There is an XSS vulnerability in the ActiveSupport::JSON.encode method in Ruby on Rails.

When a `Hash` containing user-controlled data is encode as JSON (either through
`Hash#to_json` or `ActiveSupport::JSON.encode`), Rails does not perform adequate
escaping that matches the guarantee implied by the `escape_html_entities_in_json`
option (which is enabled by default). If this resulting JSON string is subsequently
inserted directly into an HTML page, the page will be vulnerable to XSS attacks.

For example, the following code snippet is vulnerable to this attack:

    <%= javascript_tag "var data = #{user_supplied_data.to_json};" %>

Similarly, the following is also vulnerable:

    <script>
      var data = <%= ActiveSupport::JSON.encode(user_supplied_data).html_safe %>;
    </script>

Workarounds
-----------
To work around this problem add an initializer with the following code:

    module ActiveSupport
      module JSON
        module Encoding
          private
          class EscapedString
            def to_s
              self
            end
          end
        end
      end
    end

Attached patches resolve this issue.

Acknowledgements:

Red Hat would like to thank Ruby upstream developers for reporting this issue. Upstream acknowledges Francois Chagnon of Shopify as the original reporter.