Invicti identified a Remote Code Evaluation (RoR - JSON) by capturing a DNS A request, which occurs when input data is run as code.
The JSON Parsing code in Rails 2.3 and 3.0 support multiple parsing backends. One of the backends involves transforming the JSON into YAML, and passing that through the YAML parser. Using a specially crafted payload, attackers can trick the backend into decoding a subset of YAML.
ActiveSupport::JSON.backend = "JSONGem"