cfn-lint

cfn-lint is a template validator for AWS CloudFormation and AWS SAM, best suited to teams that want checks in their infrastructure workflow. Its rules and integrations make it a practical, focused safeguard, but best-effort handling of complex values means it is not a guarantee that every expression is understood.

Overview

The linter checks CloudFormation YAML and JSON against resource provider schemas, valid property values, and best-practice rules. That focus helps catch template problems before deployment; it is not a substitute for testing provisioned infrastructure or checking deployed resources.

When it encounters complex values it cannot interpret or translate, cfn-lint does not fail for that reason alone. This avoids treating an unknown expression as a confirmed error, but means users should not read a clean result as proof that every value has been fully validated.

Key features

• Rules and customization: Specific rules perform checks and validations, and user-defined rules let teams apply their own standards. That makes cfn-lint more adaptable than a fixed set of checks, though custom rules require teams to define and maintain their own policy.
• Configuration choices: Command-line options, configuration files, and template metadata provide several ways to configure the linter, suiting both ad hoc use and repeatable team workflows.
• SAM support: AWS SAM templates are transformed with AWS SAM before processing, extending the tool to serverless templates as well as CloudFormation.
• Workflow integrations: Editor integrations cover Atom, Emacs, NeoVim/Vim, Sublime, Visual Studio Code, and IntelliJ IDEA; a GitHub Action and pre-commit hook can also bring checks into development workflows.
• Output formats: Quiet, parseable, JSON, JUnit, pretty, and SARIF output give users options for command-line reading and automated workflows.
• Policy capabilities: cfn-lint supports policy as code, policy testing, reporting, and configuration compliance for AWS. Its stated scope does not extend to runtime enforcement, provisioning tests, or deployed checks.

Pricing

cfn-lint costs nothing under the MIT-0 license. There is no paid plan or trial to weigh against its capabilities. The license permits use, copying, modification, distribution, sublicensing, and sale without restriction, making it a straightforward choice for individual and commercial use alike.

Installation is documented through pip, Homebrew for macOS, or Docker. Python 3.10 through 3.14 are supported, a practical constraint for teams choosing a Python-based installation. Users seeking help can join the project's Discord community.

Platforms

cfn-lint supports Linux, macOS, Windows, and self-hosted use. Its documented installation routes include pip, Homebrew on macOS, and Docker, so teams can select an approach suited to their environment.

Who it's for

This is a strong fit for AWS infrastructure developers and teams who author CloudFormation or SAM templates and want configurable checks before changes move through their workflow. Custom rules, CI/CD integration, and multiple output formats are useful where teams need validation to fit existing conventions and automation.

Look elsewhere if the need is to test provisioning behavior, inspect deployed infrastructure, or enforce policy at runtime: cfn-lint does not cover those jobs. It is also a focused AWS template tool, not a general validator for Terraform or Kubernetes configurations.

Pros and cons

• Pro: Checks schemas, property values, and best practices, giving template authors more than basic syntax feedback.
• Pro: Custom rules and several configuration methods let teams adapt validation to their own standards.
• Pro: SAM processing, editor options, a GitHub Action, and a pre-commit hook make it suitable for multiple points in a development workflow.
• Con: Complex values are only handled on a best-effort basis, so some expressions may not be interpreted.
• Con: It does not provide runtime enforcement, provisioning tests, or deployed checks, limiting it to template-focused validation.

Alternatives

For a wider comparison, see Infrastructure Testing Tools and Infrastructure Policy as Code Tools.

• kube-score is another free, open-source option for readers considering a different infrastructure tool.
• Conftest is a free, Apache-licensed alternative for readers considering another open-source tool.
• AWS CloudFormation is worth considering when the goal is the CloudFormation service itself; the service is free, but underlying AWS resources are billed at their own rates.
• Test Kitchen is a free, Apache-licensed alternative for readers comparing open-source infrastructure tools.
• Chef InSpec may suit readers seeking a freemium alternative with a free trial; its free plan is limited to non-production workloads and personal, non-commercial use.
• Cinc Auditor is a free distribution of Chef InSpec for readers who can accept no formal warranties or support.
• kubeconform is another free option for readers considering an alternative infrastructure validator.
• OpenSCAP is a free open-source option for readers comparing infrastructure tools.

Verdict

Choose cfn-lint if you build AWS CloudFormation or SAM templates and want configurable validation integrated into your editing and CI workflow at no cost. Its focused checks and custom rules are compelling; look elsewhere when you need tests of provisioning or deployed systems, or when complex values must be fully evaluated.