Authenticated REST API with Azure Functions


If you need to authenticate with Azure AD when using a serverless or a message broker that is acting as the API Gateway the fastest approach is to use Azure’s built-in toolset.

In this post I’ll demonstrate how to Authenticate Azure Functions with Azure AD.

Manage CORS

If you’ve created a new REST web service, when accessing the REST API from a browser, a CORS error will appear similar to the image below.

In order to resolve this error, the Origin ‘’ will need to be added to the ‘Allowed Origin’ list.

Note that here you are allowing the requested endpoint to accept requests from the server ‘’.

Once this is complete, the CORS error should be resolved.

*Note that you might still receive the CORS error if you have the ‘App Service Authentication‘ set to On. This seems to be a bug, but turning it off and on again seemed to fix it for me.

Add AAD Authentication

Authentication with Azure Active Directory is relatively straight-forward. If you are creating a Function using Visual Studio and C#, ensure that you set the correct authorization level in the HTTP Trigger.

public static HttpResponseMessage (run [HttpTrigger( AuthorizationLevel.Anonymous)]

Under Settings, go to Authentication/Authorization

There are three items to configure here:

  1. Turn on ‘App Service Authentication‘.
  2. Under ‘Action to take when request is not authenticated‘ set the value to: ‘Log in with Azure Active Directory‘.
  3. Under ‘Authentication Providers‘, this setting needs to be configured.

To configure Azure Active Directory, do the following:

  1. Under ‘Management Mode‘ select ‘Express‘.

2. Ensure the button ‘Create New AD App‘ has been selected.

That’s all that is required. After saving the changes – you should see something similar to the screen below.

Test Authentication

When accessing the requested web service endpoint you will notice now that you are required to login.

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