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Introduction

In today’s businesses, web applications are vital in communication and transaction between companies and individuals. However, with the emergence of cyberattacks, web applications must deal with constant threats from cybercriminals seeking to exploit system weaknesses such as unauthorized access, data breaches, or other unsafe activities. Cybercriminals use some popular ways to attack an organization’s system. This article will explore web shells widely used by cyber attackers. Let’s find out what they are, how they work, and best practices to prevent this dangerous threat.

What is Web Shell?

A web shell refers to harmful code or script attackers sneaking into an exposed web server or applications. Then the injected code will work as a remote tool, permitting attackers to access and grant the administration level to the compromised system. When criminals install the web shells, they can execute arbitrary commands, navigate file systems, modify or exfiltrate data, and gain continued access for upcoming attacks.

Web Shells Lifecycle

The lifecycle of web shells has several stages, including:

Injection

Attackers find and utilize vulnerabilities, such as software bugs or weak password setup, to gain unauthorized access to the web server. They will put the web shell code into the target application or server when inside the system.

Command and Control (C2)

After successfully injecting the code, the web shell becomes a communication bridge between criminals and the web server. Attackers will have complete control to issue commands remotely to the hacked system.

Execution

By using the interface of web shells, attackers carry out arbitrary commands by performing actions like file modification, additional malware upload, DDos attacks launching, or sensitive data extraction.

Web Shells Variants

To prevent and have a solution for web shells, you should be aware of different versions of it. They come in various forms and programming languages, modified explicitly to attackers’ objectives and skills. Some common variants include:

PHP Shells

PHP-based web shells are prevalent because many companies use this language as a server-side scripting tool. Attackers can inject malicious PHP code into the system vulnerabilities in the applications.

ASP/ASP.NET Shells

These web shells are specific to Microsoft’s Active Server Pages (ASP) and ASP.NET frameworks. Attackers exploit vulnerabilities in ASP/ASP.NET applications to gain control over the server.

JSP Shells

Java Server Pages (JSP) shells are designed to target Java-based web applications. Attackers leverage vulnerabilities in JSP applications to upload malicious JSP code.

Best Security Practices against Web Shells

To protect your web applications from web shell attacks, here are some best practices includes:

Data & File Validation

Installing robust input validation filtering, such as File Integrity Monitoring (FIM), will help businesses prevent unusual input attempts from reaching the application. This technique ensures the system only proceeds valid and expected data and instantly blocks harmful patterns or potential malicious threats.

In addition, businesses should be careful when uploading files to the web server by setting up a strict file validation rule to ensure the system only accept safe files. It is necessary to use both server-side and client-side validation to prevent bypassing security measures.

Access Control

Many cyberattacks happen because the organization does not strictly control the access and permission to web servers. Therefore, criminals can utilize Web Shells to grant unauthorized access. To avoid this incident, here are some suggestions:

Find solid principle to restrict the permissions and licenses granted to web application components, processes, and users.

Only provide necessary permission to the servers, avoiding the potential threats of a compromised member.

Establish strong password practices such as complex passwords, multi-factor authentication, or face ID. Also regularly review and revoke long-term inactive users’ accounts.

Restrict access to the highest confidential directories, folders, and files

Security Hardening

Here are some suggestions that can strengthen your web application system:

Remove unnecessary services and ports.

Regularly update the latest patch of the operating system and server.

Conduct regular security audits and penetration testing by security professionals.

Implement Web Application Firewalls (WAF) as a protection layer to filter and eliminate suspicious traffic patterns.

Create strong coding practices using parameterized queries to prevent SQL attacks.

Constantly monitor security and perform log analysis to identify unusual activities such as file uploads, file access, or unauthorized connections to the server.

Conclusion

Web Shell is such a powerful tool for criminals to carry out cyberattacks. They can easily inject malicious code and take control of your web server via this tool. By having profound knowledge about web shell and its best practices for prevention, you can set up a protective layer to harden your security system. At CyStack, we are one of the best security companies in Vietnam and have experience with every attack, including Web Shell.