Asking for CAPTCHA only once with MvcReCaptcha

Question

I'm using MvcRecaptcha to prevent bot posts for a complex unauthenticated client form on an ASP.NET MVC 2.0 site.

I only want to require one correct CAPTCHA entry from an unauthenticated client, even if some of the form's inputs are incorrect.

I have tried using a Session["CaptchaSuccess"] = true; variable to suppress Html.GenerateCaptcha() in my view following a successful entry, but the presence of the [CaptchaValidator] attribute on my [HttpPost] view causes an error because it naturally requires some ReCaptcha form inputs.

What is the simplest way to achieve this reliably, including on mobile browsers?

Answer 1

Solved by modifying the [CaptchaValidatorAttribute] OnActionExecuting method, where CaptchaSuccessFieldKey refers to the constant string value "CaptchaSuccess":

public override void OnActionExecuting(ActionExecutingContext filterContext)
{
            bool? bCaptchaSuccess = filterContext.HttpContext.Session[CaptchaSuccessFieldKey] as bool?;
            if (bCaptchaSuccess.HasValue && bCaptchaSuccess.Value)
            {
                filterContext.ActionParameters["captchaValid"] = true;
            }
            else
            {

                var captchaChallengeValue = filterContext.HttpContext.Request.Form[ChallengeFieldKey];
                var captchaResponseValue = filterContext.HttpContext.Request.Form[ResponseFieldKey];
                var captchaValidtor = new Recaptcha.RecaptchaValidator
                                          {
                                              PrivateKey = ConfigurationManager.AppSettings["ReCaptchaPrivateKey"],
                                              RemoteIP = filterContext.HttpContext.Request.UserHostAddress,
                                              Challenge = captchaChallengeValue,
                                              Response = captchaResponseValue
                                          };

                var recaptchaResponse = captchaValidtor.Validate();

                // this will push the result value into a parameter in our Action
                filterContext.ActionParameters["captchaValid"] = recaptchaResponse.IsValid;
            }

            base.OnActionExecuting(filterContext);

            // Add string to Trace for testing
            //filterContext.HttpContext.Trace.Write("Log: OnActionExecuting", String.Format("Calling {0}", filterContext.ActionDescriptor.ActionName));
        }