There is a possible DoS vulnerability in the multipart parser in Rack before 2.0.6. Specially crafted requests can cause the multipart parser to enter a pathological state, causing the parser to use CPU resources disproportionate to the request size.
https://groups.google.com/forum/#%21msg/rubyonrails-security/U_x-YkfuVTg/xhvYAmp6AAAJ