Introduction

This write-up details the “Cat” machine from Hack The Box, a Medium-rated Linux challenge.

Objective on Cat Machine

The goal is to complete the “Cat” machine by accomplishing the following objectives:

User Flag:

To obtain the user flag, an attacker first exploits a Stored Cross-Site Scripting (XSS) vulnerability in the user registration form, which allows stealing the administrator’s session cookie. With this stolen session, the attacker accesses the admin panel and exploits an SQL Injection flaw to extract sensitive user credentials from the database. After cracking these credentials, SSH access is gained as a regular user, enabling the retrieval of the user flag—a secret token proving user-level access.

Root Flag:

For the root flag, privilege escalation is performed by finding a vulnerable image processing script owned by the root user. The attacker crafts a malicious image payload that executes unauthorised commands with root privileges. This leads to obtaining a root shell—the highest level of system access—allowing capture of the root flag, which confirms full control over the machine.

Reconnaissance and Enumeration on Cat Machine

Establishing Connectivity

I connected to the Hack The Box environment via OpenVPN using my credentials, running all commands from a Parrot OS virtual machine. The target IP address for the Dog machine was 10.10.11.53.

Initial Scanning

To identify open ports and services, I ran an Nmap scan:

nmap -sC -sV 10.10.11.53 -oA initial

nmap -sC -sV 10.10.11.53 -oA initial

Nmap Output:

┌─[dark@parrot]─[~/Documents/htb/cat]
└──╼ $ nmap -sC -sV -oA initial -Pn 10.10.11.53
# Nmap 7.94SVN scan initiated Tue Jun 17 10:05:26 2025 as: nmap -sC -sV -oA initial -Pn 10.10.11.53
Nmap scan report for 10.10.11.53
Host is up (0.017s latency).
Not shown: 998 closed tcp ports (conn-refused)
PORT   STATE SERVICE VERSION
22/tcp open  ssh     OpenSSH 8.2p1 Ubuntu 4ubuntu0.11 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey: 
|   3072 96:2d:f5:c6:f6:9f:59:60:e5:65:85:ab:49:e4:76:14 (RSA)
|   256 9e:c4:a4:40:e9:da:cc:62:d1:d6:5a:2f:9e:7b:d4:aa (ECDSA)
|_  256 6e:22:2a:6a:6d:eb:de:19:b7:16:97:c2:7e:89:29:d5 (ED25519)
80/tcp open  http    Apache httpd 2.4.41 ((Ubuntu))
|_http-title: Did not follow redirect to http://cat.htb/
|_http-server-header: Apache/2.4.41 (Ubuntu)
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
# Nmap done at Tue Jun 17 10:05:33 2025 -- 1 IP address (1 host up) scanned in 7.38 seconds

┌─[dark@parrot]─[~/Documents/htb/cat]
└──╼ $ nmap -sC -sV -oA initial -Pn 10.10.11.53
# Nmap 7.94SVN scan initiated Tue Jun 17 10:05:26 2025 as: nmap -sC -sV -oA initial -Pn 10.10.11.53
Nmap scan report for 10.10.11.53
Host is up (0.017s latency).
Not shown: 998 closed tcp ports (conn-refused)
PORT   STATE SERVICE VERSION
22/tcp open  ssh     OpenSSH 8.2p1 Ubuntu 4ubuntu0.11 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey: 
|   3072 96:2d:f5:c6:f6:9f:59:60:e5:65:85:ab:49:e4:76:14 (RSA)
|   256 9e:c4:a4:40:e9:da:cc:62:d1:d6:5a:2f:9e:7b:d4:aa (ECDSA)
|_  256 6e:22:2a:6a:6d:eb:de:19:b7:16:97:c2:7e:89:29:d5 (ED25519)
80/tcp open  http    Apache httpd 2.4.41 ((Ubuntu))
|_http-title: Did not follow redirect to http://cat.htb/
|_http-server-header: Apache/2.4.41 (Ubuntu)
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
# Nmap done at Tue Jun 17 10:05:33 2025 -- 1 IP address (1 host up) scanned in 7.38 seconds

Analysis:

• Port 22 (SSH): OpenSSH 8.2p1 on Ubuntu 4ubuntu0.11 risks remote code execution if unpatched (e.g., CVE-2021-28041).
• Port 80 (HTTP): Apache 2.4.41, vulnerable to path traversal (CVE-2021-41773), redirects to cat.htb, hinting at virtual host misconfigurations.

