Welcome back fellow geeks.

In this post I’ll be covering the behavior of the network security controls available for Azure Event Hub and the “quirk” (trying to be nice here) in enforcement of those controls specifically for Event Hub. This surfaced with one of my customers recently, and while it is publicly documented, I figured it was worthy of a post to explain why an understanding of this “quirk” is important.

As I’ve mentioned in the past, my primary customer base consists of customers in regulated industries. Given the strict laws and regulations these organizations are subject to, security is always at the forefront in planning for any new workload. Unless you’ve been living under a rock, you’ve probably heard about the recent “issue” identified in Microsoft’s CosmosDB service, creatively named ChaosDB. I’ll leave the details to the researchers over at Wiz. Long story short, the “issue” reinforced the importance of practicing defense-in-depth and ensuring network controls are put in place where they are available to supplement identity controls.

You may be asking how this relates to Event Hub? Like CosmosDB and many other Azure PaaS (platform-as-a-service) services, Event Hub has more than one method to authenticate and authorize to access to both the control plane and the data plane. The differences in these planes are explained in detail here, but the gist of it is the control plane is where interactions with the Azure Resource Manager (ARM) API occurs and involves such operations as creating an Event Hub, enabling network controls like Private Endpoints, and the like. The data plane on the other hand consists of interactions with the Event Hub API and involves operations such as sending an event or receiving an event.

Azure AD authentication and Azure RBAC authorization uses modern authentication protocols such as OpenID Connect and OAuth which comes with the contextual authorization controls provided by Azure AD Conditional Access and the granularity for authorization provided by Azure RBAC. This combination allows you to identify, authenticate, and authorize humans and non-humans accessing the Event Hub on both planes. Conditional access can be enforced to get more context about a user’s authentication (location, device, multi-factor) to make better decisions as to the risk of an authentication. Azure RBAC can then be used to achieve least privilege by granting the minimum set of permissions required. Azure AD and Azure RBAC is the recommended way to authenticate and authorize access to Event Hub due to the additional security features and modern approach to identity.

The data plane has another method to authenticate and authorize into Event Hubs which uses shared access keys. There are few PaaS services in Azure that allow for authentication using shared access keys such as Azure Storage, CosmosDB, and Azure Service Bus. These shared keys are generated at the creation time of the resource and are the equivalent to root-level credentials. These keys can be used to create SAS (shared access signatures) which can then be handed out to developers or applications and scoped to a more limited level of access and set with a specific start and expiration time. This makes using SAS a better option than using the access keys if for some reason you can’t use Azure AD. However, anyone who has done key management knows it’s an absolute nightmare of which you should avoid unless you really want to make your life difficult, hence the recommendation to use Azure AD for both the control plane and data plane.

Whether you’re using Azure AD or SAS, the shared access keys remain a means to access the resource with root-like privileges. While access to these keys can be controlled at the control plane using Azure RBAC, the keys are still there and available for use. Since the usage of these keys when interacting with the data plane is outside Azure AD, it means conditional access controls aren’t an option. Your best bet in locking down the usage of these keys is to restrict access at the control plane as to who can retrieve the access keys and use the network controls available for the service. Event Hubs supports using Private Endpoints, Service Endpoint, and the IP-based restrictions via what I will refer to as the service firewall.

If you’ve used any Azure PaaS service such as Key Vault or Azure Storage, you should be familiar with the service firewall. Each instance of a PaaS service in Azure has a public IP address exposes that service to the Internet. By default, the service allows all traffic from the Internet and the method of controlling access to the service is through identity-based controls provided by the supported authentication and authorization mechanism.

