Securing Azure Web Apps – Part III VNET Integration

In previous articles (Part I and Part II) I’ve described how an Azure web application can be secured and isolated from Internet traffic using Private Integration Endpoints.

If the security requirements are not draconian, a simpler solution can provide the same results, meaning public internet access to web application is forbidden and traffic between web apps is routed only in the VNET.

This is a simple use case, where I have a frontend web app and a backend one. Frontend web app is publicly accessible but the backend web app must be accessible only from the frontend and traffic to it must not be routed over public Internet.

Note: If you want additional protection for the frontend web app (like WAF, DDoS protection, rate limiting) an Azure Application Gateway can be placed in front of it and in Access restriction you will allow traffic only from Application Gateway IP.

The setup looks like below.

Traffic from frontend web app to backend web app is no longer outed trough Internet but on the Azure subnet from within a VNET. In the same time, backend web app will have access restrictions in place to only allow incoming traffic from the VNET (so no Internet traffic allowed). In this way, the only publicly accessible web app is the frontend end one and traffic from the frontend to the backend will go only on Azure infrastructure.

Setup is pretty simple.

1. Create a VNET, if don’t already have one.
2. In the above mentioned VNET, create a subnet with enough IPs (/24).
3. Go to created subnet and setup the following:
   1. Service endpoints: Select Microsoft.Web from the list of service endpoints.
   2. Subnet delegation: Select Microsoft.Web/serverFarms from dropdown
   3. Click Save

• 4. If you have an NSG for the subnet make sure you have the appropriate Allow rules in place to allow inbound/outbound traffic from the VNET you have created and also to allow outbound traffic to the Microsoft.Web service point.

4. Once you have created the VNET and subnet, you can proceed to web app creation.
   1. Create the frontend web app and the associated service plan
   2. Create the backend web app and use the same service plan as in the frontend one
   3. For each of the web app, go to Networking->VNET Integration and integrate the web app with VNET created at Step 1 and subnet created at Step 2.
5. Once created the web apps, proceed to access restrictions for the backend web app. From web app -> Networking -> Access restrictions create a new Allow rule and select Type as Virtual Network. Select Azure subscription in which you are working, select the VNET created at Step 1 and subnet created at Step 2. Give it a name (VNET Allow rule) and a priority (100) and click Save. Now, traffic to backend web app is restricted to only traffic coming from the VNET (when you create an Allow rule in Access restrictions, a default Deny All rule is created so all other traffic is now denied).
6. In the same time, because both frontend and backend web apps are integrated with the same VNET and subnet, traffic from to another will not go over Internet but over the integrated VNET and subnet.
7. If you have an Application Gateway in front of the frontend web app, then you can also restrict traffic to it by creating an Allow rule to permit inbound traffic only from Application Gateway IP. Frontend web app will no longer directly accessible from Internet, all traffic to it will have to pass by Application Gateway, providing additional protection (WAF, etc) and load balancing.

Setup is simpler, backend web applications are isolated from being directly accessible over internet, traffic is being redirected over Azure infrastructure (VNET and subnet).

UPDATE:

When I needed a VM to access to web app backend application, I’ve seen things getting a bit complicated. You cannot deploy the VM in the same subnet created above, where you have integrated backend web app, as the integration at web app service plan level kind of “blocks” the subnet for other deployments. So, for a setup like below:

You need to create a second subnet and deploy the VM inside. After creating the subnet, also add Microsoft.Web service end point to it.

To force traffic from the VM to the web application on the web service points and subnets, VM should have a Deny rule for all outbound internet traffic. Otherwise, traffic will go over internet and it will never reach the backend web app because we have configured it to Deny all incoming traffic, except one coming from the VNET (and it’s subnets).