Web Enumeration:

Perform directory fuzzing to uncover hidden files and directories.

gobuster dir -u http://cat.htb -w /opt/common.txt

gobuster dir -u http://cat.htb -w /opt/common.txt

Let’s perform directory enumeration with Gobuster to identify any potentially useful resources.

Gobuster Output:

Web Path Discovery (Gobuster):

• /.git Directory: Exposed Git repository risks source code leakage, revealing sensitive data like credentials or application logic.
• /admin.php, /join.php, and Other Paths: Discovered sensitive endpoints may lack authentication, enabling unauthorised access or privilege escalation.

The website features a typical interface with user registration, login, and image upload functionalities, but the presence of an exposed .git directory and accessible admin endpoints indicate significant security vulnerabilities.

Git Repository Analysis with git-dumper

Utilised the git-dumper tool to clone the exposed Git repository by executing the command git-dumper http://cat.htb/.git/ git. Subsequently, employed a Git extraction tool to retrieve critical source code files, including join.php, admin.php, and accept_cat.php, for further analysis.

Within the cloned Git repository, several PHP files were identified, meriting further examination for potential vulnerabilities or insights.

Source Code Analysis and Review on Cat Machine

Source Code Review of accept_cat.php

The accept_cat.php file is intended to let the admin user 'axel' Accept a cat by inserting its name into the accepted_cats table and deleting the corresponding entry from the cats table. The script correctly verifies the user’s session and restricts actions to POST requests, which is good practice. However, it constructs the insertion SQL query by directly embedding the $cat_name variable without any sanitisation or use of prepared statements:

$sql_insert = "INSERT INTO accepted_cats (name) VALUES ('$cat_name')";
$pdo->exec($sql_insert);

$sql_insert = "INSERT INTO accepted_cats (name) VALUES ('$cat_name')";
$pdo->exec($sql_insert);

This exposes the application to SQL injection attacks, as malicious input in catName could manipulate the query and compromise the database. On the other hand, the deletion query is properly parameterised, reducing risk. To secure the script, the insertion should also use prepared statements with bound parameters. Overall, while session checks and request validation are handled correctly, the insecure insertion query represents a critical vulnerability in accept_cat.php.

Vulnerability Review of admin.php

This admin page lets the user ‘axel’ manage cats by viewing, accepting, or rejecting them. It correctly checks if the user is logged in as ‘axel’ before allowing access and uses prepared statements to fetch cat data from the database safely. The cat details are displayed with proper escaping to prevent cross-site scripting attacks.

However, the page sends AJAX POST requests to accept_cat.php and delete_cat.php without any protection against Cross-Site Request Forgery (CSRF). This means an attacker could potentially trick the admin into performing actions without their consent. Also, based on previous code, the accept_cat.php script inserts data into the database without using prepared statements, which can lead to SQL injection vulnerabilities.

To fix these issues, CSRF tokens should be added to the AJAX requests and verified on the server side. Additionally, all database queries should use prepared statements to ensure user input is handled securely. While the page handles session checks and output escaping well, the missing CSRF protection and insecure database insertion are serious security concerns.

Security Audit of view_cat.php

The view_cat.php script restricts access to the admin user 'axel' and uses prepared statements to safely query the database, preventing SQL injection. However, it outputs dynamic data such as cat_name, photo_path, age, birthdate, weight, username, and created_at directly into the HTML without escaping. This creates a Cross-Site Scripting (XSS) vulnerability because if any of these fields contain malicious code, it will execute in the admin’s browser.

The vulnerable code includes:

Cat Details: <?php echo $cat['cat_name']; ?>
<img src="<?php echo $cat['photo_path']; ?>" alt="<?php echo $cat['cat_name']; ?>" class="cat-photo">
<strong>Name:</strong> <?php echo $cat['cat_name']; ?><br>
<strong>Age:</strong> <?php echo $cat['age']; ?><br>
</code>

Cat Details: <?php echo $cat['cat_name']; ?>
<img src="<?php echo $cat['photo_path']; ?>" alt="<?php echo $cat['cat_name']; ?>" class="cat-photo">
<strong>Name:</strong> <?php echo $cat['cat_name']; ?><br>
<strong>Age:</strong> <?php echo $cat['age']; ?><br>
</code>

To mitigate this, all output should be passed through htmlspecialchars() to encode special characters and prevent script execution. Additionally, validating the image src attribute is important to avoid loading unsafe or external resources. Without these measures, the page remains vulnerable to XSS attacks.

Input Validation Analysis of join.php

The provided PHP code is vulnerable to several security issues, primarily due to improper input handling and weak security practices. Below is an explanation of the key vulnerabilities, followed by the relevant code snippets:

1. Cross-Site Scripting (XSS): The code outputs $success_message and $error_message without sanitisation, making it susceptible to XSS attacks. User inputs (e.g., $_GET['username'], $_GET['email']) are directly echoed, allowing malicious scripts to be injected.

