kdigger: context discovery tool for Kubernetes penetration testing

kdigger

kdigger, short for “Kubernetes digger”, is a context discovery tool for Kubernetes penetration testing. This tool is a compilation of various plugins called buckets to facilitate pentesting Kubernetes from inside a pod.

Please note that this is not the ultimate Kubernetes penetration testing. Some plugins perform really simple actions, that could be performed manually by calling the mount command or listing all devices present in dev with ls /dev for example. But some others automate scanning processes, such as the admission controller scanner. In the end, this tool aims to humbly speed up the pentesting process.

Details

Usage warning

Be careful when running this tool, some checks have side effects, like scanning your available syscalls or trying to create pods to scan the admission control. By default these checks will not run without the --active or -a flag.

For example, syscalls scans may succeed to perform some syscalls with empty arguments, and it can alter your environment or configuration. For instance, if the hostname syscall is successful, it will replace the hostname with the empty string. So please, NEVER run with sufficient permissions (as root for example) directly on your machine.

Results warning

Some tests are based on details of implementation or side effects on the environment that might be subject to changes in the future. So be cautious with the results. For example, CoreDNS is considering removing the wildcard feature, see CoreDNS issue 4984.

On top of that, some results might need some experience to be understood and analyzed. To take a specific example, if you are granted the CAP_SYS_ADMIN capability inside a Kubernetes container, there is a good chance that it is because you are running in a privileged container. But you should definitely confirm that by looking at the number of devices available or the other capabilities that you are granted. Indeed, it might be necessary to get CAP_SYS_ADMIN to be privileged but it’s not sufficient and if it is your goal, you can easily trick the results by crafting very specific pods that might look confusing regarding this tool results.

It might not be the most sophisticated tool to pentest a Kubernetes cluster, but you can see this as a Kubernetes pentest 101 compilation!

Why another tool?

I started researching Kubernetes security a few months ago and participated in the 2021 Europe KubeCon Cloud-Native Security Day CTF. I learned a lot by watching various security experts conferences and demonstrations and this CTF was a really beginner-friendly entry point to practice what I learned in theory. During a live solving session, I had the opportunity to see how Kubernetes security experts were trying to solve the challenge, how they were thinking, what they were looking for.

So I decided to create a tool that compiles most of the checks we usually do as pentesters when in a Kubernetes pod to acquire information very quickly. There already are various tools out there. For example, a lot of experts were using amicontained, a famous container introspection tool by Jessie Frazelle. This tool is truly awesome, but some features are outdated, like the PID namespace detection, and it is not specialized in Kubernetes, it is only a container tool that can already give a lot of hints about your Kubernetes situation.

That is why, in kdigger, I included most of amicontained features. You can:

• Try to guess your container runtime.
• See your capabilities.
• Scan for namespace activation and configuration.
• Scan for the allowed syscalls.

But you can also do more Kubernetes specific operations:

• Retrieve service account token.
• Scan token permissions.
• List interesting environment variables.
• List available devices.
• Retrieve all available services in a cluster.
• Scan the admission controller chain!

Anyway, this tool is obviously not an automatically hack your Kubernetes cluster application, it is mostly just a compilation of tedious tasks that can be performed automatically very quickly. You still need a lot of expertise to interpret the digest and understand what the various outputs mean. And also, during pentest and challenges, you do not always have Internet access to pull your favorite toolchain, so you can also see this compilation as a checklist that you can somehow perform manually with a basic installation and a shell.

How this tool is built?

In addition to all the available features, this tool was built with a plugin design so that it can be easily extended by anyone that wants to bring some expertises.

For example, you are a security researcher on Kubernetes, and when you are doing CTFs or pentesting real infrastructure, you are often performing specific repetitive actions that could be automated or at least compiled with others. You can take a look at /pkg/plugins/template/template.go to bootstrap your own plugins and propose them to the project to extend the features! You only need a name, optionally some aliases, a description, and filling the Run() function with the actual logic.

Areas for improvement

The expertize proposed by the tool could be refined and more precise. For now, it’s mostly dumping raw data for most of the buckets and rely on the user to understand what it implies.

Generally the output format is not the best and could be reworked. The human format via array lines does not fit all the use cases. The tool also proposes a JSON output format, it has the advantage to exist but is really quirky and uses arrays so extracting information might be a bit unpredictable.

How can I experience with this tool?

Good news! We created a mini Kubernetes CTF with basic steps to experience with the tool and resolve quick challenges. For more information go to the minik8s-ctf repository.

Install & Use

Copyright 2021 Mahé Tardy