IAM policy validator for AWS CloudFormation

Use the cfn-policy-validator CLI to validate the IAM policies in a CloudFormation template against AWS IAM Access Analyzer before deploying. The tool extracts identity- and resource-based policies from the template, resolves intrinsic functions and pseudo parameters, and runs them through Access Analyzer. Install with pip install cfn-policy-validator.

Discover the exact commands and flags by running cfn-policy-validator --help and cfn-policy-validator <command> --help.

When to use which command

• validate — general "review these policies": correctness and best-practice findings. The default for an open-ended review request.
• check-no-new-access — confirm a policy grants nothing beyond an approved reference policy.
• check-access-not-granted — confirm specific actions or resources are not granted.
• check-no-public-access — confirm a resource policy cannot grant public access. Applies to resource policies only; a template with none has nothing for it to evaluate.
• parse — print the resolved policies without calling Access Analyzer. Use to troubleshoot what the tool extracted and how intrinsics were evaluated.

Preparing input

• Raw CloudFormation: pass the template directly; the tool resolves !Ref/!Sub/!GetAtt itself.
• CDK: run cdk synth first and pass a cdk.out/*.template.json. Do not synthesize implicitly; that is the user's step.
• When a policy depends on template parameters, supply them (--parameters or a template-configuration file) so the policy resolves.

Reading the result

Each check returns JSON with BlockingFindings and NonBlockingFindings and exits 2 when any blocking finding is present, 0 otherwise. Report blocking findings first. In CI, gate on the exit code.

Example

Reviewing a template before deploy:

cfn-policy-validator validate --template-path ./template.yaml

A problem policy produces a BlockingFindings entry and exit code 2:

{
  "BlockingFindings": [
    {"findingType": "ERROR", "code": "INVALID_ACTION", "resourceName": "MyRole"}
  ],
  "NonBlockingFindings": []
}

A clean template returns empty findings and exit code 0.

Credentials

The checks call Access Analyzer (including access previews), so they need AWS credentials and a region (--region, else AWS_REGION / AWS_DEFAULT_REGION). An AccessDenied error names the missing Access Analyzer or IAM permission — grant it and retry. The full required policy is in the project README: https://github.com/awslabs/aws-cloudformation-iam-policy-validator#iam-policy-required-to-run-the-iam-policy-validator-for-aws-cloudformation