fosres

Posted on Feb 13 • Originally published at github.com

Week 6 Quiz Audit XSS Vulnerabilities

#security #python #webdev #appsec

📚 Part of Week 6 in the SecEng-Exercises curriculum - a free, open-source path to becoming a Security Engineer. Star the repo to follow along with weekly challenges!

The $5 Million XSS Disaster

It was 3 AM when the security team at TechFinance Corporation received the alert. Their customer portal—processing millions of dollars in transactions daily—had been compromised. The attack vector? A simple Cross-Site Scripting (XSS) vulnerability in their search functionality.

The attacker had crafted a malicious URL containing JavaScript that stole session cookies. They shared it on social media disguised as a "special promotion." Within hours, thousands of customers clicked the link. The malicious script silently harvested authentication tokens, granting the attacker access to customer accounts.

By morning, the damage was catastrophic:

15,000+ customer accounts compromised
$5.2 million in unauthorized transfers
Complete database exfiltration including social security numbers
Stock price plummeted 40% after disclosure
Class action lawsuit filed within 72 hours

The company's Chief Security Officer resigned. The engineering team worked 72-hour shifts to contain the breach. Regulatory fines exceeded $12 million. All because of a single XSS vulnerability that should have been caught in code review.

Don't let this happen to your application.

🎓 Learning Security Engineering?

This challenge is from my free, open-source Security Engineering curriculum with:

✅ LeetCode-style exercises with 60+ test cases

✅ Real CVEs and production-incident inspired scenarios

✅ Complete solutions with best practices

✅ New challenges added weekly

⭐ Star SecEng-Exercises on GitHub - Join developers mastering AppSec fundamentals!

🎯 Part of the SecEng-Exercises Collection

This XSS challenge is Week 6 of my comprehensive Security Engineering curriculum. I'm building a public collection of LeetCode-style security exercises with:

✅ 60+ test cases per exercise for thorough validation
✅ Real-world vulnerability patterns from production systems
✅ Hands-on pentesting practice against intentionally vulnerable code
✅ Complete solutions with security best practices

⭐ Star the repo on GitHub to follow along as I add new exercises weekly covering AppSec, cryptography, API security, and more.

💡 Why star the repo? You'll get notified when I publish new challenges covering SQL injection, authentication bypasses, cryptographic vulnerabilities, and advanced API attacks. All exercises include comprehensive test suites and are production-incident inspired.

📋 What You'll Learn in This Post

Real XSS attack scenario and its $5M+ impact
Three vulnerable code challenges to test your skills
Complete vulnerability analysis with fix implementations
Prevention patterns used by security teams at top tech companies

Estimated completion time: 30-45 minutes | Difficulty: Intermediate | Prerequisites: Basic Python, HTML/JavaScript

Your Challenge: Find the XSS Vulnerabilities

Below are three real-world code snippets, each containing multiple XSS vulnerabilities. Your task is to:

Identify all security vulnerabilities in each snippet
Understand how an attacker could exploit them
Fix the code to prevent XSS attacks

These exercises are based on patterns from production systems and real security incidents. Study each carefully—the vulnerabilities range from obvious to subtle.

Sources for these exercises:

Full Stack Python Security by Dennis Byrne, Chapter 14, pp. 208-226¹
API Security in Action by Neil Madden, Chapter 2, pp. 54-55²
Secure by Design by Johnsson et al., Chapter 9, pp. 247-249³
Hacking APIs by Corey Ball⁴

📖 Following the curriculum? I'm documenting my journey through these books in the SecEng-Exercises repository, creating exercises for each chapter. Week 7 covers Full Stack Python Security Chapter 15 (CSRF) and Hacking APIs Chapter 3 (API Authentication Bypasses). Star the repo for weekly updates!

