Is there and open source security platform(CIEM/CSPM) which implements or calculates AWS's effective permissions for stating out the permission boundaries of iam identities?

To grant only the permissions required to perform a task, you can generate policies based on your access activity that is logged in AWS CloudTrail. For that you can use IAM Access Analyzer to analyze the services and actions that your IAM roles use, and then generates a fine-grained policy that you can use. After you test each generated policy, you can deploy the policy to your production environment. This ensures that you grant only the required permissions to your workloads.

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I hope this helps!