Project Overview

This module demonstrates production-ready frontend security practices aligned with OWASP Top 10 2021, specifically addressing A03:2021 – Injection (DOM-based XSS).

🎯 Purpose

Showcase practical security engineering skills through a working demonstration of XSS prevention techniques

🛡️ Protection Level

Multi-layered defense: Input sanitization, safe DOM manipulation, and CSP headers

📊 Compliance

OWASP Top 10 aligned, no external dependencies, GitHub Pages compatible

Live Security Demo

Test the input sanitization and safe rendering system below. Try entering malicious payloads – they will be safely handled.

🔒 Secure Chat Interface

All user input is sanitized before rendering

System: Welcome! This chat demonstrates DOM-based XSS prevention. Try sending a message.
0/500 characters

Try These Common XSS Payloads:

✅ All payloads above are blocked and sanitized before rendering

Implemented Security Measures

This project implements multiple layers of defense to prevent DOM-based XSS attacks:

🧹

1. Input Sanitization Layer

  • Script Tag Removal: All <script> tags are stripped from user input
  • Event Handler Blocking: Inline event handlers (onclick, onerror, onload, etc.) are removed
  • JavaScript URL Prevention: "javascript:" protocol URLs are blocked
  • Length Limitation: Input restricted to 500 characters to prevent buffer overflow attacks
  • Whitespace Normalization: Excessive whitespace is trimmed
Implementation: See sanitizeInput() function in script.js
🛡️

2. Safe Output Rendering

  • textContent over innerHTML: User input is always rendered using textContent to prevent HTML injection
  • Context-Aware Rendering: System messages use controlled HTML with allowed tags only
  • Allowlist Approach: Only <a> and <br> tags permitted in controlled contexts
  • Dangerous Tag Blocking: iframe, object, embed, script tags are never allowed
  • Attribute Filtering: Event handler attributes are stripped from all elements
Implementation: See renderSafeHTML() function in script.js
📋

3. Content Security Policy (CSP)

  • Script Source Restriction: Only scripts from 'self' (same origin) are allowed
  • Inline Script Blocking: Inline scripts are blocked by default (no eval, no inline handlers)
  • Style Source Control: Styles limited to same origin with necessary inline exceptions
  • Default Deny: All other resource types default to 'self' only
Implementation: See <meta http-equiv="Content-Security-Policy"> in HTML head
🔐

4. Defense in Depth Strategy

  • Multiple Layers: If one layer fails, others provide backup protection
  • Input Validation: Validate before processing (client-side enforcement)
  • Output Encoding: Escape data before rendering to DOM
  • Secure Defaults: Fail securely – if unsure, treat as unsafe
  • No External Dependencies: Reduces supply chain attack surface
Strategy: Sanitize + Safe Rendering + CSP = Layered Protection

Threat Model

Primary Threat: DOM-based Cross-Site Scripting (XSS)

Attack Vector: An attacker attempts to inject malicious JavaScript code through user input fields that gets executed in the victim's browser context.

Impact: Successful XSS attacks can lead to:

  • Session hijacking (cookie theft)
  • Credential harvesting (keylogging)
  • Defacement of web content
  • Phishing attacks via injected content
  • Malware distribution

Mitigation Strategy

Step 1: Input Phase

Sanitize and validate all user input before accepting it

↓
Step 2: Processing Phase

Never execute or evaluate user-controlled data as code

↓
Step 3: Output Phase

Use textContent for user data, never innerHTML

↓
Step 4: Browser Enforcement

CSP header blocks any malicious scripts that bypass other layers

Attack Surface Analysis

Entry Point Risk Level Protection Mechanism
User Input Field Mitigated Input sanitization + textContent rendering
URL Parameters Low No URL parameter processing in this demo
Third-party Scripts None Zero external dependencies + CSP blocks external scripts
Inline Event Handlers None CSP blocks inline handlers + code uses addEventListener