In this guide, we’ll cover how to secure your C# / ASP.NET Core 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!
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 forbidden requests.
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.
Note: If your Twilio webhook URLs start with
https:// instead of
http://, your request validator may fail locally when you use Ngrok or in production, if your stack terminates SSL connections upstream from your app. This is because the request URL that your ASP.NET application sees does not match the URL Twilio used to reach your application.
To fix this for local development with ngrok, use
http:// for your webook instead of
https://. To fix this in your production app, your method will need to reconstruct the request's original URL using request headers like
X-Forwarded-Proto, if available.
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.
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 MVC application in general, check out the security considerations page in the official ASP.NET docs.