Cross-origin resource sharing (CORS) is great; it allows your visitors to asynchronously upload files to e.g. Filepicker or Amazon S3, without the files having to round-trip through your web server. Unfortunately, giving your users complete write access to your online storage also exposes you to malicious intent. To combat harmful usage, good upload services that allow client-side upload, support a mechanism that allows you to validate and sign all upload requests to your online storage. By validating every request, you can give your visitors a nice upload experience, while keeping the bad visitors at bay. The CORS gem comes with support for the Amazon S3 REST API.