cf-lint

CloudFormation Lint

Validate a CloudFormation template against best practices.

Input

$ARGUMENTS = path to a CloudFormation YAML template.

Process

1. Read the Template

Parse the template and identify all sections (Parameters, Resources, Outputs, etc.).

2. Structural Checks

• Has Description
• Has AWSTemplateFormatVersion
• Parameters have AllowedValues or AllowedPattern constraints
• Outputs exist for values other stacks might need
• No hardcoded values that should be parameters (account IDs, AMI IDs, ARNs)

3. Security Checks

• No secrets in Parameters default values or resource properties
• Security groups don't allow 0.0.0.0/0 on non-HTTP ports
• IAM policies follow least privilege (no Action: "" or Resource: "")
• Encryption enabled on storage resources (S3, EBS, RDS)
• CloudTrail or logging enabled where applicable

4. Reliability Checks

• Stateful resources have DeletionPolicy: Retain
• Multi-AZ enabled for databases and critical services
• Auto-scaling configured where appropriate

5. Best Practice Checks

• All resources tagged (Environment, Project, ManagedBy)
• Uses !Ref and !GetAtt over hardcoded values
• No circular dependencies
• Stack outputs exported for cross-stack references where needed

6. Run cfn-lint (if available)

cfn-lint $ARGUMENTS — report any findings.

Output

For each finding:

• Category: Structure / Security / Reliability / Best Practice
• Severity: Error / Warning / Info
• Location: Resource logical ID or line reference
• Issue: What's wrong
• Fix: How to fix it