Fix vulnerable.

app/controllers/ru_captcha/captcha_controller.rb

@@ -7,9 +7,11 @@ module RuCaptcha
       if params[:format] == "wav" and RuCaptcha.espeak?
         data = generate_speech_rucaptcha
         opts = { disposition: 'inline', type: 'audio/wav' }
-      else
+      elsif params[:format].blank? || params[:format] == "gif"
         data = generate_rucaptcha
         opts = { disposition: 'inline', type: 'image/gif' }
+      else
+        render :body => nil, :status => 404 and return
       end
       send_data data, opts
     end