Securing Azure Functions and Logic Apps

Here is a scenario that I recently encountered. Imagine we are building micro-services using serverless (a mix on Azure Function Apps and Logic Apps) with APIM in the front. Lets say we went with the APIM standard instance and all the logic and function apps are going to be running on consumption plan (for cost reasons as its cheaper). This means we wont be getting any vnet capability and our function and logic apps will be exposed out to the world (remember to get vnet with APIM we have to go with the premium version, we are going APIM standard here for cost saving reasons).

So how do we restrict our function and logic apps to only go through the APIM, in another words all our function and logic apps must only go through the APIM and if anyone tries to access them directly they should be getting a “HTTP 403 Forbidden”.

Lets visualize this scenario; We have some WAF capable ingress endpoint, in this case its Azure Front Door, that is forwarding traffic to APIM which then sends the requests to the serverless apps.
Reason for having Front Door before APIM is because APIM doesn’t have WAF natively so we will need to put something in front of it that has that capability to be secure.

There are few options like Azure Firewall, Application Gateway etc, but for the purposes of this scenario we have Azure Front Door in front of APIM (and we can have an APIM policy that will only accept traffic from Azure Font Door, we wont be going in to that, we will keep it to securing our function apps to just being available via APIM for today)