CORS policy validation- and signing library

Cross-origin resource sharing (CORS) is great; it allows your visitors to asynchronously upload files to e.g. 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 functionality of CORS is now provided by the ruby AWS SDK. We recommend using that instead, as CORS will no longer receive updates:

Amazon S3 SDK PresignedPost

UploadManifest = CORS::Policy::S3.create do |policy|
  policy.required "method", "PUT"
  policy.optional "md5" do |value|
    Base64.strict_decode64(value)
  end
  policy.required "content-type", %r|image/|
  policy.required "x-amz-date" do |date|
    "2012-10-22T16:10:47+02:00" == date
  end
  policy.required "filename", %r|uploads/|
end

manifest = UploadManifest.new(params)

response = if manifest.valid?
  { success: manifest.sign(access_key, secret_access_key) }
else
  { error: manifest.errors }
end

Out-of-the box, the CORS library comes with support for the Amazon S3 REST API.

• Amazon S3 REST API Authentication Header
• Amazon S3 REST API POST Upload Policy

Copyright (c) 2012 Kim Burgestrand

MIT License

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