Create and retrieve Azure Functions function keys in ARM template

One of my favorite services in Azure is Functions, which allow you to create serverless micro-services. Triggered by events, after which they run their code, Functions are perfect for the event-driven architectures we strive for these days. These events can come from various sources, like when a message is available in Service Bus, timers, an event sent from Event Grid, etc. However, the one we still use a lot is an HTTP trigger, where we expose the Function as a REST endpoint, available for consumers to call into.

Often we will have an architectural guideline, that every REST endpoint needs to be exposed through Azure API Management. Therefor, we expose these HTTP triggered Functions via API Management as well. We also have guidelines that everything is deployed as Infrastructure as Code, so we do this through ARM templates. In this post we dive into the security side of this, and how to set this up in ARM.

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Implementing smart caching of secrets in Azure API Management policies

In a previous blog post, we have seen how to retrieve secrets from Azure Key Vault from an API Management policy. This works great, however, we might start to run into throttling due to the limitations which Key Vault imposes.

This might be due to having an API exposed which we need to call frequently, or because we retrieve secrets from Key Vault in multiple implementations, all of which adds to the restrictions. Luckily, API Management has another policy expression which helps us out here, namely the caching policy.

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Retrieve Azure Key Vault secrets from API Management policies

When working with Azure API Management, often we need to include secrets in our policies. For example, we may need to send a password in our authentication header, or to validate a key in a JWT token. There are several options to store these secrets. We could hardcode them into our policy, however this would mean anyone with access to our API Management instance could read them. An not just them, but also everyone who can look into our source control. because we deploy our policies as Infrastructure as Code.

The second option is to place the secret in a named value. This even provides us with the option to set the value as a secret, meaning it will not show the actual value in the overview.

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However, anyone with access to API Management can still come into the instance, and untick the secret option, and grab the secret. Consequently, this is still not a good option, as we want the management of our secrets to be separate from our API Management administration. Therefor, we will instead store the secret in Azure Key Vault, and retrieve it in our policy.

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