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What to collect before changing a policy
Record the details of one failed request so you can test the same operation after making a targeted change:
- The complete error message, including any policy type it names.
- The exact S3 API action and whether it targets a bucket or an object. A read, write, list, and multipart operation can require different permissions.
- The bucket or object ARN involved, and the ARN of the assumed role used by the function.
- Whether the bucket is in another AWS account, whether the object uses SSE-KMS, and whether the request travels through a VPC endpoint.
A Lambda function reaches AWS services using its execution role. AWS defines that role as the IAM role granting the function permission to access services and resources. Confirm the function is using the role you intend to troubleshoot before evaluating its permissions.
Understand the denial before choosing a fix
A denial can be explicit or implicit. That distinction helps focus the investigation, but it does not remove the need to check every policy layer that applies to the request.
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| Denial type | What it means | Where to look first |
|---|---|---|
| Explicit | A matching policy statement contains Deny. |
Find the named policy type or matching deny statement, then check other applicable controls too. |
| Implicit | No applicable policy grants the requested action. | Check whether the needed action is allowed for the correct principal and bucket or object resource. |
If the error identifies an SCP, permissions boundary, session policy, resource policy, or VPC endpoint policy, inspect that layer first. The message may call out one policy type even though other policies also constrain the request.
Trace the request through each authorization layer
-
Verify the action, resource, and principal
Match the failing operation to the permission it needs and the resource it targets. Bucket-level and object-level operations do not necessarily use the same resource ARN. Confirm that the request is made by the expected Lambda execution role, not a different principal.
-
Check the execution role’s identity policies
Review whether an attached identity policy allows the exact S3 action against the required bucket or object ARN. Do not assume that an allow for one kind of operation also permits listing, reading, writing, or multipart work. AWS recommends IAM Access Analyzer to help identify permissions an execution role needs.
-
Review bucket and access point controls
Inspect the applicable bucket or access point policy for the right principal, action, resource, and condition values, as well as explicit denies. Check relevant S3 Block Public Access settings. For cross-account access, validate permissions on both the caller side and the resource side. AWS notes that requests crossing accounts outside the same AWS organization may return only a generic Access Denied message.
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Check key authorization for SSE-KMS
For an object encrypted with SSE-KMS using a customer-managed key, S3 permission by itself may not be enough. AWS specifies
kms:GenerateDataKeyfor uploads andkms:Decryptfor downloads and multipart uploads; the KMS key policy must also allow the required operation. Objects using SSE-S3 do not require additional KMS permission. -
Inspect guardrails, conditions, and network routing
A permissions boundary, session policy, Organizations service control policy or resource control policy, VPC endpoint policy, or policy condition can limit a permission that appears allowed elsewhere. If the bucket policy permits access only through a particular VPC endpoint, verify that the function’s request actually traverses that endpoint and that its endpoint policy permits the operation.
Make a narrow correction and retest
Change only the specific principal, action, resource, condition, or denying policy that the investigation identifies. Then repeat the same S3 operation and inspect the new error or relevant event. Avoid adding broad wildcard permissions as a diagnostic shortcut: they can grant more access than the function needs and still fail to address a separate denial elsewhere.
A generic 403 explanation cannot identify the faulty policy in a particular AWS account. The right fix depends on the request, account relationship, object encryption, policy configuration, and network path. AWS’s Lambda and S3 authorization guidance checked on October 4, 2026 explains these general checks, not the configuration of an unspecified function.
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