Kubernetes – Basics of container security

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Container security is a deep subject area and in itself can fill its own book. Having said this, we will cover some of the high-level concerns and give you a starting point so that you can start thinking about this area.

In the A brief overview of containers section of Chapter 1, Introduction to Kubernetes, we looked at some of the core isolation features in the Linux kernel that enable container technology. Understanding the details of how containers work is the key to grasping the various security concerns in managing them.

A good paper to dive deeper is NCC’s Whitepaper, Understanding and Hardening Linux Containers. In section 7, the paper explores the various attack vectors of concern for container deployments, which I will summarize.

Keeping containers contained 

One of the most obvious features that is discussed in the paper we mentioned in the preceding section is that of escaping the isolation/virtualization of the container construct. Modern container implementations guard against using namespaces to isolate processes as well as allowing the control of Linux capabilities that are available to a container. Additionally, there is an increased move toward secure default configurations of the out-of-the-box container environment. For example, by default, Docker only enables a small set of capabilities. Networking is another avenue of escape and it can be challenging since there are a variety of network options that plug into most modern container setups.

The next area discussed in the paper is that of attacks between two containers. The User namespace model gives us added protection here by mapping the root user within the container to a lower-level user on the host machine. Networking is, of course, still an issue, and something that requires proper diligence and attention when selecting and implementing your container networking solution.

Attacks within the container itself are another vector and, as with previous concerns, namespaces and networking are key to protection here. Another aspect that is vital in this scenario is the application security itself. The code still needs to follow secure coding practices and the software should be kept up to date and patched regularly. Finally, the efficiency of container images has an added benefit of shrinking the attack surface. The images should be built with only the packages and software that’s necessary.

Resource exhaustion and orchestration security

Similar to the denial-of-service (DoS) attacks, we’ve seen in various other areas of computing that resource exhaustion is very much a pertinent concern in the container world. While cgroups provide some limitations on resource usage for things such as CPU, memory, and disk usage, there are still valid attack avenues for resource exhaustion. Tools such as Docker offer some starting defaults to the cgroups limitations, and Kubernetes also offers additional limits that can be placed on groups of containers running in the cluster. It’s important to understand these defaults and to adjust for your deployments.

While the Linux kernel and the features that enable containers give us some form of isolation, they are fairly new to the Linux operating system. As such, they still contain their own bugs and vulnerabilities. The built-in mechanisms for capabilities and namespaces can and do have issues, and it is important to track these as part of your secure container operations.

The final area covered in the NCC paper is the attack of the container management layer itself. The Docker engine, image repositories, and orchestration tools are all significant vectors of attack and should be considered when developing your strategy. We’ll look in more depth at how we can address the repositories and Kubernetes as an orchestration layer in the following sections.

If you’re interested in knowing more about the specific security features of Docker’s implementation, take a look here: https://docs.docker.com/engine/security/security/.

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