cfn_nag is a static analysis tool for CloudFormation templates that can discover obvious security flaws before deployment. This can include things like open security groups, wildcard IAM permissions or missing encryption settings. To learn more about the underlying principles of cfn_nag, please see: https://stelligent.com/2016/04/07/finding-security-problems-early-in-the-development-process-of-a-cloudformation-template-with-cfn-nag/