AWS CloudFormation Linter (cfn-lint) v1

Introduction

The CloudFormation Linter, cfn-lint, is a powerful tool designed to enhance the development process of AWS CloudFormation templates. It serves as a static analysis tool that checks CloudFormation templates for potential errors and best practices, ensuring that your infrastructure as code adheres to AWS best practices and standards. With its comprehensive rule set and customizable configuration options, cfn-lint provides developers with valuable insights into their CloudFormation templates, helping to streamline the deployment process, improve code quality, and optimize AWS resource utilization.

What’s Changing?

With cfn-lint v1, we are introducing a set of major enhancements that involve breaking changes. This upgrade is particularly significant as it converts from using the CloudFormation spec to using CloudFormation registry resource provider schemas. This change is aimed at improving the overall performance, stability, and compatibility of cfn-lint, ensuring a more seamless and efficient experience for our users.

Key Features of cfn-lint v1

1. CloudFormation Registry Resource Provider Schemas: The migration to registry schemas brings a more robust and standardized approach to validating CloudFormation templates, offering improved accuracy in linting. We use additional data sources like the AWS pricing API and botocore (the foundation to the AWS CLI and AWS SDK for Python (Boto3)) to improve the schemas and increase the accuracy of our validation. We extend the schemas with additional keywords and logic to extend validation from the schemas.
2. Rule Simplification: for this upgrade, we rewrote over 100 rules. Where possible, we rewrote rules to leverage JSON schema validation, which allows us to use common logic across rules. The result is that we now return more common error messages across our rules.
3. Region Support: cfn-lint supports validation of resource types across regions. v1 expands this validation to check resource properties across all unique schemas for the resource type.

Transition Guidelines

To facilitate a seamless transition, we advise following these steps:

Review Templates

While we aim to preserve backward compatibility, we recommend reviewing your CloudFormation templates to ensure they align with the latest version. This step helps preempt any potential issues in your pipeline or deployment processes. If necessary, you can enforce pinning to cfn-lint v0 by running pip install --upgrade "cfn-lint<1"

Handling cfn-lint configurations

Throughout the process of rewriting rules, we’ve restructured some of the logic. Consequently, if you’ve been ignoring a specific rule, it’s possible that the logic associated with it has shifted to a new rule. As you transition to v1, you may need to adjust your template ignore rules configuration accordingly. Here is a subset of some of the changes with a focus on some of the more significant changes.

• In v0, rule E3002 validated valid resource property names but it also validated object and array type checks. In v1 all type checks are now in E3012.
• In v0, rule E3017 validated that when a property had a certain value other properties may be required. This validation has been rewritten into individual rules. This should allow more flexibility in ignoring and configuring rules.
• In v0, rule E2522 validated when at least one of a list of properties is required. That logic has been moved to rule E3015.
• In v0, rule E2523 validated when only one property from a list is required. That logic has been moved to rule E3014.

Adapting extensions to cfn-lint

If you’ve extended cfn-lint with custom rules or utilized it as a library, be aware that there have been some API changes. It’s advisable to thoroughly test your rules and packages to ensure consistency as you upgrade to v1.

Upgrade to cfn-lint v1

Upon the release of the new version, we highly recommend upgrading to cfn-lint v1 to capitalize on its enriched features and improvements. You can upgrade using pip by running pip install --upgrade cfn-lint.

Stay Updated

Keep yourself informed by monitoring our communication channels for announcements, release notes, and any additional information pertinent to cfn-lint v1. You can follow us on Discord. cfn-lint is an open source solution so you can submit issues on GitHub or follow our v1 discussion on GitHub.

Dependencies

cfn-lint v1 uses Python optional dependencies to reduce the amount of dependencies we install for standard usage. If you want to leverage features like graph, or output formats junit and sarif, you will have to change your install commands.

• pip install cfn-lint[graph] – will include pydot to create graphs of resource dependencies using --build-graph
• pip install cfn-lint[junit] – will include the packages to output JUnit using --output junit
• pip install cfn-lint[sarif] – will include the packages to output SARIF using --output sarif

cfn-lint v0 support

We will continue to update and support cfn-lint v0 until early 2025. This includes regular releases to new CloudFormation spec files. We will only add new features into v1.

Thank You for Your Continued Support

We appreciate your continued trust and support as we work to enhance cfn-lint. Our team is committed to providing you with the best possible experience, and we believe that cfn-lint v1 will elevate your CloudFormation template development process.

If you have any questions or concerns, please don’t hesitate to reach out on our GitHub page.