Shellcode

Shellcode Definition
Shellcode is a small, self-contained piece of code used to exploit vulnerabilities in a program or on the whole system and force it to carry out unauthorized commands. It’s called a “shell” code because early versions often controlled the target device by opening a command shell. A command shell is the program that runs a Command-Line Interface or CLI, like Windows Command Prompt or a Linux terminal.
Shellcode can be used both locally and remotely. Local shellcode is run directly on the target device. Remote shellcode targets a different device on the same network, which can be a local area network, an intranet, or the internet. As well as malicious attacks, shellcode can be used for cybersecurity research or testing.
How Shellcodes Work
Shellcodes run only when an attacker can make a bugged or vulnerable program execute their code. This requires a software bug that lets the attacker control what the program does, for example, a flaw that allows injected data to overwrite memory. The attacker designs shellcode specifically for the target device environment, like its operating system or processor architecture, then delivers it as part of the exploit payload.
Shellcode is typically written in assembly language and then translated into machine code (instructions that a device’s CPU follows directly). This allows it to sneak in and get executed inside an already-running program. What the shellcode does when executed depends on its intended purpose. For example, it might add a new user account for the attacker or open a remote command shell. This gives the attacker a degree of control over the target device.
Attackers rely on several methods to get shellcode into vulnerable devices. A common example is a buffer overflow attack, which exploits errors that can happen when a program’s memory is overfilled. Shellcode can also be hidden in files, network data packets, or plugins.
Shellcode Types and Examples
- Privilege escalation shellcode: Helps raise the attacker’s access privileges on a compromised system.
- Connect-back shellcode: Forces the target device to open an outgoing connection to the attacker’s device. Also called a reverse shell.
- Bindshell: Allows the attacker to establish an incoming connection to a port on the target device.
- Download-and-execute shellcode: Makes the device download the attacker’s malicious executable file from the internet and immediately run it.
Hackers also have different techniques for implementing shellcode in very limited memory space:
- Staged shellcode: Separates the shellcode into multiple steps. The first step is smaller than the rest. It downloads and runs larger payloads until the device executes the whole shellcode.
- Egg-hunt shellcode: Injects shellcode into a process, but doesn’t immediately run it. A second injected shellcode searches (“hunts”) for the first one (the “egg”) and executes it.
- Omelet shellcode: Uses very limited buffer space to inject smaller parts of shellcode (“eggs”) that do nothing on their own. Then, it runs a final shellcode that assembles the parts into an executable shellcode (the “omelet”).
Read More
FAQ
Shellcodes are typically written in machine code. That’s the raw, hexadecimal bytes a CPU executes directly. Cyberattackers might write shellcode in assembly language first, as it’s a human-readable representation of machine code. They could also initially write it in a more complex language, like C, before translating the code into assembly and then machine code.
Not necessarily. They can be used in cybersecurity to check if a system has an exploitable vulnerability. This involves making sure inputs to a program can’t cause buffer overflow, as well as testing against different types of shellcodes.
They usually are. Different processors may operate on a different set of machine code instructions. Similarly, operating systems have unique methods of performing tasks. That’s why one important step of using the shellcode is figuring out the target device’s specifications.