Hardening your Kubernetes Cluster: Don’t run as root!

People sometimes ask my why I do not read for pleasure. As my career entails ingesting the NSA/CISA technical report on Kubernetes Hardening Guidance and translating it into actionable material, I ask that you let me enjoy hobbies that do not involve the written word.

The NSA/CISA technical report on came out on Tuesday, August 3. Quite coincidentally, my colleagues and I have started asking questions about our internal standards for securing Kubernetes clusters for production. Coupled with my current home lab experimentation, I figured it was a good idea to read through this document and see what I could do to secure my lab.

Hopefully, I will get through this document and how I’ve applied everything to my home lab (or, at least, the production cluster in my home lab). For now, though, I thought it prudent to look at the Pod Security section. And, as one might expect, the first recommendation is…

Don’t run as root!

For as long as I can remember working in Linux, not running as root was literally “step one.” So it amazes me that, by default, containers are configured to run as root. All of my home-built containers are pretty much the same: simple docker files that copy the results of an external dotnet publish command into the container and then run the dotnet entry point.

My original docker files used to look like this:

FROM mcr.microsoft.com/dotnet/aspnet:5.0-focal AS base
COPY . /app

ENTRYPOINT ["dotnet", "my.dotnet.dll"]

With some StackOverflow assistance, now, it looks something like this:

FROM mcr.microsoft.com/dotnet/aspnet:5.0-focal AS base
COPY . /app
# Create a group and user so we are not running our container and application as root and thus user 0 which is a security issue.
RUN addgroup --system --gid 1000 mygroup \
    && adduser --system --uid 1000 --ingroup mygroup --shell /bin/sh nmyuser
# Serve on port 8080, we cannot serve on port 80 with a custom user that is not root.
# Tell docker that all future commands should run as the appuser user, must use the user number
USER 1000

ENTRYPOINT ["dotnet", "my.dotnet.dll"]

What’s with the port change?

The docker file changes are pretty simple: add the commands to add a new group and a new user, and using the USER command, tell the docker file to execute as the new user. But why change the ASPNETCORE_URLS and port change? When running as a non-root user, you are restricted to ports above 1024, so I had to change the exposed port. This necessitated some changes to my helm charts and their service deployment, but, overall, the process was straightforward.

My next steps

When I find some spare time in next few months, I’m going to revisit Pod Security Policies and, more specifically, it’s upcoming replacement: PSP Replacement Policy. I find it amusing that the NSA/CISA released guidelines that specify usage of what is now a deprecated feature. I also find it typical of our field that our current name for the new version is, quite simply, “Pod Security Policy Replacement Policy.” I really hope they get a better name for that…

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