III. Embed Least Privileged approach

Only provide enough privileges that are necessary

Before Docker arrived we used to run software on bare-metal servers or virtual machines at best. Your host Operating Systems (OS) had to be carefully prepared with all the dependencies pre-installed. The chances of running multiple applications on the same host, even built on similar stack, were slim due to dependency clashes - also known as Dependency Hell.

What’s worst these applications often had to operate in an elevated administrator privilege context due to its requirements to write to file system or having to open up communication channel on a specific port.

How containers solve the elevated privileges problem?

You design your application with containers in mind by virtue of adopting Dockerfile declaration for your application. From the previous chapter we know that containerised applications build on top of Base Images, such as Core OS (Windows Server Core or Alpine Linux) or a Framework (.NET Core or Node) - which in turn are built on core OS…

The Least Privileged restrictions can be applied in two ways:

  1. Inside Dockerfile composition stage for Image Builds
  2. Inside YAML Declaration for Kubernetes Deployments.

Note: useful reference points on Docker Security and Resource Constraints.

What are my options for the Least Privileged restrictions?

Run Container as unprivileged user

This is simply saying don't run this workload as Administrator. As result, whoever manages to enter your workload will not be able to cause too much damage other than whatever is in the remit of the unprivileged user running it.

Prevent privilege escalation

In addition to the above, you can also explicitly state that privilege escalation is to be completely banned inside of the container workload.

Add or remove Container capabilities

As with any OS, Docker Container is no exception - it has several processes running inside. These are defined as capabilities and range from opening ports, firewalls, file system commands or networking. If your container workload doesn’t need any of these OS capabilities, you should simply disable them before these get exploited.

Limit computer resources available to Container

Only assign CPU and RAM levels that are required for the application to function properly - and nothing more. As result, you also prevent your host from Denial-of-service attack since both CPU and RAM are capped to a fixed ceiling.

Read-only file system

If your container workload is stateless by nature and doesn't need to modify its file system, then simply turn off the ability to write to it.

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