Vulnerable Code Snippet #1: Flask Search Application

from flask import Flask, request

app = Flask(__name__)

@app.route('/search')
def search():
    """Search endpoint - handles user queries"""
    query = request.args.get('q', '')

    # Simulate database search
    results = []
    if query:
        query_lower = query.lower()
        if 'laptop' in query_lower:
            results = [
                {'name': 'Gaming Laptop Pro', 'price': 1299.99, 'stock': 12},
                {'name': 'Business Laptop Elite', 'price': 899.99, 'stock': 8},
            ]

    # Generate HTML response
    html = f"""
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Search Results</title>
</head>
<body>
    <h1>Search Results for: <span>{query}</span></h1>
    <div class="results">
    """

    if results:
        for product in results:
            html += f"""
            <div class="product">
                <h3>{product['name']}</h3>
                <div class="price">${product['price']}</div>
                <div class="stock">In stock: {product['stock']} units</div>
            </div>
            """
    else:
        html += f"""
        <div class="no-results">
            <h2>No Results Found</h2>
            <p>Your search for '<strong>{query}</strong>' did not match any products.</p>
        </div>
        """

    html += """
    </div>
</body>
</html>
    """

    return html

if __name__ == '__main__':
    app.run(debug=True, port=5000, host='127.0.0.1')

Vulnerable Code Snippet #2: FastAPI Comment Board

from fastapi import FastAPI, Request
from fastapi.responses import HTMLResponse
from pydantic import BaseModel
import sqlite3
import html

app = FastAPI()

class Comment(BaseModel):
    author: str
    content: str

@app.get("/", response_class=HTMLResponse)
async def home(request: Request):
    """Main page with comment form"""
    return """
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Comment Board</title>
</head>
<body>
    <h1>Community Comment Board</h1>
    <form id="commentForm">
        <input type="text" id="author" placeholder="Your name" required>
        <textarea id="content" placeholder="Your comment" required></textarea>
        <button type="submit">Post Comment</button>
    </form>
    <div id="commentsContainer"></div>

    <script>
        // Load comments when page loads
        loadComments();

        // Handle form submission
        document.getElementById('commentForm').addEventListener('submit', async (e) => {
            e.preventDefault();

            const author = document.getElementById('author').value;
            const content = document.getElementById('content').value;

            const response = await fetch('/api/comments', {
                method: 'POST',
                headers: {'Content-Type': 'application/json'},
                body: JSON.stringify({ author, content })
            });

            if (response.ok) {
                document.getElementById('author').value = '';
                document.getElementById('content').value = '';
                loadComments();
            }
        });

        // Load and display comments
        async function loadComments() {
            const response = await fetch('/api/comments');
            const comments = await response.json();

            const container = document.getElementById('commentsContainer');
            container.innerHTML = '';

            comments.forEach(comment => {
                const div = document.createElement('div');
                div.className = 'comment';

                div.innerHTML = `
                    <div class="comment-author">${comment.author}</div>
                    <div class="comment-time">${comment.timestamp}</div>
                    <div class="comment-content">${comment.content}</div>
                `;
                container.appendChild(div);
            });
        }
    </script>
</body>
</html>
    """

@app.post("/api/comments")
async def create_comment(comment: Comment):
    """Create a new comment"""
    conn = sqlite3.connect('comments.db')
    cursor = conn.cursor()

    # Escape HTML to prevent XSS
    author = html.escape(comment.author, quote=True)
    content = html.escape(comment.content, quote=True)

    # SQL Injection vulnerable query
    query = f"INSERT INTO comments (author, content, timestamp) VALUES ('{author}', '{content}', datetime('now'))"
    cursor.execute(query)

    conn.commit()
    conn.close()

    return {"status": "success"}

@app.get("/api/comments")
async def get_comments():
    """Get all comments"""
    conn = sqlite3.connect('comments.db')
    cursor = conn.cursor()

    # SQL Injection vulnerable query
    query = "SELECT author, content, timestamp FROM comments ORDER BY id DESC"
    cursor.execute(query)

    comments = []
    for row in cursor.fetchall():
        comments.append({
            "author": row[0],
            "content": row[1],
            "timestamp": row[2]
        })

    conn.close()
    return comments

Vulnerable Code Snippet #3: Session Management API

from fastapi import FastAPI
from fastapi.responses import HTMLResponse
import sqlite3

app = FastAPI()

@app.post("/auth/register")
async def register(username: str, password: str):
    """Register a new user"""
    conn = sqlite3.connect('security_app.db')
    cursor = conn.cursor()

    # Store user in database
    cursor.execute("INSERT INTO users (username, password) VALUES (?, ?)", 
                   (username, password))

    user_id = cursor.lastrowid
    conn.commit()
    conn.close()

    return {"user_id": user_id, "username": username}

@app.post("/sessions/create")
async def create_session(user_id: int, metadata: str):
    """Create a new session"""
    conn = sqlite3.connect('security_app.db')
    cursor = conn.cursor()

    import secrets
    session_id = secrets.token_urlsafe(32)

