Cross-Site Scripting (XSS) is one of the most common and impactful vulnerabilities in modern web applications. It allows attackers to inject malicious scripts into web pages viewed by other users, often leading to stolen cookies, session hijacking, or defacement.
At Parrot CTFs, XSS challenges are a staple in our CTF event hosting packages. By solving them in a competitive, safe environment, participants learn how XSS works in practice — and how to defend against it.
What is XSS?
XSS occurs when a web application includes untrusted input in its output without proper validation or sanitization. Attackers use this flaw to execute scripts in a victim’s browser.
Example:
A web page displays comments from users:
<p>Comment: <?php echo $_GET['comment']; ?></p>
If input isn’t sanitized, an attacker could submit:
<script>alert('XSS');</script>
When another user visits the page, the script runs in their browser.
Types of XSS
| XSS Type | Description |
|---|---|
| Stored XSS | Malicious input is permanently stored (e.g., in a database) and served to all users. |
| Reflected XSS | Payload is reflected immediately in the response, often via a URL parameter. |
| DOM-Based XSS | Exploitation occurs entirely in the client-side JavaScript/DOM without server involvement. |
Why XSS Matters
- Account Hijacking: Steal session cookies and impersonate users.
- Credential Theft: Capture login details via fake forms.
- Malware Delivery: Inject malicious scripts that deliver payloads.
- Defacement: Alter content to damage reputation.
Despite being well-known, XSS consistently appears in the OWASP Top 10, proving how often it still occurs.
How CTF Events Teach XSS
At Parrot CTFs-hosted events, players encounter XSS challenges ranging from beginner-friendly to advanced.
- Basic Reflected XSS: Insert a
<script>payload into a vulnerable query parameter. - Stored XSS: Exploit a blog comment form to persistently execute malicious code.
- DOM-Based XSS: Manipulate client-side JavaScript variables for injection.
- Bypass Challenges: Use encoding, filters, and event handlers (
onerror=,onclick=, etc.) to evade protections.
Example XSS CTF Challenge
- Scenario: A vulnerable “feedback” form doesn’t sanitize inputs.
- Goal: Inject a script that triggers an alert box containing the challenge flag.
- Steps:
- Identify where user input is reflected on the page.
- Test simple payloads like
<script>alert(1)</script>. - Escalate to filter bypasses if protections exist.
- Submit the captured flag to the scoreboard.
This hands-on format teaches participants not only how to exploit XSS but also why validation and sanitization matter.
Hosting XSS Challenges with Parrot CTFs
Whether you’re running a university cyber class, a corporate training workshop, or a conference CTF, XSS scenarios are an essential part of the event lineup.
| Event Type | XSS Example Challenge |
|---|---|
| University Event | Students learn Reflected XSS by manipulating query parameters. |
| Corporate Training | Developers practice fixing Stored XSS in a web app form. |
| Conference Event | Attendees attempt advanced DOM-based XSS filter bypasses. |
By running these challenges in a controlled environment, participants learn both attack techniques and defensive strategies.
Leave a Reply