Secure microservice communication needs more than an encrypted connection: protect traffic with TLS, authenticate the calling workload, and have each receiving service authorize access to its own operations. When a call carries a user’s identity, validate that context separately from the service identity—and do not mistake a valid identity assertion for permission.
What a secure service-to-service call must establish
A service call has at least two security questions: who is making the request, and may that caller perform the requested operation on the specific resource? If a service acts on a user’s behalf, there is a third question: which user context is being carried, and can the receiver validate it?
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- Protect the connection: use TLS for sensitive traffic so it is encrypted and protected against tampering in transit.
- Authenticate the workload: let the receiving service verify the calling service rather than relying only on network location or an asserted name.
- Authorize the operation: the service that owns the protected operation checks whether this caller, user context, and requested action are allowed.
These controls address different risks. TLS does not decide whether a caller is permitted to read a record, and authentication alone does not grant that permission.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesUse TLS to protect traffic and verify the peer
For sensitive web-service communication, OWASP recommends well-configured TLS. A client should validate the server certificate: it must chain to a trusted authority, be within its validity period, not be revoked, match the service domain, and demonstrate possession of the corresponding private key. A connection that is encrypted but fails peer validation can still connect a client to the wrong endpoint.
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TLS provides server authentication and protects confidentiality and integrity in transit. It does not, by itself, authenticate the calling service to the receiver. That distinction matters for internal APIs too: being inside a private network is not proof of workload identity.
Choose how services authenticate each other
Two common patterns establish workload identity at different layers. Mutual TLS (mTLS) uses connection credentials; a signed service token is validated by the application handling the request. Either pattern still needs a credential lifecycle and an authorization policy.
| Approach | What it establishes | Where validation happens | Operational work |
|---|---|---|---|
| mTLS | Both peers present credentials; the connection provides confidentiality, integrity, and mutual identification. | During the TLS connection, before application-level authorization. | Provision certificates, bootstrap trust, and handle revocation and rotation. |
| Signed service token | A service presents a token obtained from a security token service using its own identity; the token can carry caller identity and permissions. | The receiving application validates the token online or offline. | Manage service credentials used to obtain tokens and the token’s issuance, validation, expiry, and revocation approach. |
These patterns are not substitutes for transport protection. Token validation does not encrypt a request, so use TLS for sensitive traffic even when a service token is present. OWASP describes both mTLS and token-based service authentication in its Microservices Security Cheat Sheet.
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When mTLS fits
mTLS is useful when the platform can reliably issue and distribute workload certificates and maintain trust between services. Because each side authenticates the other, a receiver can distinguish an authenticated workload from an unknown client at the connection boundary. The protection depends on managing the full certificate lifecycle; issuing certificates without a plan for bootstrap, rotation, and revocation leaves an operational gap.
When signed service tokens fit
A token-based pattern can fit systems that already have a security token service and application-level validation. A service obtains a signed token using its own service identity and includes it in requests; receivers verify it and apply their policy. Keep the token’s purpose and permissions appropriately scoped, and treat validation as authentication input—not as an automatic approval for every resource operation.
Keep authorization at the service that owns the operation
An API gateway can screen inbound traffic and reject requests that fail coarse-grained checks. It is a useful ingress control, but it should not be the only place where authorization happens. Downstream services may have resource details or business rules the gateway cannot see, and internal calls may not pass through the same ingress path.
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Each service should enforce authorization for its own protected operations, including requests from other services. Also ensure that network routes do not let callers bypass ingress controls that the architecture depends on. OWASP discusses both edge-level and service-level checks in its microservices guidance.
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Carry user identity without confusing it with service identity
When a service calls another on behalf of a user, the receiver needs a representation of the authenticated user context that it can validate. At the same time, it should authenticate the calling workload separately. The user assertion answers who the request is on behalf of; workload authentication answers which service delivered it.
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A signature or other integrity protection can show that an identity assertion has not been altered. It does not, by itself, authorize access to a particular resource or action. The downstream service must make that decision using its own policy and the context relevant to the operation. OWASP’s Microservices Security Cheat Sheet covers propagation of user identity between services.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Decide whether a service mesh belongs in your architecture
A service mesh is an infrastructure-layer option for applying security configuration consistently without requiring every microservice to implement the same connection-level mechanisms in its own code. NIST describes this approach in SP 800-204A, Building Secure Microservices-based Applications Using Service-Mesh Architecture. Google Cloud’s Cloud Service Mesh security documentation describes TLS-based service-to-service encryption and authentication alongside authorization configuration.
A mesh is not a universal requirement and does not remove the need to define who may perform each operation. Compare it with application-level controls against your team’s platform, policy needs, and capacity to operate identity and credentials.
- Operational ownership: decide who provisions identities, maintains trust, and responds to credential failures.
- Policy management: determine whether rules belong in application code, infrastructure configuration, or a deliberate combination.
- Credential lifecycle: plan for issuance, bootstrap, rotation, revocation, and validation for the chosen approach.
- Service-specific context: preserve authorization checks where the protected resource and business rules are understood.
Check the complete request path
For each sensitive service call, trace the request from its origin through every receiver. Confirm that transport, workload identity, any forwarded user context, and authorization are all accounted for at the point where they matter.
Quick Recap
- Secure the connection: use TLS and verify the server certificate’s trust, validity, revocation status, domain match, and proof of private-key possession, as outlined in the OWASP Web Service Security Cheat Sheet.
- Identify the caller: choose mTLS or a signed service-token pattern and ensure the receiver validates the workload identity.
- Validate forwarded context: if the call represents a user, make the user assertion verifiable and keep it distinct from the calling service’s identity.
- Authorize at the receiver: have the service that owns the operation decide whether the authenticated identities and request context permit the action.
- Maintain credentials: document how credentials are provisioned, rotated, revoked, and checked, and verify that intended ingress controls cannot be bypassed through direct routes.
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