Refile Gem: Ignore exception when remote_image_url is invalid or doesn't exist

Question

We are using refile to allow users to upload images to our S3 back end. In addition, we allow users the option of entering a URL to any image on the internet (through the remote_image_url property.)

This works fine, as long as the URL entered is pointing to an actual file. However, in the case where there was a mistake in the URL, or some nonsensical input was given, Refile will throw the following exception:

Errno::ENOENT (No such file or directory @ rb_sysopen - thiswillnotwork):
  app/controllers/my/deals_controller.rb:17:in `create'
  appsignal (0.11.2) lib/appsignal/rack/listener.rb:13:in `call'

Is there an option to ignore the cases where the URL entered is invalid (akin to how the validate_download option in CarrierWave works) and, ideally, use our fall back image instead?

We have tried mounting the attacher with the raise_errors option set to false, but with the same results.

Our project uses Rails 4.2.0 and Refile 0.5.3.

Edit:

I have confirmed that this exception is a lower level SystemCallError, coming from Kernel.open, and this exception type is not being rescued by Refile:

rescue OpenURI::HTTPError, RuntimeError => error
  raise if error.is_a?(RuntimeError) and error.message !~ /redirection loop/
  @errors = [:download_failed]
  raise if @raise_errors
end

I am working on a pull request to refile to fix this.

Edit 2:

While working on this, we discovered a major security issue in Refile, enabling a potential attacker to use remote code execution.

The Refile gem has a feature where a URL will be supplied and the remote file will be uploaded. This can be done by adding a field like remote_image_url in a form, where image is the name of the attachment. This feature was using open-uri to make this HTTP request without validating the passed URI. An attacker can craft a URI which executes arbitrary shell commands on the host machine.

If you are using Refile versions 0.5.0 - 0.5.3, please upgrade to the latest version. Upgrading will also solve the issue above.

Answer 1

After talking to the maintainer of Refile, this will be fixed in the next iteration. For now, we've created the following, dead simple Ruby class as a work around.

# app/services/remote_url.rb
class RemoteUrl
  def initialize(url)
    @url = url
  end

  def valid?
    URI.parse(@url).kind_of?(URI::HTTP)
  rescue URI::InvalidURIError
    false
  end
end

and I use it in my controller like so:

# app/controllers/model_controller.rb
def model_params
  sanitize_remote_url!
  params.require(:model).permit(:description, ..., :image, :remote_image_url)
end

def sanitize_remote_url!
  params[:model].delete(:remote_image_url) unless RemoteUrl.new(params[:model][:remote_image_url]).valid?
end

It is quite a primitive approach, but considering that it will be patched in the next Gem version, I consider this good enough for now.

Disclaimer:

Although this offsets some of the vulnerability mentioned in edit 2 of the original question, an attacker will still be able to do remote code execution, if they can create a folder named http: on the file system, as a string like http:/../etc/passwd will pass the URL validation, but still read a local file through Kernel.open.

The Gem has now been patched (0.5.4), and an upgrade is the correct solution.