    # Store raw session ID in database
    cursor.execute(
        "INSERT INTO sessions (session_id, user_id, metadata) VALUES (?, ?, ?)",
        (session_id, user_id, metadata)
    )

    conn.commit()
    conn.close()

    return {"session_id": session_id}

@app.get("/", response_class=HTMLResponse)
async def home():
    """Frontend with session management"""
    return """
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Session Manager</title>
</head>
<body>
    <h1>Session Management System</h1>

    <div id="register_section">
        <h2>Register</h2>
        <input type="text" id="register_username" placeholder="Username">
        <input type="password" id="register_password" placeholder="Password">
        <button onclick="registerUser()">Register</button>
        <div id="register_result"></div>
    </div>

    <div id="login_section">
        <h2>Login</h2>
        <input type="text" id="login_username" placeholder="Username">
        <input type="password" id="login_password" placeholder="Password">
        <button onclick="loginUser()">Login</button>
        <div id="login_result"></div>
    </div>

    <div id="profile_section">
        <h2>Get Profile</h2>
        <input type="text" id="profile_user_id" placeholder="User ID">
        <button onclick="getProfile()">Get Profile</button>
        <div id="profile_result"></div>
    </div>

    <script>
        async function registerUser() {
            const username = document.getElementById('register_username').value;
            const password = document.getElementById('register_password').value;

            const response = await fetch('/auth/register', {
                method: 'POST',
                headers: {'Content-Type': 'application/json'},
                body: JSON.stringify({username, password})
            });

            const data = await response.json();
            const result = document.getElementById('register_result');

            if (response.ok) {
                result.innerHTML = `<div class="result">✓ Registered! User ID: ${data.user_id}, Username: ${data.username}</div>`;
            } else {
                result.innerHTML = `<div class="result error">✗ ${data.detail}</div>`;
            }
        }

        async function loginUser() {
            const username = document.getElementById('login_username').value;
            const password = document.getElementById('login_password').value;

            const response = await fetch('/auth/login', {
                method: 'POST',
                headers: {'Content-Type': 'application/json'},
                body: JSON.stringify({username, password})
            });

            const data = await response.json();
            const result = document.getElementById('login_result');

            if (response.ok) {
                result.innerHTML = `<div class="result">✓ Login successful! User ID: ${data.user_id}</div>`;
            } else {
                result.innerHTML = `<div class="result error">✗ ${data.message}</div>`;
            }
        }

        async function getProfile() {
            const userId = document.getElementById('profile_user_id').value;

            const response = await fetch(`/auth/profile/${userId}`);
            const data = await response.json();
            const result = document.getElementById('profile_result');

            if (response.ok) {
                result.innerHTML = `
                    <div class="result">
                        <h4>Profile Information</h4>
                        <p><strong>Username:</strong> ${data.username}</p>
                        <p><strong>Bio:</strong> ${data.bio}</p>
                        <p><strong>Created:</strong> ${data.created_at}</p>
                    </div>
                `;
            } else {
                result.innerHTML = `<div class="result error">✗ ${data.detail}</div>`;
            }
        }
    </script>
</body>
</html>
    """

🔒 Test Yourself First

Before scrolling to the answers, try identifying the vulnerabilities yourself! Each exercise in the SecEng-Exercises repository includes:

Automated test suites with 60+ test cases
Vulnerable and fixed versions for comparison
Step-by-step exploitation guides
Production-ready security patterns

⭐ Star the repo to get the full test suites and run the exploits yourself. The hands-on practice is 10x more valuable than just reading solutions.

