A potentially dangerous Request.Form value was detected from the client

How to Handle Potentially Dangerous Request.Form Values in Your Web Application 🌐💻

So, you're building a web application, and you've encountered the dreaded "Potentially Dangerous Request.Form Value" exception. 😱 Don't panic! This blog post has got you covered with a solution that will not only handle this problem elegantly but also keep your application secure. 🛡️

Understanding the Issue 😕

The exception occurs when a user submits a form that contains the characters < or > in any of the fields. By default, ASP.NET throws this exception to protect your application from potential cross-site scripting (XSS) attacks. While it's great to have this protection, we also want to handle it in a more user-friendly manner. 🚫🔍

The Not-So-Professional "Solution" 🤷‍♂️

You've may have come across suggestions to trap the exception and display a generic error message, asking the user to go back and re-enter the form without using <. But let's face it, that's not the most professional or user-friendly approach. Let's find a better way to handle this. 💁‍♀️💡

The Ideal Approach ✨

The ideal solution would be to automatically HTML encode any posted value in the Form collection, thereby preventing the exception from being thrown in the first place. This way, the content is sanitized, and your application remains secure. 🧪🔒

Handling Potentially Dangerous Form Values with a Handler 🛠️

To achieve this, we can create a custom handler that intercepts the form submission and performs the necessary HTML encoding. Let's call it the FormSanitizerHandler. Here's a step-by-step guide on how to implement it: 📝👇

1. Create a new class called FormSanitizerHandler in your ASP.NET project. Inherit from the IHttpHandler interface.

2. Implement the ProcessRequest method required by the IHttpHandler interface.

3. In the ProcessRequest method, access the form values from the HttpContext.Current.Request.Form collection.

4. Iterate over the form values and HTML encode each one using the HttpUtility.HtmlEncode method.

5. Assign the encoded values back to the form fields in the Form collection.

6. Proceed with the normal postback processing.

Here's a code snippet to give you a better idea of what the ProcessRequest method might look like:

public void ProcessRequest(HttpContext context)
{
    var form = context.Request.Form;
    
    foreach (string key in form)
    {
        string encodedValue = HttpUtility.HtmlEncode(form[key]);
        form[key] = encodedValue;
    }

    // Proceed with normal postback processing...
}

1. Finally, register the FormSanitizerHandler in your application's web.config file:

<system.webServer>
  <handlers>
    <add name="FormSanitizerHandler" path="FormSanitizer.ashx" verb="*" type="YourNamespace.FormSanitizerHandler" />
  </handlers>
</system.webServer>

Wrapping Up and Taking Action 🎉🚀

With the FormSanitizerHandler in place, you've successfully handled potentially dangerous form values without compromising security. Your users will no longer encounter the intimidating exception page, and your application will remain protected against XSS attacks. It's a win-win situation! 🙌❤️

So, what are you waiting for? Implement the FormSanitizerHandler today and provide a seamless experience to your users. Share this blog post with other developers who might find it helpful, and let's make the web a safer place together! 💪🌐

Got any questions or suggestions? Leave a comment below! 👇📝