Securing Terraform and You Part 1 -- rego, Tfsec, and Terrascan

9/20: The open source version of Terraform is now OpenTofu 

 

Sometimes, I write articles even when things don't work. It's about showing a learning process.

 Using IaC means consistency, and one thing you don't want to do is have 5 open S3 buckets on AWS that anyone on the internet can reach. 

That's where tools such as Terrascan and Tfsec come in, where we can make our own policies and rules to be checked against our code before we init.

 

What I Did (Short Version)

• Integrate checks in the pipeline → if it didn’t meet criteria, it wasn’t created with terraform.
• Reiterated via test failures → If it shouldn’t have been created, I tweaked the check.
• Maintained consistency → The same naming convention was essential.

 

 As this was contract work, I can't show you the exact code used, but I can tell you that this blog post by Cesar Rodriguez of Cloud Security Musings was quite helpful, as well as this one by Chris Ayers.

The issue is using Rego; I found a cool VS Code Extension; Terrascan Rego Editor, as well as several courses on Styra Academy; Policy Authoring and Policy Essentials.

The big issue was figuring out how to tell Terrascan to follow a certain policy; I made it, put it in a directory, and ran the program while in that directory, and it didn't seem to see it.

If you need help, check out Styra Acdemy's OPA courses (Free!) and joining the Slack. Anders Eknertalso helped quite a lot!

 rego is probably excellent for those who know their way around a programming language, but I kept coming across some many different ways to stylize it, I got confused, and not yet having the knowledge on where the program should be looking made rego frustrating. 

There must be a command to allow Terrascan to check for a policy in the active directory -- I just haven't found it yet.

But somehow, the syntax still wasn't clicking for me; So I went back to Chris Ayers' series about it and decided to try tfscan by Aqua instead.

Look at this syntax; The fact that it took a few hours instead of the weeks of poking at rego to solve my problem is amazing. A small sample is below:

---
checks:
- code: CUS001
  description: Custom check to ensure the CostCentre tag is applied to EC2 instances
  impact: By not having CostCentre we can't keep track of billing
  resolution: Add the CostCentre tag
  requiredTypes:
  - resource
  requiredLabels:
  - aws_instance
  severity: ERROR
  matchSpec:
    name: tags
    action: contains
    value: CostCentre
  errorMessage: The required CostCentre tag was missing
  relatedLinks:
  - http://internal.acmecorp.com/standards/aws/tagging.html

That's YAML! I remember it looking different when using CloudFormation templates. You make it, make sure it ends in _tfchecks.yaml (or .json), and put it in the .tfsec root check path -- and if you want to test it quickly without looking for that path, use tfsec --custom-check-dir . to run it in your current directory.

 I was getting the error even when my buckets fit the criteria, and when I asked for help on Twitter, I was told that, essentially, tfsec had been integrated into Trivy, and would not be updated anymore, a little more than a year after they bought tfsec. 

The GitHub announcement page has a chorus of thumbs-down in response. It's a real pity support has been dropped for something that almost worked perfectly out of the box.

More to come.

If any of this helps you, give this post a share.