Answer Key: Vulnerabilities & Fixes

Exercise 1: Flask Search Application

Identified Vulnerabilities:

Reflected XSS Vulnerability (Critical)
- Location: Lines where {query} is directly embedded in HTML
- Impact: Attacker can inject malicious JavaScript via the search parameter
- Example Attack: ?q=<script>alert(document.cookie)</script>
- Reference: Full Stack Python Security, Chapter 14, pp. 208-212¹
Missing Security Headers
- No X-XSS-Protection header
- No X-Content-Type-Options: nosniff header
- No Content-Security-Policy header
- Reference: Secure by Design, Chapter 9, pp. 247-249³
Missing Rate Limiting
- Attacker can overwhelm server with excessive requests
- No protection against brute-force attacks
Debug Mode Enabled in Production
- debug=True exposes sensitive stack traces and system information
- Should always be False in production environments

Fixed Code:

from flask import Flask, request
from flask_talisman import Talisman
from flask_limiter import Limiter
from flask_limiter.util import get_remote_address
import html

app = Flask(__name__)

# Configure security headers with Talisman
talisman = Talisman(
    app,
    content_security_policy={
        'default-src': ["'self'"]
    }
)

# Add rate limiting
limiter = Limiter(
    key_func=get_remote_address,
    app=app,
    default_limits=["100/hour"],
    storage_uri="memory://",
)

# Enable security headers
talisman.x_xss_protection = True
talisman.x_content_type_options = True

@app.route('/search')
@limiter.limit("30/minute")
def search():
    """Search endpoint - handles user queries"""
    query = request.args.get('q', '')

    # ✅ FIX: HTML escape user input to prevent XSS
    query_escaped = html.escape(query, quote=True)

    # Simulate database search
    results = []
    if query:
        query_lower = query.lower()
        if 'laptop' in query_lower:
            results = [
                {'name': 'Gaming Laptop Pro', 'price': 1299.99, 'stock': 12},
                {'name': 'Business Laptop Elite', 'price': 899.99, 'stock': 8},
            ]

    # Generate HTML response with escaped query
    html_content = f"""
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Search Results</title>
</head>
<body>
    <h1>Search Results for: <span>{query_escaped}</span></h1>
    <div class="results">
    """

    if results:
        for product in results:
            html_content += f"""
            <div class="product">
                <h3>{html.escape(product['name'])}</h3>
                <div class="price">${product['price']}</div>
                <div class="stock">In stock: {product['stock']} units</div>
            </div>
            """
    else:
        html_content += f"""
        <div class="no-results">
            <h2>No Results Found</h2>
            <p>Your search for '<strong>{query_escaped}</strong>' did not match any products.</p>
        </div>
        """

    html_content += """
    </div>
</body>
</html>
    """

    return html_content

if __name__ == '__main__':
    # ✅ FIX: Disable debug mode in production
    app.run(debug=False, port=5000, host='127.0.0.1')

Key Security Improvements:

Used html.escape() to sanitize all user input¹
Added Flask-Talisman for Content Security Policy
Implemented rate limiting with Flask-Limiter
Disabled debug mode
Added X-XSS-Protection and X-Content-Type-Options headers

💡 Want the complete test suite? The GitHub repository includes 60+ pytest test cases for this exercise, including edge cases like double-encoded attacks, null byte injection, and DOM clobbering attempts. Star the repo to access all test files!

Exercise 2: FastAPI Comment Board

Identified Vulnerabilities:

Stored/DOM XSS Vulnerability (Critical)
- Location: Line where div.innerHTML is set with unescaped comment data
- Impact: Malicious JavaScript stored in database executes when loaded
- Attack Flow:
  1. Attacker posts comment with XSS payload
  2. Payload stored in database (even though it's HTML-escaped on server)
  3. Server returns escaped HTML entities to client
  4. Client-side JavaScript inserts into DOM via innerHTML
  5. Browser unescapes HTML entities and executes script
- Reference: Full Stack Python Security, Chapter 14, pp. 213-220¹
SQL Injection Vulnerability (Critical)
- Location: String interpolation in SQL queries
- Despite HTML escaping, the query uses f-string formatting which is vulnerable
- Reference: API Security in Action, Chapter 2, pp. 54-55²
Missing Rate Limiting
- No protection against comment spam or DoS attacks
Inline Scripts Without CSP Nonces
- Inline <script> tags violate Content Security Policy best practices
- Should use nonces or move scripts to external files
- Reference: https://www.johal.in/fastapi-content-security-csp-policies-nonce-hashes-strict-dynamic-2026/

Fixed Code:

from fastapi import FastAPI, Request
from fastapi.responses import HTMLResponse
from pydantic import BaseModel
from slowapi import Limiter
from slowapi.util import get_remote_address
import sqlite3
import html

app = FastAPI()