<?php if ($success_message != ""): ?>
   <div class="message"><?php echo $success_message; ?></div>
   <?php endif; ?>
   <?php if ($error_message != ""): ?>
   <div class="error-message"><?php echo $error_message; ?></div>
   <?php endif; ?>

<?php if ($success_message != ""): ?>
   <div class="message"><?php echo $success_message; ?></div>
   <?php endif; ?>
   <?php if ($error_message != ""): ?>
   <div class="error-message"><?php echo $error_message; ?></div>
   <?php endif; ?>

2. Insecure Password Storage: Passwords are hashed using MD5 (md5($_GET['password'])), which is cryptographically weak and easily cracked.

$password = md5($_GET['password']);

$password = md5($_GET['password']);

3. SQL Injection Risk: While prepared statements are used, the code still processes unsanitized $_GET inputs, which could lead to other injection vulnerabilities if not validated properly.
4. Insecure Data Transmission: Using $_GET for sensitive data like passwords, exposing them in URLs risks interception.

To mitigate these, use htmlspecialchars() for output, adopt secure hashing (e.g., password_hash()), validate inputs, and use $_POST for sensitive data.

Workflow Evaluation of contest.php

The PHP code for the cat contest registration page has multiple security flaws due to weak input handling and poor security practices. Below are the key vulnerabilities with relevant code snippets:

Cross-Site Scripting (XSS): The $success_message and $error_message are output without sanitization, enabling reflected XSS attacks via crafted POST inputs (e.g., cat_name=<script>alert(‘XSS’)</script>).

<?php if ($success_message): ?>
    <div class="message"><?php echo $success_message; ?></div>
<?php endif; ?>
<?php if ($error_message): ?>
    <div class="error-message"><?php echo $error_message; ?></div>
<?php endif; ?>

<?php if ($success_message): ?>
    <div class="message"><?php echo $success_message; ?></div>
<?php endif; ?>
<?php if ($error_message): ?>
    <div class="error-message"><?php echo $error_message; ?></div>
<?php endif; ?>

• Weak Input Validation: The regex (/[+*{}’,;<>()\\[\\]\\/\\:]/) in contains_forbidden_content is too permissive, allowing potential XSS or SQL injection bypasses.

$forbidden_patterns = "/[+*{}',;<>()\\[\\]\\/\\:]/";

$forbidden_patterns = "/[+*{}',;<>()\\[\\]\\/\\:]/";

• Insecure File Upload: The file upload trusts getimagesize and uses unsanitized basename($_FILES[“cat_photo”][“name”]), risking directory traversal or malicious file uploads.

$target_file = $target_dir . $imageIdentifier . basename($_FILES["cat_photo"]["name"]);

$target_file = $target_dir . $imageIdentifier . basename($_FILES["cat_photo"]["name"]);

To mitigate, sanitize outputs with htmlspecialchars(), use stricter input validation (e.g., FILTER_SANITIZE_STRING), sanitize file names, restrict upload paths, and validate file contents thoroughly.

User Registration and Login

Clicking the contest endpoint redirects to the join page, which serves as the registration page.

Let’s create a new account by completing the registration process.

The registration process was completed successfully, confirming that new user accounts can be created without errors or restrictions.

Logging in with the credentials we created was successful.

After a successful login, the contest page is displayed as shown above.

Let’s complete the form and upload a cat photo as required.

Successfully submitted the cat photo for inspection.

Exploiting XSS to Steal Admin Cookie for Cat Machine

Initialise the listener.

Injected a malicious XSS payload into the username field.

Let’s create a new account by injecting malicious XSS code into the Username field while keeping all other inputs valid.

Let’s fill out the form with normal inputs as before.

The process may take a few seconds or minutes, depending on the response time. I have attempted multiple times to ensure it works successfully.

Used Firefox Dev Tools to set the cookie and gain access to admin features

Once we obtain the token hash, we need to copy and paste it into Firefox’s inspector to proceed further.

After that, simply refresh the page, and you will notice a new “Admin” option has appeared in the menu bar.

Clicking the Admin option in the menu bar redirects us to the page shown above.

Click the accept button to approve the submitted picture.

Leveraging XSS Vulnerability to Retrieve Admin Cookie for Cat Machine

Used Burp Suite to analyze POST requests.

Use Burp Suite to examine network packets for in-depth analysis.

