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Secure microservice communication needs three distinct controls: encrypt the connection and verify its peer, authenticate the workload making the call, and have the receiving service authorize the requested operation. If a call carries a user’s identity, the receiver must validate that context too—but a valid identity assertion is not permission to access a resource.
What each service-to-service call needs
Think of a request as carrying separate questions that must each be answered:
- Is the connection protected? TLS protects sensitive traffic and lets a client verify the server endpoint.
- Which workload is calling? Authenticate the calling service, for example with mutual TLS (mTLS) or a validated service token.
- May this caller perform this operation on this resource? The receiving service makes that authorization decision using the context available to it.
- Is there a user context? If the call is on a user’s behalf, validate the propagated identity separately from the workload identity and then make an authorization decision.
These controls complement one another. TLS does not decide whether a caller may read a particular record, and an application token does not replace transport encryption for sensitive traffic. OWASP’s Web Service Security Cheat Sheet describes TLS requirements; its Microservices Security Cheat Sheet covers workload identity, authorization, and propagated user context.
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Use well-configured TLS for sensitive web-service communications. A client should not merely accept any certificate presented by the server: it needs to verify that the certificate is trusted, unexpired, not revoked, matches the service domain, and demonstrates possession of the corresponding private key. These checks help ensure the connection is encrypted with the intended service rather than an unverified endpoint.
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TLS server authentication answers, “Am I connected to the service I intended to reach?” It does not, by itself, establish the identity of the client workload. For that, use a separate application-layer identity mechanism or mutual TLS.
Authenticate workloads with mTLS or service tokens
Two common patterns establish which service is calling. Both require an operational plan for the credentials that make the identity verifiable.
| Approach | What it establishes | Where validation happens | Operational considerations |
|---|---|---|---|
| mTLS | Both sides present credentials, enabling mutual service authentication as well as connection confidentiality and integrity. | During the TLS connection, using the parties’ certificates and configured trust. | Provision certificates and keys, bootstrap trust, and manage revocation and rotation. Certificate lifecycle is part of the control, not a one-time setup. |
| Signed service token | A service can obtain a token from a security token service using its own service identity. The token can carry caller identity and permissions. | The receiving service validates the token online or offline, according to the design. | Operate the token-issuing service and define validation, token lifetime, and key or credential management. Token validation is separate from transport encryption. |
When mTLS is a fit
Use mTLS when you want the connection itself to authenticate both endpoints. The client authenticates the service it calls, and the receiving service authenticates the client workload. OWASP identifies confidentiality, integrity, and mutual identification as benefits. That mutual identification depends on a managed certificate lifecycle: decide how certificates are issued and provisioned, how workloads bootstrap trust, how compromised credentials are revoked, and how credentials are rotated.
When service tokens are a fit
A token pattern can express workload identity at the application layer. A service obtains a signed token from a security token service using its own identity and presents it with each request; the receiver validates it online or offline. The token may carry identity and permissions, but it does not encrypt the connection. Keep TLS in place for sensitive traffic, and make the receiver’s authorization policy explicit rather than treating token validity as blanket access.
Enforce authorization where the protected operation lives
An API gateway can reject unauthorized inbound requests and provide a useful coarse-grained control point. It should not be the only place authorization is enforced. A gateway may not have the downstream resource or business context needed to decide whether a particular operation is allowed, and internal calls may not pass through the same ingress path.
Each service should protect its own sensitive operations, including those reached through internal calls. The service that owns the relevant resource or business rule is best placed to use that context in its decision. Also control network routes so callers cannot bypass intended ingress checks by reaching a service directly; routing controls complement, rather than replace, service-level authorization.
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Propagate user identity without treating it as permission
When a service calls another service on behalf of a user, the downstream service needs a representation of the authenticated user context that it can validate. It also needs to authenticate the calling workload. Those identities answer different questions: the assertion describes the user context, while workload authentication identifies the service making the request.
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Decide whether a service mesh fits your platform
A service mesh is an infrastructure-layer option for applying security configuration consistently across services. NIST SP 800-204A describes a mesh as an abstraction layer for defining and implementing security requirements without requiring changes to each microservice’s code. Google Cloud’s Cloud Service Mesh security documentation describes TLS-based service-to-service encryption and authentication, along with authorization configuration.
A mesh is not a universal requirement and does not remove the need to define who may perform each protected operation. Compare the approaches against your operating model:
Quick Recap
- Policy ownership: Decide whether security policy should primarily be configured in service code, in shared infrastructure, or across both.
- Credential lifecycle: Identify who operates certificate provisioning and rotation for mTLS, or token issuance and validation for an application-layer pattern.
- Authorization context: Ensure the service that owns a resource can make decisions using its business and resource context, even when a gateway or mesh supplies additional controls.
- Platform fit: Weigh the consistency a mesh can provide against the operational responsibility of managing that layer. Application-level tokens instead place more of the identity and validation work in the services and token infrastructure.
A practical design sequence
- Map the call paths. Identify service callers, receiving services, ingress routes, and calls made on a user’s behalf. Note any direct route that could bypass an intended gateway.
- Protect sensitive connections. Configure TLS and require clients to validate the server certificate’s trust, expiry, revocation status, domain match, and proof of private-key possession.
- Choose workload authentication. Select mTLS, signed service tokens, or a platform approach such as a mesh based on where identity validation and credential lifecycle will be operated.
- Define authorization at the receiver. Have each service check the caller’s access to its protected operations and resources, including for internal requests.
- Handle user context explicitly. When forwarding a user identity, use a representation the receiver can validate; authenticate the calling service separately and authorize the operation independently.
- Operate credentials continuously. Establish how credentials are provisioned, how trust is bootstrapped, and how credentials or signing keys are rotated and revoked. Confirm that each receiver validates credentials according to the chosen pattern.
- Check for bypasses. Verify that network paths do not let a request avoid required ingress checks, and confirm that direct service calls still encounter the service’s own authorization enforcement.
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