# Add rate limiting
limiter = Limiter(key_func=get_remote_address)
app.state.limiter = limiter

class Comment(BaseModel):
    author: str
    content: str

@app.get("/", response_class=HTMLResponse)
@limiter.limit("10/minute")
async def home(request: Request):
    """Main page with comment form"""
    return """
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <meta http-equiv="Content-Security-Policy" content="default-src 'self'; script-src 'self' 'unsafe-inline'">
    <title>Comment Board</title>
</head>
<body>
    <h1>Community Comment Board</h1>
    <form id="commentForm">
        <input type="text" id="author" placeholder="Your name" required>
        <textarea id="content" placeholder="Your comment" required></textarea>
        <button type="submit">Post Comment</button>
    </form>
    <div id="commentsContainer"></div>

    <script>
        loadComments();

        document.getElementById('commentForm').addEventListener('submit', async (e) => {
            e.preventDefault();

            const author = document.getElementById('author').value;
            const content = document.getElementById('content').value;

            const response = await fetch('/api/comments', {
                method: 'POST',
                headers: {'Content-Type': 'application/json'},
                body: JSON.stringify({ author, content })
            });

            if (response.ok) {
                document.getElementById('author').value = '';
                document.getElementById('content').value = '';
                loadComments();
            }
        });

        // ✅ FIX: Use textContent instead of innerHTML to prevent XSS
        async function loadComments() {
            const response = await fetch('/api/comments');
            const comments = await response.json();

            const container = document.getElementById('commentsContainer');
            container.innerHTML = '';

            comments.forEach(comment => {
                const div = document.createElement('div');
                div.className = 'comment';

                // Create separate elements and use textContent
                const authorDiv = document.createElement('div');
                authorDiv.className = 'comment-author';
                authorDiv.textContent = comment.author;  // ✅ Safe: Uses textContent

                const timeDiv = document.createElement('div');
                timeDiv.className = 'comment-time';
                timeDiv.textContent = comment.timestamp;  // ✅ Safe: Uses textContent

                const contentDiv = document.createElement('div');
                contentDiv.className = 'comment-content';
                contentDiv.textContent = comment.content;  // ✅ Safe: Uses textContent

                div.appendChild(authorDiv);
                div.appendChild(timeDiv);
                div.appendChild(contentDiv);

                container.appendChild(div);
            });
        }
    </script>
</body>
</html>
    """

@app.post("/api/comments")
@limiter.limit("5/minute")
async def create_comment(request: Request, comment: Comment):
    """Create a new comment"""
    conn = sqlite3.connect('comments.db')
    cursor = conn.cursor()

    # ✅ FIX: Use parameterized queries to prevent SQL injection
    query = "INSERT INTO comments (author, content, timestamp) VALUES (?, ?, datetime('now'))"
    cursor.execute(query, (comment.author, comment.content))

    conn.commit()
    conn.close()

    return {"status": "success"}

@app.get("/api/comments")
@limiter.limit("10/minute")
async def get_comments(request: Request):
    """Get all comments"""
    conn = sqlite3.connect('comments.db')
    cursor = conn.cursor()

    # ✅ FIX: Use parameterized query
    query = "SELECT author, content, timestamp FROM comments ORDER BY id DESC"
    cursor.execute(query)

    comments = []
    for row in cursor.fetchall():
        comments.append({
            "author": row[0],
            "content": row[1],
            "timestamp": row[2]
        })

    conn.close()
    return comments

Key Security Improvements:

Critical: Changed from innerHTML to textContent for DOM manipulation¹
Used parameterized SQL queries to prevent SQL injection²
Added Content-Security-Policy header
Implemented rate limiting on all endpoints
Removed HTML escaping on server since client now uses textContent (defense in depth still valuable)

Why innerHTML is Dangerous:
Even though the server HTML-escapes the data, when the client retrieves it and uses innerHTML, the browser automatically unescapes HTML entities, converting <script> back to <script>, which then executes. Using textContent prevents this by treating everything as plain text.¹

🔥 Pro Tip: Exercise 2 in the SecEng-Exercises repo includes a bonus challenge on CSP nonces and hash-based script whitelisting—an advanced defense technique used by Google and Facebook. Star to unlock!

