Securing Access to Storage with Private Link

Security. Security is one of the hardest and yet most vital pieces to any application we build, whether on-premise or in the cloud. We see evidence of persons not taking security seriously all the time in the news in the form of data leaks, ransomware, etc. No cloud can automatically safeguard you from attacks but, they do offer some very sophisticated tools to aid in helping you more easily bring a strong security posture to your organization.

In Azure, the release of the Private Link feature is a crucial means to ensure we safeguard access to our PaaS (Platform as a Service) deployments. Using Private Link you can create a secure tunnel from your vNet (Virtual Network) to the PaaS service (Azure Storage for example). By using this tunnel, you can ensure that NO TRAFFIC between your assets in the vNet and the dependent services traverse the public internet. Why is this important?

Encryption can definitely aid to ensure data is kept private while in transit but, it is not perfect and for some industries (finance, insurance, healthcare, etc) the privacy of data is not something that can be left to chance.

Defense in Depth

As with any security posture, its strength lies in the layers of protection which exist whether that in NVAs checking incoming traffic, microsegmentation of networks, and/or IP whitelisting, the key is to never rely solely on one measure to keep you safe.

Private Link, for example, keeps your traffic off the internet but, does not protect you from a malicious actor accessing your storage account from a compromised machine. Ensuring your adhere to a defense in depth mentality is the best security along with assuming and accepting you will have a breach and that, the focus is not so much on preventing the breach but, rather, limiting the damage an attacker can do.

Establish your Private Link endpoint (Storage)

Private Link is available for the following services:

  • Azure Machine Learning
  • Azure Synapse Analytics
  • Azure Event Hub
  • Azure Monitor
  • Azure Data Factory
  • Azure App Configuration
  • Azure-managed Disks
  • Azure Container Registry
  • AKS (Azure Kubernetes Service)
  • Azure SQL Database
  • Azure CosmosDB
  • Azure Database for Postgres
  • Azure Database for MySQL
  • Azure Database for MariaDB
  • Azure Event Grid
  • Azure Service Bus
  • Azure IoT Hub
  • Azure Digital Twins
  • Azure Automation
  • Azure Backup
  • Azure Key Vault
  • Azure Storage (all services)
  • Azure FileSync
  • Azure Batch
  • Azure SiganlR Service
  • Azure Web Apps
  • Azure Search
  • Azure Relay
  • Azure Load Balancer (Standard Only)

Source: https://docs.microsoft.com/en-us/azure/private-link/availability

This visual shows what is going on under the hood:

Manage a Private Endpoint connection in Azure | Microsoft Docs

The Private Link provides a specific endpoint within a Private DNS Zone within the vNet, this DNS is updated with the mapping for the endpoint. Therefore, for machines WITHIN the vNet you can use the typical endpoint and connection strings for services over the Private Link as you would if it did not exist – Private DNS will ensure the address is resolved to the private link IP which in turn allows communication with the connected PaaS service.

Creating the Initial Resources

The Virtual Network offering from Azure is a way to isolate your resources deployed to Azure, most commonly Virtual Machines and other features deployed through the IaaS (Infrastructure as a Service) model. By using vNets and Network Security Groups (NSGs) you can effectively control the incoming and outgoing traffic for this custom network you have created in Azure.

Here is a good link on creating a Virtual Network: https://docs.microsoft.com/en-us/azure/virtual-network/quick-create-portal

The short story is, for the purposes here, you can take the default CIDR for the Address Space (10.0.0.0/16) and the default Subnet CIDR (10.0.0.0/24). This will work fine for this bit (for those more familiar with CIDR you can adjust this to be more practical).

This Virtual Network not being setup we can create our Storage Account. There is an option on the Networking tab for Private Endpoint. By choosing to Add private endpoint you can select the vNet and the subnet in which this Private Endpoint will be created.

A quick note, Private Link will ONLY allow access to a specific service within the storage account; you dont get unfettered access to all services, its designed to be a pipe to a specific service. For this demo, I will be select blob as my file upload code will create and retrieve objects from Azure Storage Blob Service.

Keep the default settings for the Private DNS integration section.

For details on creating Azure Storage Accounts, follow this link: https://docs.microsoft.com/en-us/azure/storage/common/storage-account-create?tabs=azure-portal – please make sure you do not allow anonymous access to containers, this is the number one way data leaks are caused.

