COURSE: Web Application Security Fundamentals
WEEK 1 — Introduction to XSS & Client-Side Attacks
INSTRUCTOR: Prof. David Reeves
SLIDE DECK: Week 1 / Lecture 1
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SLIDE 1: What is Cross-Site Scripting?
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Cross-Site Scripting (XSS) is a code injection vulnerability that allows
attackers to inject malicious scripts into web pages viewed by other users.

Types of XSS:
  1. Reflected XSS  — payload in request, reflected in response
  2. Stored XSS     — payload persisted in the database
  3. DOM-Based XSS  — payload executed via client-side JS

SLIDE 2: Attack Surface
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Common injection points:
  - URL query parameters
  - Form inputs (search boxes, comments)
  - HTTP headers (User-Agent, Referer)
  - JSON API responses rendered without encoding

SLIDE 3: Vulnerable Code Pattern
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  PHP:  echo $_GET['name'];                 // No sanitization
  JS:   document.innerHTML = location.hash  // DOM sink

Safe alternative:
  PHP:  echo htmlspecialchars($_GET['name'], ENT_QUOTES, 'UTF-8');
  JS:   element.textContent = value;        // Not innerHTML

SLIDE 4: Lab Exercise
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Target: http://vuln-app.local/search?q=
Goal:   Trigger alert(1) in the browser

Try:
  <script>alert(1)</script>
  <img src=x onerror=alert(1)>
  "><svg/onload=alert(1)>

Assessment: Submit your working payload via the Assignments section.