Exercise 3: Session Management API

Identified Vulnerabilities:

Multiple DOM-Based XSS Vulnerabilities (Critical)
- Locations: Every use of .innerHTML in JavaScript functions
- Lines with result.innerHTML = ...${data.user_id}... and similar
- Impact: API responses containing malicious data execute as JavaScript
- Reference: Full Stack Python Security, Chapter 14, pp. 220-226¹
Session ID Storage Vulnerability (High)
- Location: create_session() stores raw session ID in database
- Impact: If database is compromised, all session IDs are exposed
- Best Practice: Store only hashed session IDs²
Missing Rate Limiting (Medium)
- No protection against brute-force login attempts
- No limits on session creation
Password Storage Issue (Critical, implied)
- Password likely stored in plaintext (not shown hashing in register)
- Should use bcrypt or similar

Fixed Code:

from fastapi import FastAPI
from fastapi.responses import HTMLResponse
import sqlite3
import secrets
import hashlib

app = FastAPI()

# Add rate limiting (import statements omitted for brevity)

@app.post("/auth/register")
async def register(username: str, password: str):
    """Register a new user"""
    conn = sqlite3.connect('security_app.db')
    cursor = conn.cursor()

    # ✅ FIX: Hash password before storing
    import bcrypt
    password_hash = bcrypt.hashpw(password.encode(), bcrypt.gensalt())

    cursor.execute("INSERT INTO users (username, password_hash) VALUES (?, ?)", 
                   (username, password_hash))

    user_id = cursor.lastrowid
    conn.commit()
    conn.close()

    return {"user_id": user_id, "username": username}

@app.post("/sessions/create")
async def create_session(user_id: int, metadata: str):
    """Create a new session"""
    conn = sqlite3.connect('security_app.db')
    cursor = conn.cursor()

    session_id = secrets.token_urlsafe(32)

    # ✅ FIX: Store hashed session ID, not raw value
    session_hash = hashlib.sha256(session_id.encode()).hexdigest()

    cursor.execute(
        "INSERT INTO sessions (session_hash, user_id, metadata) VALUES (?, ?, ?)",
        (session_hash, user_id, metadata)
    )

    conn.commit()
    conn.close()

    # Return raw session ID to client (only once)
    return {"session_id": session_id}

@app.get("/", response_class=HTMLResponse)
async def home():
    """Frontend with session management"""
    return """
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <meta http-equiv="Content-Security-Policy" content="default-src 'self'; script-src 'self' 'unsafe-inline'">
    <title>Session Manager</title>
</head>
<body>
    <h1>Session Management System</h1>

    <div id="register_section">
        <h2>Register</h2>
        <input type="text" id="register_username" placeholder="Username">
        <input type="password" id="register_password" placeholder="Password">
        <button onclick="registerUser()">Register</button>
        <div id="register_result"></div>
    </div>

    <div id="login_section">
        <h2>Login</h2>
        <input type="text" id="login_username" placeholder="Username">
        <input type="password" id="login_password" placeholder="Password">
        <button onclick="loginUser()">Login</button>
        <div id="login_result"></div>
    </div>

    <div id="profile_section">
        <h2>Get Profile</h2>
        <input type="text" id="profile_user_id" placeholder="User ID">
        <button onclick="getProfile()">Get Profile</button>
        <div id="profile_result"></div>
    </div>

    <script>
        // ✅ FIX: Helper function to safely display text
        function createTextElement(tag, className, text) {
            const element = document.createElement(tag);
            element.className = className;
            element.textContent = text;  // Safe: textContent prevents XSS
            return element;
        }

        async function registerUser() {
            const username = document.getElementById('register_username').value;
            const password = document.getElementById('register_password').value;

            const response = await fetch('/auth/register', {
                method: 'POST',
                headers: {'Content-Type': 'application/json'},
                body: JSON.stringify({username, password})
            });

            const data = await response.json();
            const result = document.getElementById('register_result');
            result.innerHTML = '';  // Clear previous content

            if (response.ok) {
                const div = document.createElement('div');
                div.className = 'result';
                // ✅ FIX: Use textContent instead of innerHTML
                div.textContent = `✓ Registered! User ID: ${data.user_id}, Username: ${data.username}`;
                result.appendChild(div);
            } else {
                const div = document.createElement('div');
                div.className = 'result error';
                div.textContent = `✗ ${data.detail}`;
                result.appendChild(div);
            }
        }

