Git hooks are scripts that extend Git functionality when certain events and actions occur during code development. Developer teams often use Git hooks to perform quality checks before they commit their code changes. For example, see the blog post Use Git pre-commit hooks to avoid AWS CloudFormation errors for a description of how the AWS Integration and Automation team uses various pre-commit hooks to help reduce effort and errors when they build AWS Quick Starts.

This blog post shows you how to extend your Git hooks to validate your AWS CloudFormation templates against policy-as-code rules by using AWS CloudFormation Guard. This can help you verify that your code follows organizational best practices for security, compliance, and more by preventing you from commit changes that fail validation rules.

We will also provide patterns you can use to centrally maintain a list of rules that security teams can use to roll out new security best practices across an organization. You will learn how to configure a pre-commit framework by using an example repository while you store Guard rules in both a central Amazon Simple Storage Service (Amazon S3) bucket or in versioned code repositories (such as AWS CodeCommit, GitHub, Bitbucket, or GitLab).

Prerequisites

To complete the steps in this blog post, first perform the following installations.

  1. Install AWS Command Line Interface (AWS CLI).
  2. Install the Git CLI.
  3. Install the pre-commit framework by running the following command.pip install pre-commit
  4. Install the Rust programming language by following these instructions.
  5. (Windows only) Install the version of Microsoft Visual C++ Build Tools 2019 that provides just the Visual C++ build tools.

Solution walkthrough

In this section, we walk you through an exercise to extend a Java service on an Amazon EKS example repository with Git hooks by using AWS CloudFormation Guard. You can choose to upload your Guard rules in either a separate GitHub repository or your own S3 bucket.

First, download the sample repository that you will add the pre-commit framework to.

To clone the test repository

Next, create Guard rules that reflect the organization’s policy-as-code best practices and store them in an S3 bucket.

To set up an S3 bucket with your Guard rules

  1. Create an S3 bucket by running the following command in the AWS CLI.

    aws s3 mb s3://<account-id>-cfn-guard-rules --region <aws-region>

    where  is the ID of the AWS account you’re using and  is the AWS Region you want to use.

  2. (Optional) Alternatively, you can follow the Getting started with Amazon S3 tutorial to create the bucket and upload the object (as described in step 4 that follows) by using the AWS Management Console.

    When you store your Guard rules in an S3 bucket, you can make the rules accessible to other member accounts in your organization by using the aws:PrincipalOrgID condition and setting the value to your organization ID in the bucket policy.

  3. Create a file that contains a Guard rule named rules.guard, with the following content.

    let eks_cluster = Resources.* [ Type == 'AWS::EKS::Cluster' ]
    rule eks_public_disallowed when %eks_cluster !empty {
          %eks_cluster.Properties.ResourcesVpcConfig.EndpointPublicAccess == false
    }
    

    This rule will verify that public endpoints are disabled by checking that resources that are created by using the AWS::EKS::Cluster resource type have the EndpointPublicAccess property set to false. For more information about authoring your own rules using Guard domain-specific language (DSL), see Introducing AWS CloudFormation Guard 2.0.

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