Azure Relay

How Azure Relay works

Azure Relay is a brokered connectivity pattern, not a direct network tunnel. The relay lets an external caller reach a service behind a firewall or NAT by sending traffic to a cloud endpoint that the on-premises gateway has already connected to. That preserves inbound reachability without exposing the internal host directly.

This design is useful when an organisation needs selective inbound access, but it also means the relay becomes part of the trust boundary. The relay is only one piece of the path, the gateway, authentication layer, and downstream service must still validate who is calling and what the request is allowed to do.

Why this pattern exists

Relay-based connectivity solves a common deployment problem: internal services often need to be reachable from outside, but opening a firewall port or publishing the host directly increases exposure. A brokered model reduces direct attack surface and can simplify remote integration, partner access, and hybrid application design.

That convenience comes with architectural trade-offs. The relay introduces another dependency, another control plane, and another place where configuration errors can weaken the overall security posture. If the relay is treated as a shortcut around normal access controls, it becomes a governance problem rather than a connectivity feature.

Security implications of untrusted payloads

The primary security concern is not the relay transport itself, but what the gateway and backend do with the forwarded request. If the service accepts untrusted data without strong parsing, validation, and deserialisation safeguards, the relay can become a delivery path for injection, deserialisation abuse, or logic flaws in the internal application.

Because the request arrives through an expected cloud channel, defenders may over-trust it. The real question is whether the downstream application enforces its own authentication, authorisation, input handling, and output safety, regardless of how the request entered the environment.

Operational use cases and control boundaries

Azure Relay is most defensible when it is used as a narrow bridge for a clearly defined workload, with explicit ownership of the relay, gateway, and backend service. It works best when the internal endpoint remains private, the exposed interface is minimal, and the application is designed to tolerate hostile input even from “known” upstream components.

Well-governed usage also depends on visibility. Teams should be able to inventory which services are reachable through the relay, who can initiate those flows, and which internal resources are ultimately exposed. The relay is not a substitute for segmentation, service-level authentication, or least-privilege design.

Risk and Threat Considerations

Brokered connectivity reduces direct exposure, but it can also hide the true attack surface if organisations assume the relay itself makes the backend safe. Any weakness in request handling, deserialisation, or backend authorisation can turn the relay into a controlled ingress path for abuse.

Failure mechanism: An attacker abuses the trusted relay path to deliver malicious or malformed payloads to the gateway or backend, then leverages weak input handling or over-permissive processing to trigger compromise or unauthorised action.

Impact: The result can be internal service compromise, unintended data access, command or logic abuse, and broader exposure of systems that were never meant to be internet-facing.

Practitioner Guidance

Why practitioners should care: Azure Relay should be governed as a trust-boundary component, not just a plumbing choice. The security decision is whether the downstream service can safely receive external requests through an intermediary without assuming those requests are benign.

Common misunderstanding: Teams sometimes treat “no open inbound firewall rule” as equivalent to “secure.” In practice, the relay only removes one exposure; it does not remove the need for strong service authentication, request validation, logging, and backend hardening.

Practitioner takeaway: Use the relay to reduce network exposure, but design the application as if every forwarded request may still be hostile.

Related resources from NHI Mgmt Group

• How should security teams reduce NTLM relay risk in Active Directory?
• When should organisations prioritise relay and coercion controls?
• How should security teams reduce Azure managed identity abuse risk?
• Why is Azure IMDS abuse hard to detect?