Secure your C# / ASP.NET WEB API app by validating incoming Twilio requests

(warning)

Warning

This guide is for ASP.NET Web API on the .NET Framework. For ASP.NET Core, see this guide. For ASP.NET MVC on the .NET Framework, see this guide.

In this guide, we'll cover how to secure your C# / ASP.NET Web API application by validating incoming requests to your Twilio webhooks are, in fact, from Twilio.

With a few lines of code, we'll write a custom filter attribute for our ASP.NET app that uses the Twilio C# SDK's validator utility. This filter will then be invoked on the controller actions that accept Twilio webhooks to confirm that incoming requests genuinely originated from Twilio.

Let's get started!

Create a custom filter attribute

The Twilio C# SDK includes a RequestValidator class we can use to validate incoming requests.

We could include our request validation code as part of our controller, but this is a perfect opportunity to write an action filter attribute. This way we can reuse our validation logic across all our controller actions which accept incoming requests from Twilio.

ASP.NET Web API filter attribute to validate Twilio requests

Confirm incoming requests to your controllers are genuine with this filter.

1using System;
2using System.Collections.Generic;
3using System.Configuration;
4using System.IO;
5using System.Linq;
6using System.Net;
7using System.Net.Http;
8using System.Threading;
9using System.Threading.Tasks;
10using System.Web.Http.Controllers;
11using System.Web.Http.Filters;
12using Twilio.Security;
13
14namespace ValidateRequestExample.Filters
15{
16    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
17    public class ValidateTwilioRequestAttribute : ActionFilterAttribute
18    {
19        private readonly string _authToken;
20        private readonly string _urlSchemeAndDomain;
21
22        public ValidateTwilioRequestAttribute()
23        {
24            _authToken = ConfigurationManager.AppSettings["TwilioAuthToken"];
25            _urlSchemeAndDomain = ConfigurationManager.AppSettings["TwilioBaseUrl"];
26        }
27
28        public override async Task OnActionExecutingAsync(HttpActionContext actionContext, CancellationToken cancellationToken)
29        {
30            if (!await IsValidRequestAsync(actionContext.Request))
31            {
32                actionContext.Response = actionContext.Request.CreateErrorResponse(
33                    HttpStatusCode.Forbidden,
34                    "The Twilio request is invalid"
35                );
36            }
37
38            await base.OnActionExecutingAsync(actionContext, cancellationToken);
39        }
40
41        private async Task<bool> IsValidRequestAsync(HttpRequestMessage request)
42        {
43            var headerExists = request.Headers.TryGetValues(
44                "X-Twilio-Signature", out IEnumerable<string> signature);
45            if (!headerExists) return false;
46
47            var requestUrl = _urlSchemeAndDomain + request.RequestUri.PathAndQuery;
48            var formData = await GetFormDataAsync(request.Content);
49            return new RequestValidator(_authToken).Validate(requestUrl, formData, signature.First());
50        }
51
52        private async Task<IDictionary<string, string>> GetFormDataAsync(HttpContent content)
53        {
54            string postData;
55            using (var stream = new StreamReader(await content.ReadAsStreamAsync()))
56            {
57                stream.BaseStream.Position = 0;
58                postData = await stream.ReadToEndAsync();
59            }
60
61            if(!String.IsNullOrEmpty(postData) && postData.Contains("="))
62            {
63                return postData.Split('&')
64                    .Select(x => x.Split('='))
65                    .ToDictionary(
66                        x => Uri.UnescapeDataString(x[0]),
67                        x => Uri.UnescapeDataString(x[1].Replace("+", "%20"))
68                    );
69            }
70
71            return new Dictionary<string, string>();
72        }
73    }
74}
75

To validate an incoming request genuinely originated from Twilio, we first need to create an instance of the RequestValidator class passing it our Twilio Auth Token. Then we call its Validate method passing the requester URL, the form parameters, and the Twilio request signature.

That method will return True if the request is valid or False if it isn't. Our filter attribute then either continues processing the action or returns a 403 HTTP response for invalid requests.

Use the filter attribute with our Twilio webhooks

Now we're ready to apply our filter attribute to any controller action in our ASP.NET application that handles incoming requests from Twilio.

Apply the request validation filter attribute to a set of ApiController methods

1using System.Net.Http;
2using System.Text;
3using System.Web.Http;
4using Twilio.TwiML;
5using Twilio.TwiML.Messaging;
6using ValidateRequestExample.Filters;
7
8namespace ValidateRequestExample.Controllers
9{
10    public class TwilioMessagingRequest
11    {
12        public string Body { get; set; }
13    }
14
15    public class TwilioVoiceRequest
16    {
17        public string From { get; set; }
18    }
19
20    public class IncomingController : ApiController
21    {
22        [Route("voice")]
23        [AcceptVerbs("POST")]
24        [ValidateTwilioRequest]
25        public HttpResponseMessage PostVoice([FromBody] TwilioVoiceRequest voiceRequest)
26        {
27            var message =
28                "Thanks for calling! " +
29                $"Your phone number is {voiceRequest.From}. " +
30                "I got your call because of Twilio's webhook. " +
31                "Goodbye!";
32
33            var response = new VoiceResponse();
34            response.Say(message);
35            response.Hangup();
36
37            return ToResponseMessage(response.ToString());
38        }
39
40        [Route("message")]
41        [AcceptVerbs("POST")]
42        [ValidateTwilioRequest]
43        public HttpResponseMessage PostMessage([FromBody] TwilioMessagingRequest messagingRequest)
44        {
45            var message =
46                $"Your text to me was {messagingRequest.Body.Length} characters long. " +
47                "Webhooks are neat :)";
48
49            var response = new MessagingResponse();
50            response.Append(new Message(message));
51
52            return ToResponseMessage(response.ToString());
53        }
54
55        private static HttpResponseMessage ToResponseMessage(string response)
56        {
57            return new HttpResponseMessage
58            {
59                Content = new StringContent(response, Encoding.UTF8, "application/xml")
60            };
61        }
62    }
63}
64

