Test your configuration files

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So you have been following Test Driven Development and have a steady and reliable suite of test cases for your application. All your tests in your CICD pipeline work well and you have good sensible test coverages to show for it in your tool of choice. Now you start looking into the git repo you have your application code in and you realize that you have a lot of configuration files in your repo.

  • A few HCL config files for terraform-ing your infrastructure
  • A Dockerfile
  • A few YAML files for your Kubernetes deployments.

Below is a sample deployment YAML from the Kubernetes documentation. Anyone familiar with the K8s deployment YAMLs would notice a few things missing from the spec including the securityContext , memory & cpu limits which are vital to improve the security posture.

As I started writing these configuration files for deployment, I wanted to find some good practices around it. Talking to the security folks around the team helped me understand the things I should include at a minimum like avoiding privileged escalation or ensuring there is a resource request/limit set for some of these configuration files. How do I ensure that my learnings are documented for everyone else who follows ? I could very well write these down in a wiki and assume they will be reviewed while producing new manifests.

How would you add some validations as the number of services you are creating grows and you want to add some standard validations before they go through the stages of deployment ? Kubesec was the first thing I noticed which does some of these security best practice validations.

I preferred to have a validation mechanism in place in the code or pipeline itself which could help me do this in practice and not get ahead with deployments with these vulnerabilities opened up.One of the easy options is using Conftest.

What is Conftest ?

Conftest is a utility to help you write tests against structured configuration data. Conftest relies on the Rego language from Open Policy Agent for writing policies. If you’re unsure what exactly a policy is, or unfamiliar with the Rego policy language, the Policy Language documentation provided by the Open Policy Agent documentation site is a great resource to read.


Conftest is available for Windows, macOS and Linux on the releases page. On Linux and macOS you can download as follows:

How can it be used

The policy language is easy to understand once you stitch up the path to the files or configuration elements to validate. Let us take an example for a Kubernetes deployment YAML.

Let us start with a file with an extension “.rego” and add some of the examples Conftest already have in their examples list.

Conftest looks for deny, violation, and warn rules. The above “policy” is looking for a specific element called “runAsNonRoot” in the element tree below in a kind of a resource called” Deployment”.

You can keep all of these policy rules in a folder called “policy” where Conftest looks by default or you can specify the path using a runtime argument if needed.

How do you run it ?

Once the installation is complete you can run the below command in your terminal.

I have a sample K8s YAML and a few examples of the policy rules you can write as part of your validations here. Some of these rules were readily available in the Conftest examples and creating others wasn’t such a big task.

If you are getting used to the Rego language or specification, you can use the Rego Playground to test your rego policies before integrating it with your suite.

Validation results

Below are the results of running tests on my original YAML. It gives me an idea what Iam missing from the deployment manifest I have created.

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  • Security policies are not in someone’s head
  • Easy integration with any CICD tool ( GitHub Actions in this case)
  • Ability to extend this as a pod admission controller in K8s if you need to prevent pods being scheduled with any of the security constraints missed
  • Easy to read and understand
  • Involve compliance and security early
  • Supporting a huge number of configuration types including HCL, Dockerfile, K8S yamls, SAM templates and so on


Written by

Advocate for developing software the right way. Can hold a conversation on Test Driven Development, DevSecOps, Test Automation and the few buzzwords

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