If you've ever tried to give safe, read-only access to an AWS S3 bucket in a self-hosted environment, you know the tension between security and convenience. One misstep, and a role meant for reading becomes a role that can write, delete, or expose your data. Done right, a self-hosted AWS S3 read-only role locks access tight without slowing down your workflows.
A read-only IAM role for S3 is simple in concept. You grant s3:GetObject, s3:ListBucket, and nothing more. In practice, the details matter. You need to define a trust policy so that only the right service or application can assume the role. You need to craft an inline or attached IAM policy that covers the exact ARNs of your buckets and folders. And you avoid wildcards unless you truly want that scope open.
When you self-host, you also deal with the mechanics of assuming the role from within your infrastructure. That could mean configuring AWS CLI with aws sts assume-role, setting up temporary credentials for containers, or wiring SDKs to call STS before making S3 requests. Every added step is an opportunity for misconfiguration, so automation matters. Export credentials with the shortest possible duration and refresh them before they expire. Never hardcode keys.
For even tighter control, enable S3 Block Public Access on the bucket. Combine it with bucket policies that allow only the specific IAM role ARN to succeed in read requests. This ensures that even if someone guesses a key, they can't bypass the role. Add AWS CloudTrail logging for role assumptions and S3 object access. Monitor and alert on unexpected source IPs, times, or request patterns.