Test the web application to determine if it is vulnerable to SQL injection attacks.

Attempting to inject the SQL command resulted in an “access denied” error, likely due to a modified or invalid cookie.

SQL Injection and Command Execution

After reconstructing the cookie, the SQL injection appears to function as anticipated.

Successfully executed command injection.

We can use the curl command to invoke the malicious file and execute it. The fact that it’s hanging is promising, indicating potential success.

It was observed that bash.sh has been transferred to the victim’s machine.

Success! A shell was obtained as the www-data user.

Database Enumeration

It’s unusual to find cat.db while searching for the database file.

Transfer the SQL file to our local machine.

We discovered that cat.db is a SQLite 3.x database.

sqlite3 cat.db opens the cat.db file using the SQLite command-line tool, allowing you to interact with the database—run queries, view tables, and inspect its contents.

The cat.db database contains three tables: accepted_cats, cats, and users, which likely stores approved cat entries, general cat data, and user information, respectively.

Immediate cracking is possible for some obtained hashes.

The screenshot shows the hashes after I rearranged them for clarity.

Breaking Password Security: Hashcat in Action

We need to specify the hash mode, which in this case could be MD5.

We successfully cracked the hash for the user Rosa, revealing the password: soyunaprincesarosa.

Boom! We successfully gained access using Rosa’s password.

The access.log file reveals the password for Axel.

The user Axel has an active shell account.

The credentials for Axel, including the password, were verified successfully.

Access is achievable via either pwncat-cs or SSH.

Executing the appropriate command retrieves the user flag.

Escalate to Root Privileges Access on Cat Machine

Privilege Escalation

The Axel user does not have sudo privileges on the cat system.

Email Analysis

We can read the message sent from Rosa to Axel.

The emails are internal updates from Rosa about two upcoming projects. In the first message, Rosa mentions that the team is working on launching new cat-related web services, including a site focused on cat care. Rosa asks Axel to send details about his Gitea project idea to Jobert, who will evaluate whether it’s worth moving forward with. Rosa also notes that the idea should be clearly explained, as she plans to review the repository herself. In the second email, Rosa shares that they’re building an employee management system. Each department admin will have a defined role, and employees will be able to view their tasks. The system is still being developed and is hosted on their private Gitea platform. Rosa includes a link to the repository and its README file, which has more information and updates. Both emails reflect early planning stages and call for team involvement and feedback.

Checking the machine’s open ports reveals that port 3000 is accessible.

Therefore, we need to set up port forwarding for port 3000.

Gitea Exploitation on Cat Machine

The service running on port 3000 is the Gitea web interface.

Using Axel’s credentials, we successfully logged in.

Gitea service is running version 1.22.0, which may contain specific features and known vulnerabilities relevant for further evaluation.

Start the Python server to serve files or host a payload for the next phase of the assessment.

Inject the XSS payload as shown above.

The fake email is sent to the user jobert to test the functionality.

Obtained a base64-encoded cookie ready for decoding.

The decoded cookie appears to contain the username admin.

Edit the file within the Gitea application.

Obtained the token as shown above.

<?php
$valid_username = 'admin';
$valid_password = 'IKw75eR0MR7CMIxhH0';

if (!isset($_SERVER['PHP_AUTH_USER']) || !isset($_SERVER['PHP_AUTH_PW']) || 
    $_SERVER['PHP_AUTH_USER'] != $valid_username || $_SERVER['PHP_AUTH_PW'] != $valid_password) {
    
    header('WWW-Authenticate: Basic realm="Employee Management"');
    header('HTTP/1.0 401 Unauthorized');
    exit;
}

<?php
$valid_username = 'admin';
$valid_password = 'IKw75eR0MR7CMIxhH0';

if (!isset($_SERVER['PHP_AUTH_USER']) || !isset($_SERVER['PHP_AUTH_PW']) || 
    $_SERVER['PHP_AUTH_USER'] != $valid_username || $_SERVER['PHP_AUTH_PW'] != $valid_password) {
    
    header('WWW-Authenticate: Basic realm="Employee Management"');
    header('HTTP/1.0 401 Unauthorized');
    exit;
}

This PHP script enforces HTTP Basic Authentication by verifying the client’s username and password against predefined valid credentials: the username “admin” and the password “IKw75eR0MR7CMIxhH0.” Upon receiving a request, the script checks for authentication headers and validates them. If the credentials are missing or incorrect, it responds with a 401 Unauthorised status and prompts the client to authenticate within the “Employee Management” realm.

The password discovered grants root access and functions as an administrator password on Windows machines.

Executing the appropriate command retrieves the root flag.