Secure your C# / ASP.NET WEB API app by validating incoming Twilio requests
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.
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 cal its Validate method passing the requester URL, the form params, 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.
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:
<add key="TwilioAuthToken" value="your_auth_token" />
<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.
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.
Need some help?
We all do sometimes; code is hard. Get help now from our support team, or lean on the wisdom of the crowd by visiting Twilio's Stack Overflow Collective or browsing the Twilio tag on Stack Overflow.
