When upgrading web applications from .NET 2.0 to .NET 4.0, developers often encounter unexpected validation errors that can be puzzling and frustrating. These errors typically manifest as HttpRequestValidationException messages, indicating that the application has detected potentially dangerous request data. This change in behavior represents a significant shift in how ASP.NET handles request validation, with important implications for application security and functionality.

The Validation Challenge

The error message you’re likely seeing looks something like this:

System.Web.HttpRequestValidationException (0x80004005): 
A potentially dangerous Request.Form value was detected from the client 
(ctl00$Body$ucSettings$tbText="<Settings><TypeID>10...").
at System.Web.HttpRequest.ValidateString(String value, String collectionKey, RequestValidationSource requestCollection)
at System.Web.HttpRequest.ValidateHttpValueCollection(HttpValueCollection collection, RequestValidationSource requestCollection)

This error occurs even when you’ve explicitly set ValidateRequest="false" on your page directive. The reason for this change lies in ASP.NET 4.0’s enhanced security model, which implements stricter validation rules by default.

The Root Cause

In .NET 4.0, Microsoft made significant changes to the request validation system to improve security. The previous approach of simply setting ValidateRequest="false" on a page is no longer sufficient. This change was implemented to provide better protection against cross-site scripting (XSS) attacks and other security vulnerabilities.

The new validation system is more thorough and checks for potentially dangerous content at multiple levels, including:

  • Form submissions
  • Query strings
  • Cookies
  • Server variables

Implementing the Solution

To address this issue, you have two options:

Global Solution

If you need to maintain the .NET 2.0 validation behavior across your entire application, add the following to your web.config file:

<system.web>
    <httpRuntime requestValidationMode="2.0" />
</system.web>

Page-Specific Solution

If you only need to disable strict validation for specific pages, you can use the location element in your web.config:

<location path="[PageRequiring].aspx">
    <system.web>
        <httpRuntime requestValidationMode="2.0" />
    </system.web>
</location>

Security Considerations

While these solutions will resolve the validation errors, it’s crucial to understand the security implications. The .NET 4.0 validation changes were implemented for good reasons, and disabling them should be done with caution.

When disabling request validation, you should:

  1. Implement alternative validation mechanisms
  2. Carefully sanitize all user input
  3. Use proper encoding when displaying user-provided content
  4. Consider implementing a custom validation solution

Best Practices

Instead of simply reverting to the .NET 2.0 validation mode, consider these alternatives:

  1. Use HTML encoding for user input
  2. Implement custom validation for specific fields
  3. Use the [AllowHtml] attribute for specific model properties
  4. Consider using a rich text editor that sanitizes input

Moving Forward

The changes to request validation in ASP.NET 4.0 represent a step forward in web application security. While the immediate solution might be to revert to the previous behavior, it’s worth considering whether your application could benefit from the enhanced security features of the new validation system.

If you do choose to disable the new validation, ensure you have proper security measures in place. Remember that request validation is just one layer of security, and you should implement additional safeguards to protect your application from potential attacks.

Note: Always test thoroughly after making changes to request validation settings, as these changes can have significant security implications for your application.