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When a developer hears the word “shell,” it doesn’t automatically evoke calming oceans waves and warm, luscious sand. More often, developers hear the word shell and their minds automatically transition to shell scripting. While shell script syntax may feel clunky by today’s modern coding standards, shell enables productivity and collaboration. 

On the other hand, many attackers have at least basic coding skills, meaning that they know shell, too. With these technical skills, they continue to deploy malicious packages that enable them to deploy a reverse shell on machines. 

Developers who know the difference between reverse shell and bind shell can understand better why threat actors prefer reverse shell when deploying software supply chain attacks.  

What is a shell?

A shell is a software that interprets commands for an operating system so users can execute commands, run scripts, and automate tasks, like:

  • Controlling processes
  • Executing programs
  • Managing files

Some of the most common shells are:

  • Windows PowerShell
  • Windows Command Prompt
  • bash
  • sh
  • dash
  • Born
  • Korn

The shell and its scripting language often help streamline complex tasks like:

  • Setting up a network 
  • Managing software installation
  • Backing up files 
  • Remotely managing servers
  • Monitoring system performance
  • Updating software
  • Managing configurations

Types of Shell Attacks

Although shell has many legitimate uses, attackers often use it to take control of a victim machine. In a shell connection, the exploited system has a utility networking utility, like netcat, that reads and writes data across network connections running in listener mode, meaning that the function only responds when an event occurs. Whenever a user makes a web request or a network connection is established, the listeners provide information about it. 

Bind Shell

A bind shell only listens to a specific port, waiting for an incoming connection request. Once a user establishes a connection, the bind shell provides a shell interface, giving the user a way to remotely execute commands on the machine. System admins often use bind shells for server, networked devices, or system remote management.

With a bind shell, anyone – including attackers – can connect to the port to take control over the target machine.  For example, if attackers deliver a malicious payload to a target machine, they can launch a command shell that listens to the local port and takes control of it. 

Reverse Shell

A reverse shell establishes a connection between a remote machine and a target machine, allowing the remote machine to send a connection request to the target machine. The target machine listens for this request, then establishes the connection. System admins may use reverse shell for legitimate reasons, like remote server administration.

With a reverse shell, attackers can often bypass security controls, like firewalls. While firewalls may prevent bind shell attacks because they filter incoming traffic, they often provide fewer limits for outgoing connections, meaning they won’t capture the malicious activity. For example, attackers inserted malicious payloads into application inputs that a log4j