A minimal, no-drama, friction-less C# HTTP verification client for Cloudflare's Turnstile API.
The problem with current Turnstile libraries in .NET is that all of them take a hard dependency on the underlying web framework like ASP.NET WebForms, ASP.NET MVC 5, ASP.NET Core, or ASP.NET Razor Pages.
Furthermore, current Turnstile libraries for .NET are hard coded against the HttpContext.Request to retrieve the remote IP address of the visitor. Unfortunately, this method doesn't work if your website is behind a service like Cloudflare where the CF-Connecting-IP header value is the real IP address of the visitor on your site.
BitArmory.Turnstile is a minimal library that works across all .NET web frameworks without taking a hard dependency on any web framework. If you want to leverage platform specific features, like MVC Action Filters, you'll need to implement your own ActionFilter that leverages the functionality in this library.
- .NET Standard 1.3 or later
- .NET Framework 4.5 or later
- Managed
- Non-interactive
- Invisible
- 🐕 Dogecoin:
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NuGet Package BitArmory.Turnstile
Install-Package BitArmory.TurnstileYou'll need a Cloudflare account. Get started by following the directions here! After you sign up and you're signed into the dashboard; you'll need two important pieces of information:
- Your
sitekey - Your
secretkey
This library supports all widget types:
- Managed
- Non-interactive
- Invisible
Be sure to read the documentation before implementing.
Then, to protect an HTML form submission on your website, add the following:
<html>
<head>
<script src="https://challenges.cloudflare.com/turnstile/v0/api.js" async defer></script>
</head>
<body>
<form method='POST' action='/my-form-post-endpoint'>
...
<div class="cf-turnstile" data-sitekey="your_site_key"></div>
<input type="submit" value="Submit">
</form>
</body>
</html>When the user visits the HTML form, a hidden form field cf-turnstile-response will be added to the HTML form above. The cf-turnstile-response represents a token of the captcha challenge result and will need to be verified server-side when the HTML form is posted to your server.
When the HTML form POST is received on the server:
- Get the client's IP address. If you're using Cloudflare, be sure to use the
CF-Connecting-IPheader value. - Extract the hidden form field
cf-turnstile-responsefrom the browser's form POST submission. - Use the
TurnstileServiceto verify that the client's Turnstilecf-turnstile-responsechallenge is valid.
The following example shows how to verify the captcha during an HTTP form POST back in ASP.NET Core: Razor Pages.
//1. Get the client IP address in your chosen web framework
string secret = "your_secret_key";
string clientIp = GetClientIpAddress();
string browserChallengeToken = null;
//2. Extract the `cf-turnstile-response` hidden field from the HTML form in your chosen web framework
//Tip: You can also use Constants.ClientResponseFormKey instead of the magic string below
if( this.Request.Form.TryGetValue("cf-turnstile-response", out var hiddenFormField) )
{
browserChallengeToken = hiddenFormField;
}
//3. Validate the Turnstile challenge with Cloudflare
var turnstileApi = new TurnstileService();
var verifyResult = await turnstileApi.VerifyAsync(browserChallengeToken, secret, clientIp);
if( verifyResult.IsSuccess is false )
{
this.ModelState.AddModelError("captcha", "The Cloudflare challenge is not valid.");
return new BadRequestResult();
}
else{
//continue processing, everything is okay!
}Notes:
- The
TurnstileService.VerifyAsyncsupports an optional idempotency parameter; you can read more about that here. - The
clientIpis technically optional but providing the client's IP address prevents abuses by ensuring that the current HTTP request is the one that received the token.
GetClientIpAddress() in ASP.NET Core
Note: If your site is behind Cloudflare, be sure you're using the CF-Connecting-IP header value instead.
public string GetClientIpAddress(){
return this.HttpContext.Connection.RemoteIpAddress.ToString();
}GetClientIpAddress() in ASP.NET WebForms
Note: If your site is behind Cloudflare, be sure you're using the CF-Connecting-IP header value instead.
public string GetClientIpAddress(){
return this.Request.UserHostAddress;
}That's it! Happy verifying! 🎉
Cloudflare Turnstile has some convenient testing keys and secrets that respond/behave differently for various UI interactions and server-side verification responses to help test your website. Be sure to read the following testing documentation here.
The testing keys and secrets can be used from the TestingSiteKeys and TestingSecretKeys constant classes:
Turnstile/BitArmory.Turnstile/Constants.cs
Lines 30 to 73 in 1d59d26
- Download the source code.
- Run
build.cmd.
Upon successful build, the results will be in the \__compile directory. If you want to build NuGet packages, run build.cmd pack and the NuGet packages will be in __package.