Secure your C# / ASP.NET Core app by validating incoming Twilio requests
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!
If you don't want to develop your own validation filter, you can install the Twilio helper library for ASP.NET Core and use the library's [ValidateRequest] attribute instead that has more features. This library also contains an endpoint filter and a middleware validator.
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 controllers and 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 call 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.
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 controller action, 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.
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 webhook 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-Original-Host and X-Forwarded-Proto, if available.
Before running the application, make sure you configure your Twilio Auth Token as the Twilio:AuthToken configuration, using .NET's secrets manager, environment variables, a vault service, or some other secure configuration source.
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.
To disable the request validation, you can now configure Twilio:RequestValidation:Enabled to false in your appsettings.json or appsettings.Development.json file.
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 MVC application in general, check out the security considerations page in the official ASP.NET 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.
