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remi bourgarel
remi bourgarel

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How to validate incoming command with reCaptcha on a Blazor app with a ASPNET Core backend with MediatR

The Toss project is a message board : people register and then they send messages. One of the threat with this kind of application are robot : people creates program that creates new account and then post spam message.

reCaptcha is a tool created by Google since a long time that aim to validate that a given action is done only by a human. In this blog post we'll see how I integrated this tool so it's easy for me to validate that a given action is done only by a human being.

How repatcha works

Recaptcha works this way :

  • a given action is done on your app
  • you call their JS api for getting a token, this method will fail if the user is not a human (or at least reCaptcha does not think it's a robot). We completly ignore how it does it, but I trust google to be more eficient at this task than me and my 2 hours of work on this project per week.
  • we send the token to the backend
  • the backend send this token with our private key to Google reCaptcha API
  • if the token is correct the API returns "succeed"
  • if not then it returns an error and we can stop the processing of the query

Getting the token

First thing we need to add reCaptcha script link :

<script src=''></script>
  • You can find this on your reCaptcha GUI
  • it's sad google doesn't provide a version with integrity attribute so we're sure the script doesn't get modified by hackers

For getting the token on protected action I did it that way :

Every sensible command inherit from "NotARobot" class :

public class NotARobot
    public string Token { get; set; }
public class RegisterCommand : NotARobot, IRequest<CommandResult>
    [Display(Name = "Email")]
    public string Email { get; set; }

    [StringLength(100, ErrorMessage = "The {0} must be at least {2} and at max {1} characters long.", MinimumLength = 6)]
    public string Name { get; set; }

    [StringLength(100,ErrorMessage = "The {0} must be at least {2} and at max {1} characters long.", MinimumLength = 6)]
    [Display(Name = "Password")]
    public string Password { get; set; }

    [Display(Name = "Confirm password")]
    [Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
    public string ConfirmPassword { get; set; }
  • This classes are defined in my shared assembly so I'll be able to read it on server side as well

Every time I send a command inheriting from NotARobot, I will gather a token. This is done on my HttpClient extension

 public async Task Post<T>(T data)
            if (data is NotARobot)
                (data as NotARobot).Token =  await JSRuntime.Current.InvokeAsync<string>("runCaptcha",this._uri);
            await ExecuteHttpQuery(async () =>
                var requestJson = Json.Serialize(data);
                return await _httpClient.SendAsync(await PrepareMessageAsync(new HttpRequestMessage(HttpMethod.Post, _uri)
                    Content = new StringContent(requestJson, System.Text.Encoding.UTF8, "application/json")

The js method called with interop is defined as such :

runCaptcha = function (actionName) {
    return new Promise((resolve, reject) => {
        grecaptcha.ready(function () {
            grecaptcha.execute('6LcySnsUAAAAAKFZn_ve4gTT5kr71EXVkQ_QsGot', { action: actionName })
                .then(function (token) {

  • the first argument is the public key given by Google, it's hard coded here because it's public and used only in one place
  • I use Promise as it's the way to handle async execution in JSInterop
  • Doing it this way I only need to add NotARobot to my command and the whole process will be done

Now that my server receives the token, I need to add the validation with Google API.

Validating the token with MediatR

We can see on the previous source code that my class RegisterCommand also implement the IRequest interface from MediatR. This command is executed by a handle implementing IRequestHandler. MediatR provides an easy way for adding validation before the handler get executed : pipelines.

Here is my pipeline code

public class CaptchaMediatRAdapter<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
        where TRequest : NotARobot, IRequest<TResponse>
    private ICaptchaValidator captchaValidator;

    public CaptchaMediatRAdapter(ICaptchaValidator captchaValidator)
        this.captchaValidator = captchaValidator;

    public async Task<TResponse> Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate<TResponse> next)
        await this.captchaValidator.Check(((NotARobot)request).Token);
        return await next();

It's pretty simple it just calls my validator that is supposed to raise an exception when the validation fails. Here is my validator

public class CaptchaValidator : ICaptchaValidator
    private string _secret;
    private readonly IHttpClientFactory httpClientFactory;
    private readonly IHttpContextAccessor httpContextAccessor;

    public CaptchaValidator(string secret, IHttpClientFactory httpClientFactory, IHttpContextAccessor httpContextAccessor)
        _secret = secret;
        this.httpClientFactory = httpClientFactory;
        this.httpContextAccessor = httpContextAccessor;

    public async Task Check(string token)
        var webClient = this.httpClientFactory.CreateClient();
        var content = new FormUrlEncodedContent(new[]
            new KeyValuePair<string, string>("secret", _secret),
            new KeyValuePair<string, string>("response", token),
            new KeyValuePair<string, string>("remoteip", httpContextAccessor.HttpContext.Connection.RemoteIpAddress.ToString())
        var res = await webClient.PostAsync("",content);
        var resJson = JObject.Parse(await res.Content.ReadAsStringAsync());
        var succeed = resJson["success"].ToString() == "True";
        if (!succeed)
            throw new InvalidOperationException("Captcha validation failed : " + string.Join(",", resJson["error-codes"].AsEnumerable()));
  • we need to send the client IP
  • secret is a ... secret and is defined in my secrets.json, you can find it on your reCaptcha GUI
  • here I just parse the resulting json and see if the token is ok, if it's not then I raise an Exception. I could have returned a boolean, but this case is not supposed to happen beucase I only want human on my app.

And for injecting this service I do it like that on my ConfigureServices

services.AddSingleton<ICaptchaValidator>(s => new CaptchaValidator(
  • It's a singleton as there is no state
  • Don't forget to init the HttpClient or IHttpClientFactory injection will fail

I also need to inject my pipeline, I don't know why MediatR ASPNET Core extension doesn't do this by default

services.AddScoped(typeof(IPipelineBehavior<, >), typeof(CaptchaMediatRAdapter<,>));

Now everything is bootstraped :

  • A NotARobot is detected on client side and we try to get a token (this fails if the user is not a human)
  • The token is send on the command payload
  • MediatR executes the pipeline if the command inherit from NotARobot
  • The validation will fail if the token is not correct
  • My command get executed if reCaptcha doesn't think it's a robot.

E2E tests

Now my E2E tests are failing because Selenium WebDriver is a robot :(.

I need to change the two validation process : on the client-side and server side.

On the client side, my test executes the following script

if (Browser is IJavaScriptExecutor)
    //in E2E test we disable getting the token from recaptcha
    ((IJavaScriptExecutor)Browser).ExecuteScript("runCaptcha = function(actionName) { return Promise.resolve('test'); }");
  • The js function is overriden and returns a 'test' token at every execution

On the server side I need to inject a Fake implementation of my validator

if (Configuration.GetValue<string>("test") != null)
    services.AddSingleton<ICaptchaValidator, FakeCaptchaValidator>();
public class FakeCaptchaValidator : ICaptchaValidator
        public bool NextResult { get; set; } = true;
        public Task Check(string token)
            if (!NextResult)
                throw new InvalidOperationException();
            return Task.CompletedTask;
  • Because I send "-test true" when I execute my server for E2E test, the good implementation is injected.
  • with NextResult in my FakeCaptchaValidator I can validate that my process fails when the captcha validation fails


In this article we saw again two major advantages of Blazor : the code sharing between client and server and the js interoperability which enables you to use any existing javascript api.


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