cfn2iam

Generate the IAM policy a CloudFormation stack needs, from its template, before you deploy it.

Deploy roles are usually far broader than the stack needs, often AdministratorAccess, because working out the minimum by hand means reading the docs for every resource type in the template. IAM Access Analyzer can generate a policy, but it works from CloudTrail, so you have to deploy with broad permissions first and narrow them afterwards. cfn2iam reads the template instead.

Web version · PyPI

Install

pip install cfn2iam            # or: uv tool install cfn2iam
pip install "cfn2iam[iam]"     # adds boto3, for --create-role and registry lookups of private types

Requires Python 3.9+ and network access to the schema host (see How it works).

Usage

cfn2iam template.yaml

This writes policy-<random>.json to the current directory:

{
  "Version": "2012-10-17",
  "Statement": [
    { "Effect": "Allow", "Action": ["s3:CreateBucket", "s3:GetBucketTagging", "..."], "Resource": "*" },
    { "Effect": "Deny",  "Action": ["s3:DeleteBucket", "..."], "Resource": "*" }
  ]
}

Examples:

cfn2iam template.yaml -d                       # allow deletes, e.g. for stacks you tear down
cfn2iam template.yaml -c -r MyDeployRole       # create the role as well
cfn2iam template.yaml -c -p arn:aws:iam::123456789012:policy/boundary

How it works

1. Reads the template (JSON or YAML, including short-form intrinsics like !Ref) and collects the resource types.
2. Expands SAM resources (AWS::Serverless::*) into the resources SAM creates, using rules that check the resource's properties (for example, a function with an SQS event also needs the event source mapping).
3. Fetches each type's CloudFormation resource schema from mrlikl.github.io/cfn2iam/backend/schemas. These are AWS's published schemas, refreshed weekly by a GitHub workflow. The SAM expansion rules live next to them, in backend/sam_rules.
4. For a type with no public schema (a private type, or a third-party type activated in your account), it calls CloudFormation DescribeType in your account and region. This needs cfn2iam[iam], AWS credentials and cloudformation:DescribeType. With --registry, every type is looked up this way; SAM rules still come from the published rules.
5. Takes the permissions each schema lists for its create, read, update and list handlers as Allow, and the permissions only its delete handler needs as Deny (or Allow with -d).

If any resource type can't be resolved, cfn2iam lists them, writes no policy, and exits with status 1. A partial policy would fail mid-deploy at best. Network failures can't be skipped; types found nowhere can, with --ignore-unknown.

Limitations

• Actions, not resources. Every statement uses "Resource": "*". Resource ARNs mostly don't exist until the stack does, so the policy is least-privilege on actions only.
• The delete statement is an explicit Deny. An explicit Deny overrides an Allow from any other policy attached to the same role. Use -d if the role also has to delete these resources.
• Custom resources are skipped (Custom::*, AWS::CloudFormation::CustomResource, AWS::CDK::Metadata). Anything they need, such as permission to invoke their Lambda function, isn't included.
• The policy is only as complete as AWS's schemas. The handler permission lists are maintained by AWS and can be incomplete for some resource types or configurations. Test the policy on a non-production stack first.

License

MIT. See LICENSE.