S3 AccessDenied: find the denying policy before adding permissions

An S3 403 response does not identify one universal fix. The caller may lack an allow, encounter an explicit deny or be blocked by a policy outside the bucket itself. Start with the failed API operation, resource ARN, request time and runtime identity. A test performed with an administrator’s CLI profile can obscure the identity used by the application.

Narrow the operation and resource

Reading an object and listing a bucket require different permissions and resource scopes. Check the operation that actually failed rather than granting a generic collection of S3 actions. If one prefix works and another fails, compare object ownership, policy conditions and encryption before assuming a service outage.

Retain the error’s request identifiers for troubleshooting, but do not paste credentials or signed URLs into a public question. Enhanced denial details can identify policy types in supported same-account or same-organization requests. Other contexts still return a generic denial, and the absence of detail is not evidence that a policy is irrelevant.

Follow the effective policy path

Review the caller’s identity policy and the bucket or access-point policy. Then check applicable permissions boundaries, organization controls, session restrictions and VPC endpoint policies. An explicit deny is not overridden by adding another allow. If traffic uses an S3 VPC endpoint, evaluate its policy in the context of that network path.

For objects encrypted with a customer-managed KMS key, examine the required KMS permissions and key policy as well. S3 authorization and permission to use the encryption key are distinct. Avoid broadening both at once, because doing so prevents you from knowing which change mattered.

Test a narrow correction

Write down the policy statement you believe caused the failure and the evidence connecting it to the request. Propose the smallest change that allows the intended caller, operation and resource while retaining unrelated restrictions. Have the relevant owner review a production policy change.

Repeat the same operation with the same runtime identity after the change. Also try a deliberately unauthorized resource within the test scope to confirm that the boundary still exists. Making a bucket public or attaching full administrator access can make a symptom disappear while creating a much larger problem. A successful investigation ends with a working permitted request and an understandable policy trail.