OAuth Scope Management for AWS S3 Read-Only Roles

The request came in: give a third-party system access to S3—read-only, nothing else. No more, no less.

Mismanaging AWS permissions can turn a small feature into an attack surface. OAuth scopes exist to limit exactly what a token can do. AWS IAM roles exist to define exactly what a principal can touch. Marrying the two for S3 read-only access is precise work: the scope defines the requested capability, the IAM policy enforces it in AWS.

Start by mapping OAuth scopes to your AWS actions. For S3 read-only, the minimum set is:

s3:GetObject
s3:ListBucket

These are the only two that matter for reading objects and listing buckets. Anything else—PutObject, DeleteObject—should be absent. Create an IAM policy with only these actions, tied to the specific bucket ARN. Attach the policy to a dedicated IAM role.

Next, connect that IAM role to your OAuth authorization server logic. When a client requests the "S3:ReadOnly" scope, your OAuth server should issue credentials that assume this role and nothing else. If using AWS STS and temporary credentials, ensure the session policy matches the IAM policy to avoid privilege creep.

Audit regularly. Check CloudTrail for actions outside the allowed scope. If you see a PutObject or DeleteObject from a scoped token, you have a gap to close. Keep scopes granular and policies exact. Do not reuse roles across scopes. The tighter the link between OAuth scope and AWS role, the smaller your blast radius.

Misconfiguration often hides in broad policies and vague scopes. Precision is the advantage here: clear scopes, narrow permissions, enforced in both the identity layer and AWS. Done right, OAuth scope management for AWS S3 read-only roles means zero surprise writes, zero leaked data, and full control over what an external system can do.

Want to see this flow working in minutes without writing the glue from scratch? Try it on hoop.dev and lock down your S3 read-only roles the right way.