Handling HTML Input in ASP.NET: Safely Managing < and > Characters

Introduction

In web applications, handling user input securely is crucial to prevent vulnerabilities such as Cross-Site Scripting (XSS) attacks. When developing with ASP.NET, a common issue arises when users submit HTML characters like < or >. These can trigger exceptions related to potentially dangerous inputs because the framework’s request validation feature aims to block them by default. This tutorial explores how to manage these inputs elegantly without compromising security.

Understanding Request Validation

ASP.NET includes built-in request validation to protect against XSS attacks, which occur when malicious scripts are injected into web pages viewed by other users. By default, ASP.NET inspects the incoming requests for potentially dangerous content and raises an exception if it detects characters like < or > in form submissions.

Common Approaches

1. Disabling Request Validation Globally:

   • You can disable request validation by setting validateRequest="false" in your .aspx page directive or in the web.config. However, this approach is risky as it opens up your application to XSS attacks across all pages.

2. Disabling for Specific Pages Using Web.Config:

   • Instead of disabling request validation globally, you can target specific pages by using a <location> element in your web.config:

     <configuration>
       ...
       <location path="MyFolder/SpecificPage.aspx">
         <system.web>
           <pages validateRequest="false" />
           <httpRuntime requestValidationMode="2.0" />
         </system.web>
       </location>
       ...
     </configuration>
     

   • This method ensures that only selected pages allow HTML input, maintaining security for the rest of your site.

3. Handling in ASP.NET MVC:

   • In ASP.NET MVC, you can disable request validation on specific actions using attributes:

     [HttpPost, ValidateInput(false)]
     public ActionResult Edit(FormCollection collection)
     {
         // Your code here
     }
     

   • Alternatively, use the [AllowHtml] attribute to permit HTML in specific model properties:

     [AllowHtml]
     public string Description { get; set; }
     

Best Practices

• Encode Output:
  Always encode data before rendering it as HTML. Use Server.HtmlEncode() to convert special characters into their HTML-encoded equivalents.

• Use Prepared Statements:
  When interacting with databases, use parameterized queries or ORM frameworks that automatically handle escaping and encoding.

• Validate Input Programmatically:
  Even when request validation is disabled, ensure you validate and sanitize inputs programmatically. This step prevents malicious data from being processed by your application logic.

Conclusion

Handling HTML characters in ASP.NET requires a balance between security and functionality. By selectively disabling request validation where necessary and ensuring that all outputs are encoded, you can safely manage user input without exposing your application to vulnerabilities. Remember to validate inputs at the point of use, regardless of whether request validation is enabled or disabled.