        async function loginUser() {
            const username = document.getElementById('login_username').value;
            const password = document.getElementById('login_password').value;

            const response = await fetch('/auth/login', {
                method: 'POST',
                headers: {'Content-Type': 'application/json'},
                body: JSON.stringify({username, password})
            });

            const data = await response.json();
            const result = document.getElementById('login_result');
            result.innerHTML = '';

            if (response.ok) {
                const div = document.createElement('div');
                div.className = 'result';
                // ✅ FIX: Use textContent
                div.textContent = `✓ Login successful! User ID: ${data.user_id}`;
                result.appendChild(div);
            } else {
                const div = document.createElement('div');
                div.className = 'result error';
                div.textContent = `✗ ${data.message}`;
                result.appendChild(div);
            }
        }

        async function getProfile() {
            const userId = document.getElementById('profile_user_id').value;

            const response = await fetch(`/auth/profile/${userId}`);
            const data = await response.json();
            const result = document.getElementById('profile_result');
            result.innerHTML = '';

            if (response.ok) {
                // ✅ FIX: Build DOM safely without innerHTML
                const div = document.createElement('div');
                div.className = 'result';

                const h4 = document.createElement('h4');
                h4.textContent = 'Profile Information';
                div.appendChild(h4);

                const usernamePara = document.createElement('p');
                const usernameStrong = document.createElement('strong');
                usernameStrong.textContent = 'Username: ';
                usernamePara.appendChild(usernameStrong);
                usernamePara.appendChild(document.createTextNode(data.username));
                div.appendChild(usernamePara);

                const bioPara = document.createElement('p');
                const bioStrong = document.createElement('strong');
                bioStrong.textContent = 'Bio: ';
                bioPara.appendChild(bioStrong);
                bioPara.appendChild(document.createTextNode(data.bio));
                div.appendChild(bioPara);

                const createdPara = document.createElement('p');
                const createdStrong = document.createElement('strong');
                createdStrong.textContent = 'Created: ';
                createdPara.appendChild(createdStrong);
                createdPara.appendChild(document.createTextNode(data.created_at));
                div.appendChild(createdPara);

                result.appendChild(div);
            } else {
                const div = document.createElement('div');
                div.className = 'result error';
                div.textContent = `✗ ${data.detail}`;
                result.appendChild(div);
            }
        }
    </script>
</body>
</html>
    """

Key Security Improvements:

Replaced ALL innerHTML assignments with safe DOM manipulation¹
Used textContent to prevent XSS when displaying user data
Added password hashing with bcrypt
Implemented session ID hashing before database storage²
Added Content-Security-Policy header
Should add rate limiting (not shown for brevity)

Pattern to Remember:

// ❌ DANGEROUS
element.innerHTML = `<p>${untrustedData}</p>`;

// ✅ SAFE
const p = document.createElement('p');
p.textContent = untrustedData;
element.appendChild(p);

Key Takeaways

The Three Types of XSS

Reflected XSS (Exercise 1): User input immediately reflected in response
Stored XSS (Exercise 2): Malicious payload stored in database
DOM-Based XSS (Exercise 3): Vulnerability in client-side JavaScript

Universal Prevention Rules

Never trust user input - Always assume it's malicious
Escape output - Use html.escape() in Python, textContent in JavaScript
Use parameterized queries - Prevent SQL injection
Implement CSP - Content Security Policy headers
Rate limit everything - Prevent abuse and DoS
Hash sensitive data - Session IDs, passwords, tokens
Disable debug mode - Never run production with debug=True

These exercises are inspired by real vulnerabilities and industry-standard security books. I invest significant time ensuring each exercise is:

Technically accurate - Based on actual CVEs and production incidents
Properly cited - All patterns sourced from authoritative security literature
Interview-relevant - Covers topics asked in Security Engineer interviews at FAANG+ companies

All 48 weeks of exercises available in the SecEng-Exercises repository. Star it to stay updated!

References

Byrne, Dennis. Full Stack Python Security. Manning Publications, 2021. Chapter 14: "Cross-Site Scripting", pp. 208-226.

Madden, Neil. API Security in Action. Manning Publications, 2020. Chapter 2: "Secure API development", pp. 54-55.

