AWS IAM Policy Design Defaults Silently Override Expected Safeguards

Symptom

A service account configured with a restrictive IAM policy begins accessing newly provisioned S3 buckets that were not explicitly listed in its allow statements. The security team observes unauthorised data enumeration in CloudTrail logs, despite the policy appearing to follow the principle of least privilege by specifying only known bucket ARNs.

False Assumption

Engineers often assume that an IAM policy denying all actions by default will automatically block access to any resource not explicitly permitted. This belief leads to policies that list specific allowed resources but fail to account for how AWS evaluates permissions when new resources match broader wildcard patterns or when service-level defaults shift.

Root Cause

The root cause is a misunderstanding of the IAM evaluation logic regarding explicit denies versus implicit denies and the scope of resource ARNs. If a policy uses a broad resource ARN such as arn:aws:s3:::company-data/* to allow access, it implicitly permits access to any new bucket created under that prefix. Furthermore, if the policy lacks an explicit deny for sensitive actions on unlisted resources, and another attached policy grants broader access, the explicit allow may take precedence depending on the evaluation order. The critical failure is relying on implicit denial without verifying that no other policy grants broader access, or failing to use explicit denies for high-risk actions across all resources.

Impact

Unintended access to sensitive data stores can lead to data exfiltration, compliance violations, and increased blast radius in the event of credential compromise. The silent nature of this misconfiguration means it may persist undetected for months, only surfacing during a security audit or incident response.

Diagnosis

To diagnose this issue, administrators should use the IAM Policy Simulator to test the effective permissions of the role against both existing and hypothetical new resources. Additionally, reviewing CloudTrail logs for AccessDenied events that are absent can reveal where permissions are unexpectedly granted. Comparing the effective policy document against the intended scope using AWS Access Analyzer can highlight overly broad resource ARNs.

Correction

Replace broad resource ARNs with specific bucket ARNs where possible. For dynamic environments, use condition keys such as s3:prefix or tag-based conditions to restrict access logically rather than by static ARN lists. Crucially, add an explicit deny statement for sensitive actions (such as s3:DeleteBucket or s3:PutBucketPolicy) on all resources to ensure these actions are blocked regardless of other allow statements. This creates a safety net that overrides any permissive policies attached to the same principal.

Validation

After applying the corrected policy, use the IAM Policy Simulator to confirm that the role can still access the intended buckets but is denied access to new, unlisted buckets. Verify that the explicit deny statements block sensitive actions even if a separate policy grants them. Check CloudTrail logs to ensure no unexpected access events occur during a controlled test period.

Rollback

If the corrected policy breaks legitimate application functionality, revert to the previous policy version immediately. Investigate the specific resource ARNs or condition keys that caused the failure and refine the policy iteratively. Ensure that the rollback does not remove the explicit deny statements for high-risk actions unless absolutely necessary, and instead adjust the allow statements to accommodate the required access.

Prevention

Implement Infrastructure as Code (IaC) scanning tools that flag broad resource ARNs and missing explicit denies in IAM policies. Require peer review for all IAM policy changes, focusing on the evaluation logic and potential side effects of new resource creation. Regularly run AWS Access Analyzer to identify unused permissions and overly broad access patterns, ensuring that policies remain aligned with the principle of least privilege as the environment evolves.