Configure Access Analyzer to discover which resources in an AWS account are shared with external principals outside of the account. Access Analyzer analyzes generates findings for supported resources in the region it was enabled, with the exception of IAM resources which generates findings in each region (as IAM is a global service).

Select the appropriate Zone of Trust when deploying Access Analyer. There are two options: Account and Organization, and this specifies the zone in which Access Analyzer will treat all resources as trusted and access from outside the zone as untrusted. 

Access Analyzer Supported Resources: IAM Roles, S3 Buckets, KMS Keys, Lambda Functions and Layers, and SQS Queues.

Configure Archive Rules to automatically archive new findings that meet the criteria you define when you create the rule. Archive rules can be created to whitelist certain resources, source principals, organization IDs, VPCs, and more.

Automatic notifications can be enabled with a CloudWatch Event Rule.

CloudFormation Template