Tag: Serverless Architecture

[This post is updated on 19th July 2020 to reflect the latest UI of both Azure Portal and Postman. I’d like to take this chance to correct some of my mistakes made in earlier post, as friendly pointed out by readers in the comment section.]

Today is the first working day of a new year. Today is the second half of year 2020 where I have been instructed to work from home for months. I thus decided to work on a question raised previously by the senior developer in my previous job back in 2018: How do we authenticate an Azure Function?

The authentication tool that I am using is the Azure Active Directory (Azure AD). Azure AD provides an identity platform with enhanced security, access management, scalability, and reliability for connecting users with all our apps.

Setting up Azure Function

The Azure Function that I’m discussing here is the Azure Function app with .NET Core 3.1 runtime stack and published as Code instead as Docker Container.

The whole Function creation process takes about 2 minutes. Once it is successfully created, we can proceed to add a new function to it. In this case, we are going to choose a HTTP trigger, as shown in the screenshot below. We choose to use a HTTP trigger function because later we will show only authenticated users can get the results when sending a POST request to this function.

Once the trigger is created, we will see that there is a default C# code template given which will return the caller a greeting message if a name is provided in the body of HTTP request (or through query string).

HTTP Methods and Function Keys

Before we continue, there are a few things we need to handle. The steps below are optional but I think they are useful for the readers.

Firstly, by default, the Function accepts both GET and POST requests. If you would like to only allow POST request, changing only the C# codes above is not going to help much. The correct way is to choose the accepted HTTP methods for this Function under its “Integration” section, as shown in the screenshot below.

In our case, since we will only accept POST request, we will tick only the POST option.

As you notice in the “Authorization level” dropdown which is right above the “Selected HTTP methods”, it currently says “Function”. Later we must change this but for now we keep it as it is. If you would like to manage the Function Key, or checkout the default one, you can find the keys in the “Function Keys” section of the Function.

Secondly, what is the URL of this Function? Unlike the previous version of Azure Function, the URL of the Function can be retrieved at both the Overview section and the Code + Test section of the Function. However, the URL in the Overview section has no HTTPS, so we will be using the HTTPS URL found in Code + Test, as shown in the screenshot below.

Now if we send a GET request to the Function, we shall receive 404 Not Found, as shown in the following screenshot, because we only open for POST requests.

Thus, when we send another HTTP request but make it to be a POST request, we will receive the message that is found in the C# codes in the Function, as shown in the following screenshot.

Now, everyone can send a POST request and get the message as long as they know the Function Key. So how do we add Authentication to this Function?

Authorization Level for the Function

Remember in the earlier section above, we talked about the Authorization Level of the Function? It has three options: Function, Admin, and Anonymous.

We must change the Authorization Level of the Function to be “Anonymous”, as shown in the screenshot below. This is because for both “Function” and “Admin” levels, they are using keys. What we need here is user-based authentication, hence we must choose “Anonymous” instead.

This step is very important because if we forgot to change the Authorization Level to “Anonymous”, the Function will still need the Function Key as the query string even though the request comes in with a valid access token.

Enable App Service Authorization

After that, we need to visit the App Service of the Function App to turn on App Service Authentication. This feature is at App Service level instead of the Function itself. So please pay attention to where to look for the feature.

After the Authentication is turned on, we need to specify “log in with Azure Active Directory” as the action to be taken when the request is not authenticate, as illustrated below. This step is also very important because if we forgot to change it and “Allow Anonymous requests (no action)”, then no matter whether we set the Authentication Providers or not, people can still access the Function. Hence, please remember to change this setting accordingly.

Next, please click on the Azure Active Directory which is listed as one of the Authentication Providers. It is currently labelled as “Not Configured”. Don’t worry, we will now proceed to configure it.

Firstly, we choose the Express mode as management mode. Then we can proceed to create a new Azure AD. The Portal then will help us to setup a new AD Application (or choose from existing AD Application). You can go to Advanced directly if you are experienced with Azure AD.

You should now see the page which looks like what is shown in the following screenshot.

There is one thing that may catch your attention. It is the last item in the page called “Grant Common Data Service Permissions”. Common Data Service, or CDS, is Microsoft way of providing a secure and cloud-based storage option for our data. There is a one-hour Microsoft online course about CDS, you can take the course to understand more. Grace MacJones, Microsoft Azure Customer Engineer, also gave us a short-and-sweet explanation about this setting on GitHub.

We basically can leave everything as default in the page and proceed to click the “OK” button at the bottom of the page.

After this, the Azure AD will be labelled as “Configure (Express Mode: Create)”. We can then proceed to save the changes.

After the settings are saved, we can refresh the page and realising the Azure AD is now labelled as “Configure (Express: Existing App)”. That means the Azure AD app has been created successfully.

Now, click in to the Azure AD under the Authentication Providers list again. We will be brought to the section where we specified the management node earlier. Instead of choosing Express mode, now we can proceed to choose the Advanced mode.

We will then be shown with Client ID, Issuer Url, and Client Secret, as shown in the following screenshot. According to Ben Martens’ advise, we have to add one more record, which is the domain URL of the Function, to the “Allowed Token Audiences” list to make Azure AD work with this Function, as shown in the following screenshot. (This step is no longer needed with the new interface since October 2019 so I strikethrough it)

When you leave this page, the Azure Portal may prompt you to save it. You can choose not to save it. It is optional. If you save it, the Azure AD mode will be changed from Express to Advanced and this will not affect our setup.

Testing on Postman

Now, let’s test our setup above.

When we send the same POST request to the Function again (with the code query string removed since it’s no longer necessary), this time with the App Service Authorization enabled for the Function App, we will no longer be able to get the same message back. Instead, we are told to be 401 Unauthorised and “You do not have permission to view this directory or page”, as shown in the screenshot below.

Now, let’s try to authenticate ourselves.

To do so, we will make a POST request with the body containing Grant Type, Client ID, Client Secret, and Resource to the following URL:
https://login.microsoftonline.com/<Tenant ID>/oauth2/token to retrieve the access token, as shown in the following screenshot.

If we use the access token to send POST request to our Function, we will be told that we are now authorised and the message found in C# code is presented to us, as shown in the following screenshot.

Conclusion

If you would like to get the claims in the Azure Function, you can refer to the following code which loops through all the claims. If you would like to allow a certain client app to call the Azure Function, you can check for the value of the claim having “appid” as its type.

foreach(var claim in principal.Claims)
{
    log.LogInformation($"CLAIM TYPE: {claim.Type}; CLAIM VALUE: {claim.Value}");
}

That’s all it takes to setup a simple authentication for Azure Function with Azure AD. If you find anything wrong above, feel free to correct me by leaving a message in the comment section. Thanks!

References

• How to Use Azure Active Directory (AAD) Access Tokens in Postman;
• Client App Calling An Azure Function With AAD;
• [Stack Overflow] Azure Function authentication using Azure Active Directory;
• Add OAuth2 Implicit Flow to Azure Function;
• [Stack Overflow] Azure Function 2.x – Get Current User’s Claims;
• Azure Functions – Triggers Usage – Working with Identities;
• ClaimsPrincipal Class – .NET Core 2.2.