Johnsson, Dan, et al. Secure by Design. Manning Publications, 2019. Chapter 9: "Handling output safely", pp. 247-249.

Ball, Corey. Hacking APIs. No Starch Press, 2022.

🚀 What's Next? Continue Your Security Training

This XSS challenge is just one exercise from a complete Security Engineering curriculum. The SecEng-Exercises repository includes:

Current Exercises (Updated Weekly)

✅ Week 1-5: TCP/IP fundamentals, SQL injection, Python security patterns
✅ Week 6: XSS vulnerabilities (you just completed this!)
🔄 Week 7-10: SAST/DAST tools, authentication bypasses, API security
📅 Coming soon: Cryptographic attacks, race conditions, SSRF, and more

What Makes These Exercises Different?

Battle-tested patterns - Every vulnerability is based on real CVEs and production incidents
Comprehensive testing - 60+ test cases per exercise ensure you truly understand the exploits
Career-focused - Designed to prepare you for Security Engineer interviews at top companies
Open source - Free forever, community-driven, continuously improving

📊 Exercise Difficulty Progression

Weeks 1-8: Foundation (beginner-friendly)
Weeks 9-16: Intermediate (interview-ready)
Weeks 17-24: Advanced (senior-level concepts)

Practice Challenge

Can you identify which vulnerability would cause the most damage in each scenario?

Exercise 1: Which attack vector is most dangerous—the search query or the "no results" message?
Exercise 2: Why doesn't HTML escaping on the server prevent the XSS?
Exercise 3: If you could only fix ONE vulnerability, which would have the biggest security impact?

⭐ Star the GitHub repo and share your answers in the Issues tab—I respond to every submission!

🎯 Ready to Level Up Your Security Skills?

If this exercise helped you understand XSS better, imagine mastering:

SQL injection with 15 different attack vectors
JWT authentication bypasses
Race condition exploits
Cryptographic timing attacks
API rate limit bypasses
Server-Side Request Forgery (SSRF)
XML External Entity (XXE) attacks

All available for free in the repository.

Take Action Now:

⭐ Star the SecEng-Exercises repo - Get notified of new challenges
🔀 Fork it - Customize exercises for your learning style
💬 Join discussions - Ask questions, share solutions, help others
📢 Share - Help other aspiring Security Engineers discover these resources

My Goal

I'm creating the most comprehensive, free, open-source Security Engineering curriculum available. Every week I add new exercises based on real vulnerabilities I've encountered in my career at Intel and from analyzing thousands of CVEs.

Your ⭐ star helps more developers discover these resources and motivates me to create even better content.

Remember: The TechFinance disaster started with a single XSS vulnerability. Don't let it happen to your application.

Master these patterns. Review code carefully. Always assume user input is malicious.

🎓 Continue Your Journey

This is just Week 6 of a 48-week curriculum. Star the SecEng-Exercises repository to follow the complete path from beginner to senior Security Engineer.

Next up: Week 7 covers SAST/DAST tool implementation and code review automation—exercises drop this Friday!

Built by a Security Engineer, for Security Engineers. All exercises are free and open source forever. Star the repo to support the project! ⭐

Repository: https://github.com/fosres/SecEng-Exercises

Stay secure! 🔒

DEV Community

Week 6 Quiz Audit XSS Vulnerabilities

The $5 Million XSS Disaster

🎓 Learning Security Engineering?

🎯 Part of the SecEng-Exercises Collection

📋 What You'll Learn in This Post

Your Challenge: Find the XSS Vulnerabilities

Vulnerable Code Snippet #1: Flask Search Application

Vulnerable Code Snippet #2: FastAPI Comment Board

Vulnerable Code Snippet #3: Session Management API

🔒 Test Yourself First

Answer Key: Vulnerabilities & Fixes

Exercise 1: Flask Search Application

Exercise 2: FastAPI Comment Board

Exercise 3: Session Management API

Key Takeaways

The Three Types of XSS

Universal Prevention Rules

References

🚀 What's Next? Continue Your Security Training

Current Exercises (Updated Weekly)

What Makes These Exercises Different?

📊 Exercise Difficulty Progression

Practice Challenge

🎯 Ready to Level Up Your Security Skills?

Take Action Now:

My Goal

🎓 Continue Your Journey

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