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Access to the service through the public IP can be restricted to a specific set of IPs, specific Virtual Networks, or to Private Endpoints. I want quickly to address a common point of confusion for customers; when locking down access to a specific set of Virtual Networks such as available in this interface, you are using Service Endpoints. This list should be empty because there are few very situations where you are required to use a Service Endpoint now because of the availability of Private Endpoints. Private Endpoints are the strategic direction for Microsoft and provide a number of benefits over Service Endpoints such as making the service routable from on-premises over an ExpressRoute or VPN connection and mitigating the risk of data exfiltration that comes with the usage of Service Endpoints. If you’ve been using Azure for any length of time, it’s worthwhile auditing for the usage of Service Endpoints and replacing them where possible.

Now this is where the “quirk” of Event Hubs comes in. As I mentioned earlier, most of the Azure PaaS services have a service firewall with a similar look and capabilities as above. If you were to set the the service firewall to the setting above where the “Selected Networks” option is selected and no IP addresses, Virtual Networks, or Private Endpoints have been exempted you would assume all traffic is blocked to the service right? If you were talking about a service such as Azure Key Vault, you’d be correct. However, you’d be incorrect with Event Hubs.

The “quirk” in the implementation of the service firewall for Event Hub is that it is still accessible to the public Internet when the Selected Networks option is set. You may be thinking, well what if I enabled a Private Endpoint? Surely it would be locked down then right? Wrong, the service is still fully accessible to the public Internet. While this “quirk” is documented in the public documentation for Event Hubs, it’s inconsistent to the behaviors I’ve observed in other Microsoft PaaS services with a similar service firewall configuration. The only way to restrict access to the public Internet is to add a single entry to the IP list.

So what does this mean to you? Well this means if you have any Event Hubs deployed and you don’t have a public IP address listed in the IP rules, then your Event Hub is accessible from the Internet even if you’ve enabled a Private Endpoint. You may be thinking it’s not a huge deal since “accessible” means open for TCP connections and that authentication and authorization still needs to occur and you have your lovely Azure AD conditional access controls in place. Remember how I covered the shared access key method of accessing the Event Hubs data plane? Yeah, anyone with access to those keys now has access to your Event Hub from any endpoint on the Internet since Azure AD controls don’t come into play when using keys.

Now that I’ve made you wish you wore your brown pants, there are a variety of mitigations you can put in place to mitigate the risk of someone exploiting this. Most of these are taken directly from the security baseline Microsoft publishes for the service. These mitigations include (but are not limited to):

• Use Azure RBAC to restrict who has access to the share access keys.
• Take an infrastructure-as-code approach when deploying new Azure Event Hubs to ensure new instances are configured for Azure AD authentication and authorization and that the service firewall is properly configured.
• Use Azure Policy to enforce Event Hubs be created with correctly configured network controls which include the usage of Private Endpoints and at least one IP address in the IP Rules. You can use the built-in policies for Event Hub to enforce the Private Endpoint in combination with this community policy to ensure Event Hubs being created include at least one IP of your choosing. Make sure to populate the parameter with at least one IP address which could be a public IP you own, a non-publicly routable IP, or a loopback address.
• Use Azure Policy to audit for existing Event Hubs that may be publicly available. You can use this policy which will look for Event Hub hubs with a default action of Allow or Event Hubs with an empty IP list.
• Rotate the access keys on a regular basis and whenever someone who had access to the keys changes roles or leaves the organization. Note that rotating the access keys will invalidate any SAS, so ensure you plan this out ahead of time. Azure Storage is another service with access keys and this article provides some advice on how to handle rotating the access keys and the repercussions of doing it.

If you’re an old school security person, not much of the above should be new to you. Sometimes it’s the classic controls that work best. 🙂 For those of you that may want to test this out for yourselves and don’t have the coding ability to leverage one of the SDKs, take a look at the Event Hub add-in for Visual Studio Code. It provides a very simplistic interface for testing sending and receiving messages to an Event Hub.

Well folks, I hope you’ve found this post helpful. The biggest piece of advice I can give you is to ensure you read documentation thoroughly whenever you put a new service in place. Never assume one service implements a capability in the same way (I know, it hurts the architect in me as well), so make sure you do your own security testing to validate any controls which fall into the customer responsibility column.

Have a great week!