permission categories: admin, editor, viewer. Attribute-based access control adds contextual rules: allow access if the user's department matches the resource's department and the request is during business hours. For complex systems, I combine both: RBAC for service-level access control and ABAC for resource-level fine-grained decisions.

For service-to-service authentication, each service has its own identity, typically through certificates managed by the service mesh or workload identity federation. When Service A calls Service B, it presents its service identity. Service B verifies the identity and checks whether Service A is authorized to call the specific endpoint. This prevents compromised services from accessing resources they should not.

Token propagation across services requires careful design. The user's JWT is forwarded in headers through the call chain so that downstream services know the original user's identity. For asynchronous communication through message queues, the user context is embedded in the message headers. Each service validates the token independently rather than trusting that upstream services have already validated it, providing defense in depth.