Let’s Access Our Blobs

So, right now we have a pretty useless setup. Sure its secure, but we cant do anything with our Private Endpoint. We could deploy a Virtual Machine running IIS or Nginx (or whatever you prefer), through some .NET Core code up there and interact with the endpoint.

Before doing this, consult this link to understand the changes, if any, are needed to allow your application to talk to the Private Endpoint: https://docs.microsoft.com/en-us/azure/private-link/private-endpoint-dns

That works but, I have a different idea. One of the new features in Azure AppService is the ability to allow the App Service to integrate with a Virtual Network. Since AppService is a PaaS it also means we are not required to patch and maintain and underlying OS, we can just deploy our code and indicate to Azure how much in the way of resources we need. Let’s use this.

Prepare our VNet for App Service Integration

So, disclaimer, I am not a Networking expert, though I know enough to get by, there are people who are far more versed than I am. I say this because, one of the requirements for Azure App Service VNet Integration is the App Service be injected into an empty Subnet.

To achieve this, I added a secondary address space to my VNet and created a single subnet within this new Address Space, called Apps. There is likely a better way to do this, but this is what worked for me.

Here is a screen shot of what my VNet Address space and Subnets look like:

VNet Address Space
VNet Subnets

Make sure this is created AHEAD of the next phase where we will create and join an App Service instance to this VNet.

Join an App Service to our Virtual Network

I wont go into how to create an Azure AppService, here is a link explaining it: https://docs.microsoft.com/en-us/azure/app-service/quickstart-dotnetcore?tabs=netcore31&pivots=development-environment-vs

Once the App Service is created, you can upload your code to it. For simplicity, I am posting gists with the code for reading and writing blob data from a Web API, but there are numerous examples you could draw from:

The StorageAccountConnectionString is the value straight from the Azure portal, no changes. This is the beauty of the Private DNS Zone that gets added to the Virtual Network when you add a Private Link. More on that later.

Deploy your application to your Azure AppService and add StorageAccountConnectionString to your Configuration blade, again, the value is verbatim what is listed in the Access Keys blade in the Storage Account.

You can now try to use your application at it should fail. The error, if you look in the logs will be 403 Forbidden which makes sense, we have NOT joined the App Service yet, this proves our security is working – the storage account is NOT visible to anyone outside of our Virtual Network.

I expect you will want to test things locally with the Storage Account, it is onerous to only do your testing when deployed to Azure. While the Storage Account IS setup to allow connections via the Private Link you can also poke a whole in the Storage Account firewall to allow specific IP addresses (or ranges). This can be found under the Networking blade of the Storage Account, look for the Firewall section. There is an option to add your current IP to the allowed list.

In the Azure App Service select the Networking blade. The very first option in the listing is VNet Integration – this is what you want to configure. Select the Click here to configure link.

Select the Add VNet option. This will open a panel with your available VNets and their respective Subnets. This is what we are after. You should see your Apps (or whatever you used) Subnet – select this. Press Ok.

The Configuration screen will update with the new details. Give it a few minutes for the stars to align before you test. Not super long but just long enough that if you test right away it might not work.

What are the results?

If everything is working, your App Service can talk to the Storage Account with NO CONFIGURATION changes. Yet, this is very secure since only the App Service can talk to the Storage Account over the web, lessening the chances of a data leak. There are other ways to further secure this, NSGs and Firewall rules being at the top of the list. But, this is a good start to creating a secure application.

Difference with Service Connections

You may be away of another feature in Azure known as Service Connections, and make no mistake they share a lot of similarities with Private Links but, the two are different.

The greatest difference is that Service Connections expose a Public IP, Private Links do not, they only ever user Private IPs. Thus the two address distinctly different use cases. The Service Connection can limit access to the underlying service but data is still traversing the public web. With Private Link (Endpoints) the data NEVER leaves the VNet and is thus more secure.

MVP Ravi explains this difference in more detail: https://scomandothergeekystuff.com/2020/06/22/azure-service-endpoints-versus-azure-private-links/

Regardless of which you choose I implore you to consider security first and foremost in your applications. There are enough tools and features in Azure to keep security breaches to a minimum.

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