It was an issue we discussed quite a bit when working on Bucketio. If you're S3 savvy, we recommend that you create a limited user and give it access to JUST the bucket you want us using. If you're not, then you can provide your Master keys and the system does the setup for you. We create a limited user with access to a single bucket we create, set the CORS needed for uploading, then we forget your master keys and connect using the new user ONLY.