To use the filter attribute with an existing view, just put [ValidateTwilioRequest] above the action's definition. In this sample application, we use our filter attribute with two controller actions: one that handles incoming phone calls and another that handles incoming text messages.

Configuration Options

You will need to add the following to your Web.config file, in the appSettings section:

1    <add key="TwilioAuthToken" value="your_auth_token" />
2    <add key="TwilioBaseUrl" value="https://????.ngrok.io"/>

You can get your Twilio Auth Token from the Twilio Console. The TwilioBaseUrl setting should be the public protocol and domain that you have configured on your Twilio phone number. For example, if you are using ngrok, you would put your ngrok URL here. If you are deploying to Azure or another cloud provider, put your publicly accessible domain here and include https or http, as appropriate for your application.

Disable request validation during testing

If you write tests for your controller actions, those tests may fail where you use your Twilio request validation filter. Any requests your test suite sends to those actions will fail the filter's validation check.

To fix this problem we recommend adding an extra check in your filter attribute, like so, telling it to only reject incoming requests if your app is running in production.

An improved ASP.NET Web API request validation filter attribute, useful for testing

Use this version of the custom filter attribute if you test your controllers.

1using System;
2using System.Collections.Generic;
3using System.Configuration;
4using System.IO;
5using System.Linq;
6using System.Net;
7using System.Net.Http;
8using System.Threading;
9using System.Threading.Tasks;
10using System.Web.Http.Controllers;
11using System.Web.Http.Filters;
12using Twilio.Security;
13
14namespace ValidateRequestExample.Filters
15{
16    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
17    public class ValidateTwilioRequestImprovedAttribute : ActionFilterAttribute
18    {
19        private readonly string _authToken;
20        private readonly string _urlSchemeAndDomain;
21        private static bool IsTestEnvironment =>
22            bool.Parse(ConfigurationManager.AppSettings["IsTestEnvironment"]);
23
24        public ValidateTwilioRequestImprovedAttribute()
25        {
26            _authToken = ConfigurationManager.AppSettings["TwilioAuthToken"];
27            _urlSchemeAndDomain = ConfigurationManager.AppSettings["TwilioBaseUrl"];
28        }
29
30        public override async Task OnActionExecutingAsync(HttpActionContext actionContext, CancellationToken cancellationToken)
31        {
32            if (!await IsValidRequestAsync(actionContext.Request) && !IsTestEnvironment)
33            {
34                actionContext.Response = actionContext.Request.CreateErrorResponse(
35                    HttpStatusCode.Forbidden,
36                    "The Twilio request is invalid"
37                );
38            }
39
40            await base.OnActionExecutingAsync(actionContext, cancellationToken);
41        }
42
43        private async Task<bool> IsValidRequestAsync(HttpRequestMessage request)
44        {
45            var headerExists = request.Headers.TryGetValues(
46                "X-Twilio-Signature", out IEnumerable<string> signature);
47            if (!headerExists) return false;
48
49            var requestUrl = _urlSchemeAndDomain + request.RequestUri.PathAndQuery;
50            var formData = await GetFormDataAsync(request.Content);
51            return new RequestValidator(_authToken).Validate(requestUrl, formData, signature.First());
52        }
53
54        private async Task<IDictionary<string, string>> GetFormDataAsync(HttpContent content)
55        {
56            string postData;
57            using (var stream = new StreamReader(await content.ReadAsStreamAsync()))
58            {
59                stream.BaseStream.Position = 0;
60                postData = await stream.ReadToEndAsync();
61            }
62
63            if (!String.IsNullOrEmpty(postData) && postData.Contains("="))
64            {
65                return postData.Split('&')
66                    .Select(x => x.Split('='))
67                    .ToDictionary(
68                        x => Uri.UnescapeDataString(x[0]),
69                        x => Uri.UnescapeDataString(x[1].Replace("+", "%20"))
70                    );
71            }
72
73            return new Dictionary<string, string>();
74        }
75    }
76}
77

What's next?

Validating requests to your Twilio webhooks is a great first step for securing your Twilio application. We recommend reading over our full security documentation for more advice on protecting your app, and the Anti-Fraud Developer's Guide in particular.

To learn more about securing your ASP.NET Web API application in general, check out the security considerations in the official ASP.NET Web API docs.