Technical Solutions for Resolving "A potentially dangerous Request.Path value was detected from the client (&)" Error in ASP.NET MVC

Problem Background and Error Analysis

During ASP.NET MVC 3 application development, when URL paths contain specific special characters, the system throws the "A potentially dangerous Request.Path value was detected from the client (&)" error. This validation mechanism is a built-in security protection measure in the ASP.NET framework, designed to prevent cross-site scripting (XSS) attacks.

By default, ASP.NET 4.0 and later versions strictly validate the following characters in URL paths: < > * % & : \ ?. These characters have special meanings in URLs and could be maliciously exploited to construct attack payloads.

Core Solution

To address this issue, the most effective solution is to adjust request validation behavior by modifying configuration parameters in the web.config file. The specific implementation is as follows:

<system.web>
    <httpRuntime requestPathInvalidCharacters="" requestValidationMode="2.0" />
    <pages validateRequest="false" />
</system.web>

Here, the requestPathInvalidCharacters parameter is set to an empty string, indicating that all characters are allowed to pass path validation; requestValidationMode="2.0" enables ASP.NET 2.0 compatibility mode, providing more lenient validation rules; validateRequest="false" disables page-level request validation.

Alternative Configuration Approach

If you wish to maintain a certain level of security validation, you can specify the allowed character set:

<system.web>
    <httpRuntime requestPathInvalidCharacters="<,>,*,%,&,:,\,?" />
</system.web>

This configuration method explicitly lists the dangerous characters that require validation, achieving a balance between security and functionality.

Code-Level Supplementary Measures

At the controller level, you can disable input validation for specific Actions using the [ValidateInput(false)] attribute:

public class WebsiteController : Controller
{
    [ValidateInput(false)]
    public ActionResult Home()
    {
        return View();
    }
}

This approach is suitable for scenarios that require handling special characters without globally disabling validation.

Best Practice Recommendations

Although technical solutions can resolve validation errors, from a security perspective, it is recommended to avoid using special characters in URL design. You can employ URL encoding or replacement strategies, such as replacing the & character with an underscore _ or other safe characters.

In terms of route configuration, ensure that catch-all routes are correctly set:

routes.MapRoute(
    "Default", // Route name
    "{garb1}/{garb2}", // URL parameter pattern
    new { controller = "Website", action = "Home", garb1 = UrlParameter.Optional, garb2 = UrlParameter.Optional } // Parameter defaults
);

This configuration can properly handle URL paths containing special characters while maintaining application robustness.

Security Considerations

Disabling or relaxing request validation increases the security risks of the application. When implementing these solutions, it is essential to ensure:

• Strict whitelist validation of user input
• Implementation of output encoding to prevent XSS attacks
• Regular security audits and penetration testing
• Use of lenient validation configurations only when necessary

By comprehensively applying configuration adjustments, code optimizations, and security protection measures, you can meet functional requirements while maintaining application security.