Fix ASP.Net XSS Vulnerabilities

G5 Cyber Security

2 weeks ago

TL;DR

Cross-Site Scripting (XSS) in ASP.Net happens when user input isn’t properly handled before being displayed on a webpage. This lets attackers inject malicious scripts into your site, potentially stealing data or hijacking accounts. We’ll cover how it works and practical steps to fix it.

Understanding XSS

XSS exploits the trust that browsers place in content delivered from your server. If an attacker can get their script running within a user’s browser *as if* it came from your site, they can do a lot of damage.

How XSS Works in ASP.Net

User Input: A user enters data into a form field (e.g., name, comment).
Server-Side Processing: Your ASP.Net application receives this input.
Unsafe Output: The application displays the input on a webpage *without* proper encoding or sanitisation.
Script Execution: A browser interprets the injected script as part of your page, executing it with the user’s permissions.

Types of XSS

Stored (Persistent) XSS: The malicious script is permanently stored on the server (e.g., in a database). Every visitor viewing the affected page will execute the script. This is the most dangerous type.
Reflected (Non-Persistent) XSS: The script is injected via a URL or form submission and executed immediately. It requires tricking a user into clicking a malicious link.
DOM-Based XSS: The vulnerability exists in client-side JavaScript code that processes the input without proper sanitisation.

Fixing XSS Vulnerabilities

Input Validation:
Check user input on the server-side to ensure it conforms to expected formats and lengths. This *isn’t* enough on its own, but is a good first step.

// Example C# Input Validation (basic)

string name = Request.Form["name"];

if (string.IsNullOrEmpty(name) || name.Length > 50) {

  // Handle invalid input - reject it or sanitise it.

Output Encoding:
This is the *most important* step. Encode user-supplied data before displaying it on a webpage. ASP.Net provides several encoding methods:
- HtmlEncode: Encodes characters that have special meaning in HTML (e.g., < becomes <). Use this for most general text output.
```
// Example C# Html Encoding
```
```
string safeName = Server.HtmlEncode(name);
```
- UrlEncode: Encodes characters that have special meaning in URLs. Use this when including user data in a URL.
```
// Example C# Url Encoding
```
```
string safeUrl = Server.UrlEncode(name);
```
- JavaScriptEncode: Encodes characters that have special meaning in JavaScript. Use this when including user data within a JavaScript block.
```
// Example C# Javascript Encoding
```
```
string safeScript = Server.JavaScriptEncode(name);
```
Use AntiXSS Library:
Microsoft provides the AntiXSS library which offers more robust encoding and sanitisation features than basic methods.
Install via NuGet:
```
Install-Package Microsoft.AntiXss
```
Example Usage:
```
using Microsoft.AntiXss;
```
```
string safeName = Sanitizer.GetSafeHtmlFragment(name);
```
Content Security Policy (CSP):
A security header that tells the browser which sources of content are allowed to load. This can significantly reduce the impact of XSS attacks.
Add this to your web.config file:
```
<httpHeaders>
```
```
  <add name="Content-Security-Policy" value="default-src 'self'" />
```
```
</httpHeaders>
```
(Adjust the policy to suit your application’s needs.)
Regular Security Scans:
Use automated tools and manual penetration testing to identify XSS vulnerabilities in your code.

Important Considerations

Never trust client-side validation alone. Always validate on the server.
Be careful with WYSIWYG editors. They can introduce complex HTML structures that are difficult to sanitise correctly.
Keep your ASP.Net framework and libraries up to date. Security patches often address XSS vulnerabilities.