Auth Bypass Through JavaScript File Analysis

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Analyzing JavaScript (JS) files can often lead to discovering critical vulnerabilities, including authentication (Auth) bypass flaws. As demonstrated in a recent bug bounty submission, a quick analysis of JS files allowed for an Auth bypass that was fixed within just two hours—before the triager even examined the report.

You Should Know:

  1. How to Analyze JS Files for Auth Bypass

– Use browser DevTools (F12) to inspect network requests and JS files.
– Search for keywords like:
– token, auth, session, verify, isAdmin, role, `privilege`
– Use `grep` in Linux to scan JS files: 

grep -r "token|auth|session" /path/to/js/files/

– Automate JS analysis with tools like:
– LinkFinder (Extracts endpoints from JS files): 

python3 linkfinder.py -i https://example.com/file.js -o cli

– JS-Scan (Static analysis for secrets & endpoints): 

js-scan -u https://example.com/script.js

2. Common Auth Bypass Techniques

  • Hardcoded Tokens:
  • Search for static tokens in JS: 
    curl -s https://example.com/app.js | grep -E "token\s=\s['\"].+?['\"]"
  • Logic Flaws in Authentication Checks:
  • Look for weak client-side checks like: 
    if (user.role === "guest") { admin = false; } 
// Can be manipulated via Proxy (Burp/Chrome DevTools)
  • JWT Tampering:
  • Decode JWT tokens with: 
    echo "JWT_TOKEN" | jq -R 'split(".") | .[bash],.[bash] | @base64d'
  • Test for `none` algorithm vulnerability: 
    jwt_tool <JWT_TOKEN> -X a -n

3. Exploiting Auth Bypass in Practice

  • Step 1: Identify JS files loaded on the login page: 
    wget https://example.com/login.html --no-check-certificate -O login.html && grep -Po '(?<=src=")[^"].js' login.html
  • Step 2: Download and analyze JS files: 
    wget https://example.com/auth.js -O auth.js && cat auth.js | grep -i "validate|check|verify"
  • Step 3: Modify requests using Burp Suite/Proxy:
  • Intercept a request and remove/alter `Authorization` headers.
  • Replace `role: user` with `role: admin` in POST data.

4. Defensive Measures

  • Always validate authentication server-side.
  • Obfuscate sensitive logic in JS (but assume attackers can reverse it).
  • Use CSP headers to restrict JS loading: 
    Content-Security-Policy: script-src 'self' https://trusted.cdn.com;

What Undercode Say

Auth bypass vulnerabilities remain prevalent due to misplaced trust in client-side controls. Key takeaways:
– Linux Commands for JS Analysis: 

 Extract URLs from JS files
curl -s https://example.com/file.js | grep -Eo 'https?://[^"]+'

Monitor JS changes (for recon)
while true; do curl -s https://example.com/auth.js | md5sum; sleep 60; done

– Windows Equivalent (PowerShell): 

(Invoke-WebRequest -Uri "https://example.com/auth.js").Content | Select-String "token"

– Automated Scanning: 

nikto -h https://example.com -Cgidirs /js/

– JWT Exploitation: 

jwtforge --token <JWT> --claim role=admin --alg HS256 --key weakkey123

– Proxy Tools: 

mitmproxy -s auth_bypass_script.py

Expected Output:

A successful Auth bypass exploit granting unauthorized access (e.g., admin panel entry or data leakage).

Prediction

As web apps increasingly rely on client-side logic, JS-related Auth bypass flaws will surge—especially in SPA frameworks (React, Angular). Automated tools will evolve to detect these issues faster, but manual analysis will remain critical for advanced bypasses.

(No relevant URLs extracted from the original post.)

References:

Reported By: Muhammad Mubarak – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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