Protecting ASP.NET Web APIs from XSS

TL;DR

Cross-Site Scripting (XSS) is a common web security problem where attackers inject malicious scripts into your website. This guide shows you how to protect your ASP.NET Web API by validating input, encoding output, using Content Security Policy and keeping dependencies up to date.

1. Understand the Threat

XSS attacks happen when user-supplied data isn’t properly handled before being displayed on a webpage or used in scripts. An attacker could inject JavaScript code that steals cookies, redirects users, or modifies your website’s content. In an API context, this often means injecting malicious script into responses that are later rendered by a client-side application.

2. Input Validation

The first line of defence is to validate all input before you use it. This means checking the data type, length, format and content against what you expect.

• Server-Side Validation: Always perform validation on the server side. Client-side validation can be bypassed easily.
• Whitelisting is Best: Define exactly what characters are allowed. Blacklisting (trying to block bad characters) is less effective as attackers constantly find new ways around filters.
• Example (C#): Check for expected data types and lengths.

public ActionResult MyApiMethod(string userInput)
{
    if (string.IsNullOrEmpty(userInput))
    {
        return BadRequest("Input cannot be empty.");
    }

    if (userInput.Length > 100)
    {
        return BadRequest("Input is too long.");
    }

    //Further validation - check for allowed characters if necessary.

    //Process the validated input...
    return Ok();
}

3. Output Encoding

Even after validating input, you need to encode output before displaying it or using it in scripts. This converts potentially dangerous characters into a safe format.

• HTML Encoding: Use this when displaying data within HTML tags.
• JavaScript Encoding: Use this when inserting data into JavaScript code.
• URL Encoding: Use this when including data in URLs.
• Example (C#): Using HttpUtility.HtmlEncode to encode output for HTML.

string safeOutput = HttpUtility.HtmlEncode(userInput);
//Now you can safely display 'safeOutput' in your HTML.

4. Content Security Policy (CSP)

CSP is a powerful security feature that tells the browser which sources of content are allowed to load for your website. This helps prevent XSS attacks by blocking malicious scripts from running.

• Configure in Web.config: Add a <httpHeaders> section to your web.config file.
• Example (Web.config): A basic CSP header allowing scripts from your own domain and trusted CDNs.

<system.webServer>
  <httpHeaders>
    <add name="Content-Security-Policy" value="default-src 'self'; script-src 'self' https://trustedcdn.com" />
  </httpHeaders>
</system.webServer>

5. Keep Dependencies Updated

Regularly update your ASP.NET framework, NuGet packages and other dependencies. Security vulnerabilities are often discovered in older versions of software.

• Use NuGet Package Manager: Check for updates regularly using the NuGet Package Manager in Visual Studio.
• Security Scanners: Consider using a security scanner to automatically identify vulnerable dependencies.

6. Use Anti-XSS Libraries

Microsoft provides libraries like AntiXSS which can help with encoding and sanitizing user input.

• Install via NuGet: Add the AntiXSS package to your project.
• Example (C#): Using AntiXSS.HtmlEncode for HTML encoding.

using AntiXSS;
...
string safeOutput = HtmlEncode(userInput);

7. Test Your API

Regularly test your API for XSS vulnerabilities.

• Manual Testing: Try injecting various malicious scripts into input fields and see if they are properly handled.
• Automated Scanning: Use a web security scanner to automatically identify potential vulnerabilities.