AWS S3 Support
Fix AWS S3 Access Denied (403) errors.
An S3 403 can come from six different layers: the caller's IAM policy, the bucket policy, Block Public Access, object ownership, KMS key policy or a VPC endpoint policy. Guessing wastes hours. We follow the request through each layer in order and change only what is actually blocking it.
When You Need This
The situations where this work pays for itself.
Signs it is time
- An application or user gets AccessDenied on GetObject, PutObject or ListBucket
- A request works from your laptop but fails from EC2, Lambda or a VPC
- Uploads succeed but another account cannot read the objects
- Reads of SSE-KMS objects fail while plain objects work
- A missing key returns 403 instead of 404
What you get
- The identified blocking layer with the evidence from logs or the simulator
- Corrected IAM, bucket, key or endpoint policy in infrastructure as code
- A short checklist for onboarding new principals to the bucket
- CloudTrail data event or access log setup for future debugging
How we do it
A clear, safe process you can follow along with.
1. Capture the exact request
We get the full error including the x-amz-request-id, the principal ARN from aws sts get-caller-identity, the bucket, key, Region and whether the call goes through a VPC endpoint. The principal is often not the one people assume.
2. Check for explicit denies first
An explicit Deny in any policy overrides every Allow. We read the IAM identity policies, permission boundaries, session policies, bucket policy and AWS Organizations service control policies for Deny statements and conditions such as aws:SourceVpce, aws:SecureTransport or aws:PrincipalOrgID.
3. Verify the Allow path
We confirm the right action and resource form: s3:ListBucket applies to the bucket ARN, while s3:GetObject and s3:PutObject apply to arn:aws:s3:::bucket/*. For cross-account access both the caller's policy and the bucket policy must allow the action.
4. Check KMS, Block Public Access and ownership
For SSE-KMS objects the caller also needs kms:Decrypt or kms:GenerateDataKey in both the key policy and IAM. We also review Block Public Access settings and the bucket's Object Ownership setting, since Bucket owner enforced disables ACLs.
5. Check the network path
A VPC endpoint policy can restrict buckets and actions, and a bucket policy can require a specific endpoint. We read both and confirm the route table and endpoint type for the caller.
6. Prove it with logs and the policy simulator
We use the IAM policy simulator for identity-side questions and CloudTrail for what S3 saw. Management events are on by default, but object-level calls need CloudTrail data events or S3 server access logs enabled.
7. Fix narrowly and retest
We change the one failing layer with the smallest policy edit, retest with the same principal and path, and record the cause so it does not return.
Mistakes we help you avoid
Common ways this goes wrong, and what we do instead.
Fixing only the identity policy
In cross-account cases, an Allow in the caller's account is not enough. The bucket policy must also grant access.
Opening the bucket publicly to make the error go away
That trades an outage for an exposure. We find the real cause and grant only the needed principal.
Forgetting KMS
Objects encrypted with a customer managed key need key permissions. Missing them also produces AccessDenied.
Misreading 403 for missing objects
Without s3:ListBucket permission, S3 returns 403 for a key that does not exist, rather than 404.
Frequently Asked Questions
Why do I get Access Denied even though my IAM policy allows s3:*?
Another layer is probably denying it: a bucket policy Deny, a service control policy, a permission boundary, a KMS key policy or a VPC endpoint policy. Identity policy Allow does not override any of these.
Why does S3 return 403 instead of 404 for a missing file?
If the caller lacks s3:ListBucket on the bucket, S3 hides whether a key exists and returns AccessDenied. Granting ListBucket makes missing keys return 404.
How do I see which policy denied my request?
Newer AWS error messages often name the type of policy that denied the call. Combine that with the IAM policy simulator and CloudTrail event details to find the exact statement.
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