Retiring the “Back End” Concept

For people who have been in the security industry for some time, we have grown very accustomed to the phrases “front end” and “back end.” These terms, in part, came from the basic network architecture diagram that we used to see frequently when dealing with traditional network hosting:


The phrase “front end” referred to anything located in DMZ 1, and “back end” referred to anything located in DMZ 2. This was convenient because the application layer discussion of “front” and “back” often matched nicely with the network diagram of “front” and “back.”  Your web servers were the first layer to receive traffic in DMZ 1 and the databases which were behind the web servers were located in DMZ 2. Over time, this eventually led to the implicit assumption that a “back end” component was “protected by layers of firewalls” and “difficult for a hacker to reach.”

How The Definition Is Changing

Today, the network diagram and the application layer diagrams for cloud architectures do not always match up as nicely with their network layer counterparts. At the network layer, the diagram frequently turns into the diagram below:


In the cloud, the back end service may be an exposed API waiting for posts from the web server over potentially untrusted networks. In this example, the attacker can now directly reach the database over the network without having to pass through the web server layer.

Many traditional “back end” resources are now offered as a stand alone service. For instance, an organization may leverage a third-party database as a service (DBaaS) solution that is separate from its cloud provider. In some instances, an organization may decide to make their S3 buckets public so that they can be directly accessed from the Internet.

Even when a company leverages integrated solutions offered by a cloud provider, shared resources frequently exist outside the defined, protected network. For instance, “back end” resources such as S3, SQS and DynamoDB will exist outside your trusted VPC. Amazon does a great job of keeping its AWS availability zones free from most threats. However, you may want to consider a defense-in-depth strategy where SSL is leveraged to further secure these connections to shared resources.

With the cloud, we can no longer assume that the application layer diagram and the network layer diagrams are roughly equivalent since stark differences can lead to distinctly different trust boundaries and risk levels. Security reviews of application services are now much more of a mix of network layer questions and application layer questions. When discussing a “back end” application component with a developer, here are a few sample questions to measure its exposure:

*) Does the component live within your private network segment, as a shared resource from your cloud provider or is it completely external?

*) If the component is accessible over the Internet, are there Security Groups or other controls such as authentication that limit who can connect?

*) Are there transport security controls such as SSL or VPN for data that leaves the VPC or transits the Internet?

*) Is the data mirrored across the Internet to another component in a different AWS region? If so, what is done to protect the data as it crosses regions?

*) Does your threat model take into account that the connection crosses a trust boundary?

*) Do you have a plan to test this exposed “back end” API as though it was a front end service?

Obviously, this isn’t a comprehensive list since several of these questions will lead to follow up questions. This list is just designed to get the discussion headed in the right direction. With proper controls, the cloud service may emulate a “back end” but you will need to ask the right questions to ensure that there isn’t an implicit security-by-obscurity assumption.

The cloud has driven the creation of DevOps which is the combination of software engineering and IT operations. Similarly, the cloud is morphing application security reviews to include more analysis of network layer controls. For those of us who date back to the DMZ days, we have to readjust our assumptions to reflect the fact many of today’s “back end” resources are now connected across untrusted networks.

Peleus Uhley
Lead Security Strategist