How to implement reCaptcha V3 in ASP.NET

The simplest implementation:

1. In your cshtml file (at the top)

   @section Scripts{    <script src="https://www.google.com/recaptcha/api.js?render=your site key"></script>    <script>        grecaptcha.ready(function () {            grecaptcha.execute('your site key', { action: 'homepage' }).then(function (token) {                document.getElementById("foo").value = token;            });        });    </script>}

2. In your cshtml, inside the form (just before </form>):

   <input type="hidden" id="foo" name="foo" />

3. A function inside your Pagemodel class. See the documentation for the response object:

   public static bool ReCaptchaPassed(string gRecaptchaResponse){    HttpClient httpClient = new HttpClient();    var res = httpClient.GetAsync($"https://www.google.com/recaptcha/api/siteverify?secret=your secret key no quotes&response={gRecaptchaResponse}").Result;    if (res.StatusCode != HttpStatusCode.OK)     {        return false;    }    string JSONres = res.Content.ReadAsStringAsync().Result;    dynamic JSONdata = JObject.Parse(JSONres);    if (JSONdata.success != "true" || JSONdata.score <= 0.5m)    {        return false;    }    return true;}

4. Finally, inside your OnPostAsync() handler, at the top:

   if (!ModelState.IsValid) {    return Page();}else{    if (!ReCaptchaPassed(Request.Form["foo"]))    {        ModelState.AddModelError(string.Empty, "You failed the CAPTCHA.");        return Page();    }}

Edit : I have added a demo project . Check this github repository . https://github.com/NIHAR-SARKAR/GoogleRecaptchav3-example-In-asp.net

From frontend (.aspx page) you need to send ajax request to pass the token to backend server . Using "recaptcha.execute" U can get the response , and pass the token using ajax request .Please check the code block .

 <script src="http://www.google.com/recaptcha/api.js?render=recaptchaSiteKey"></script><script> grecaptcha.ready(function() { grecaptcha.execute('recaptchaSiteKey', {action: 'homepage'}).then(function(token) {            $.ajax({                //pass the toket to Webmethod using Ajax            });          });     });</script>

Reference link: https://developers.google.com/recaptcha/docs/verifyhttps://developers.google.com/recaptcha/docs/display#js_api

Now in the aspx.cs you need to write a "[WebMethod]" to receive the token from Ajax request .

    [WebMethod]    public static void CaptchaVerify(string token)    {            var responseString = RecaptchaVerify(token);            ResponseToken response = new ResponseToken();            response = Newtonsoft.Json.JsonConvert.DeserializeObject<ResponseToken>(responseString.Result);    }

To get the response from google recapcha api u need to use async call using httpClient . you also need to create a class which will contain same properties like the response string . After getting the "responseString" u need to convert the response to ResponseToken object by using Newtonsoft.Json.response = Newtonsoft.Json.JsonConvert.DeserializeObject<ResponseToken>(responseString.Result);

private string apiAddress = "https://www.google.com/recaptcha/api/siteverify";private string recaptchaSecret = googleRecaptchaSecret;        public async Task<string> RecaptchaVerify(string recaptchaToken)        {            string url = $"{apiAddress}?secret={recaptchaSecret}&response={recaptchaToken}";            using (var httpClient = new HttpClient())            {                try                {                    string responseString=  httpClient.GetStringAsync(url).Result;                    return responseString;                }                catch (Exception ex)                {                    throw new Exception(ex.Message);                }            }        }        public class ResponseToken        {            public DateTime challenge_ts { get; set; }            public float score { get; set; }            public List<string> ErrorCodes { get; set; }            public bool Success { get; set; }            public string hostname { get; set; }        }

There are several Recaptcha libraries available for ASP.Net. I chose to use reCAPTCHA.AspNetCore because it provides an HtmlHelper.

Please note that this library only supports one ReCatpcha per page, and it doesn't support Recaptcha v3 passive monitoring on non-form pages.