Iam Effective Policy Inheritance Troubleshooting

What Policy Precedence and Deny Policies Are

In Google Cloud Identity and Access Management (IAM), access evaluation follows a strict hierarchical evaluation order where IAM deny policies always take precedence over IAM allow policies, regardless of where the policies are attached in the resource hierarchy.

How IAM Evaluates the Effective Access

When the developer attempts to execute an action that requires storage.objects.delete on the Cloud Storage bucket, IAM resolves the request using the following evaluation sequence:

1. Deny Policy Evaluation: IAM first evaluates all applicable deny policies attached to the resource and all its ancestors (organization, folder, project, and resource). Because a deny policy at the organization level explicitly denies storage.objects.delete to the principal's group, the denial is absolute.
2. Short-Circuit Denial: If any applicable deny rule matches the principal, permission, and conditions, IAM immediately rejects the request without allowing lower-level allow policies to override it.
3. Allow Policy Evaluation: Even though the bucket-level allow policy grants the Storage Object Admin (roles/storage.objectAdmin) role—which contains the storage.objects.delete permission—this grant cannot supersede the ancestor deny rule.

Key Rules of IAM Evaluation and Inheritance

• Deny overrides allow: Deny policies create absolute guardrails that override any allow policy grants across the entire hierarchy.
• Allow policies are additive: Allow policies at the organization, folder, project, and resource levels combine into a union of permissions, but they can never negate or bypass a deny policy.
• Inherited scope: Policies attached at higher levels of the resource hierarchy (such as organizations and folders) automatically apply to all descendant containers and individual resources.

Why This Is Optimal

This evaluation model ensures that central governance and compliance guardrails set by organization security administrators cannot be inadvertently bypassed or overridden by project owners or resource-level administrators. Tools like Policy Troubleshooter and gcloud beta projects get-ancestors-iam-policy --include-deny can be used to quickly inspect these interacting layers.