A separate always-on service that sits between a client application and an upstream API. It is commonly used to handle WebSocket connectivity, hold secrets away from the browser, and enforce server-side authentication. The trade-off is added infrastructure, scaling, and operational overhead.
Expanded Definition
A relay server is a purpose-built intermediary that receives requests from a client application, then forwards them to an upstream API while preserving control over transport, authentication, and secret handling. It is not the same as a generic reverse proxy or API gateway: the relay server is typically introduced because the client cannot safely hold credentials or maintain a direct, trusted connection to the target service.
In practice, relay servers are common in browser-based applications, real-time integrations, and products that need long-lived connections such as WebSockets. They can terminate sessions, translate protocols, normalise requests, and centralise access checks before the upstream service is reached. The architectural boundary matters: the relay becomes part of the trust chain, so its configuration and lifecycle are security-relevant, not just infrastructural.
Industry usage is broadly consistent on the role of the relay, but there is some overlap in how teams label adjacent components such as proxies, brokers, and backend-for-frontend services. The practical distinction is that a relay server exists to mediate a specific client-to-API path and usually protects secrets or authentication logic that should not live in the client. For that reason, NHIMG treats it as an operational security control as much as an application pattern.
Examples and Use Cases
Relay servers appear whenever a client needs controlled access to a service that should not be exposed directly. They also appear where connection state, protocol conversion, or server-side policy enforcement must sit between the user interface and the upstream system.
- A browser app uses a relay server to call an API with server-held credentials instead of embedding tokens in frontend code.
- A chat or trading interface routes WebSocket traffic through a relay so the browser does not connect directly to the backend service.
- A mobile app sends requests to a relay that applies authentication and request shaping before forwarding to an internal API.
- A SaaS integration uses a relay to convert client-facing HTTPS requests into a different upstream protocol or message format.
- A platform team places a relay in front of third-party APIs to centralise audit logging, throttling, and service-specific policy checks.
The trade-off is straightforward: the relay reduces client-side exposure, but it also introduces another always-on service that must scale, patch, and monitor reliably. If the relay becomes a bottleneck, the protected application path can fail even when the upstream API is healthy.
Security Implications
The main security value of a relay server is that it keeps secrets, trust decisions, and access logic out of the client. That is also where the risk concentrates. If the relay is misconfigured, it can become the easiest route to sensitive upstream APIs, because attackers no longer need to defeat client protections; they only need to abuse the intermediary.
Common failure modes include credential leakage through logs, overly broad forwarding rules, weak origin or request validation, and missing rate controls that allow the relay to be used as an abuse amplifier. In browser-facing deployments, the relay can also become a cross-site request target if authentication boundaries are confused. The operational symptom is often not a dramatic breach but a slow expansion of what the relay is allowed to reach.
For that reason, the relay should be treated as a security boundary with its own monitoring and lifecycle controls. NHIMG’s OWASP Non-Human Identity Top 10 is useful here because relay servers frequently mediate machine credentials and backend trust, even when the client experience looks simple.
Domain and Governance Relevance
In application architecture, the relay server matters because it reshapes who is trusted to talk to the upstream API and under what conditions. That changes governance from a purely client-side question into a server-side ownership problem: access policy, auditability, key handling, and incident response all move onto the relay.
Where the relay protects machine credentials, service tokens, or other backend access material, the NHI angle becomes material rather than incidental. The relay is then part of the control plane for non-human access, because it may be the only place where secret exposure, request attribution, and revocation can be enforced consistently. That is especially important when the relay fronts multiple integrations or brokers calls on behalf of many users or agents.
Practically, organisations should understand a relay server as a trust concentration point. If its ownership is vague, or if different teams assume the upstream API provider or the frontend team is accountable for its behaviour, security gaps appear quickly. NHIMG’s guidance is to treat relay services as governed infrastructure, not disposable glue code.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
MITRE ATT&CK address the attack and risk surface, while CIS Controls v8 and NIST CSF 2.0 set the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| CIS Controls v8 | 6 — Access Control Management | Relay servers centralise and enforce access before upstream APIs. |
| 8 — Audit Log Management | Relays should log forwarding, auth, and policy decisions for traceability. | |
| Recommendation — Enforce least privilege on relay paths and remove unused upstream access quickly. Log relay authentication and forwarding events to support detection and investigation. | ||
| NIST CSF 2.0 | PR.AC — Identity Management, Authentication and Access Control | Relay servers sit inside the access-control path for protected APIs. |
| DE.CM — Security Continuous Monitoring | Relays need monitoring for abuse, misroutes, and anomalous forwarding patterns. | |
| Recommendation — Use PR.AC controls to authenticate clients and restrict relay-to-API access. Monitor relay traffic and alert on unusual destinations, volumes, or failures. | ||
| MITRE ATT&CK | T1090 — Proxy | Relays can be abused as proxy infrastructure or abused proxy channels. |
| Recommendation — Detect proxy-like abuse and investigate unexpected relay destinations or relays. | ||
Related resources from NHI Mgmt Group
Deepen Your Knowledge
Reviewed and updated by the NHIMG editorial team on September 9, 2026.
NHI Mgmt Group — the #1 independent authority on Non-Human Identity, IAM, and Agentic AI security. nhimg.org
