Why AWS S3 Read-Only Roles Still Fail and How to Fix Them

This is the pain point many hit with AWS S3 read-only roles. The policy looks correct. The IAM role has s3:GetObject and s3:ListBucket. You expect it to work across all paths. But S3 access control is subtle. One missing permission can block a role from listing or reading even though it can technically fetch objects.

The first trap: ListBucket without the right Resource scope. If the bucket ARN lacks the trailing slash or object wildcard (arn:aws:s3:::bucket-name/*), the role can’t list. The second trap: object-level read-only still fails if AWS KMS encryption is used. Without kms:Decrypt, a role can read the object metadata but not the content. The third: access points and bucket policies. If a bucket policy denies anonymous listing, that deny overrides any role permission.

These pain points grow when teams spread buckets across accounts. Cross-account read-only depends on both sides granting matching ListBucket and GetObject privileges. A single deny on the resource policy is final. Roles set for auditing or testing often lose range because they don’t include all necessary actions tied to S3’s internal API calls.

To solve S3 read-only pain, you must design policies that map every needed permission. Include s3:GetObject for objects, s3:ListBucket for bucket listings, s3:GetObjectVersion if versioning is on, and decryption permissions for encrypted content. Test with real objects, not just AWS console checks. Policy Generator tools help, but nothing beats a live validation flow before rollout.

Avoid assumptions. “Read-only” in AWS S3 is not a single switch—it’s a precise set of permissions, bucket policies, and encryption rules working in sync.

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