Shellcode Basics

TL;DR

This guide covers what shellcode is, how it’s used, and basic ways to find and prevent it.

What is Shellcode?

Shellcode is a small piece of machine code designed to be injected into a running process. It typically performs malicious actions like spawning a shell (hence the name), creating files, or connecting to a network. It’s ‘shell’-independent – meaning it doesn’t rely on external libraries; it has to do everything itself.

How is Shellcode Used?

1. Exploiting Vulnerabilities: The most common use. Attackers find weaknesses in software (buffer overflows, format string bugs, etc.) and inject shellcode into the process’s memory to take control.
2. Payload Delivery: Shellcode is often part of a larger exploit. It’s the bit that actually *does* something after gaining access.
3. Malware: Malware authors use shellcode for various tasks, like persistence or data theft.

Finding Shellcode

Identifying shellcode isn’t always easy because it’s designed to be stealthy. Here are some basic techniques:

1. Manual Analysis (Disassembly)

If you suspect a section of memory contains shellcode, disassemble it using tools like objdump or a debugger (GDB, x64dbg). Look for:

• No Imports: Shellcode shouldn’t call any external functions.
• Raw Machine Code: It will appear as a sequence of bytes representing assembly instructions.
• Common Shellcode Patterns: Certain shellcode sequences are well-known (e.g., execve for spawning a shell on Linux).

Example using objdump:

objdump -d /path/to/suspect_file | less

2. Using YARA Rules

YARA is a pattern-matching tool used to identify malware families. You can create or use existing rules to detect shellcode based on its characteristics.

Example YARA rule (very basic):

rule detect_shellcode
{
  meta:
    description = "Detects simple shellcode"
    author = "Your Name"
  strings:
    $shellcode = { 83 ?? ?? ?? ?? } // Example: 'sub' instruction
  condition:
    $shellcode
}

Run YARA against a file or process memory:

yara /path/to/suspect_file detect_shellcode.yar

3. Memory Scanning Tools

Tools like Volatility can analyze memory dumps and identify suspicious code regions.

Preventing Shellcode Execution

1. Input Validation: Thoroughly validate all user inputs to prevent buffer overflows and other injection vulnerabilities.
2. Data Execution Prevention (DEP): Marks certain memory regions as non-executable, preventing shellcode from running. Most modern operating systems have DEP enabled by default.
3. Address Space Layout Randomization (ASLR): Randomizes the location of key memory areas, making it harder for attackers to predict where to inject shellcode.
4. Firewalls and Intrusion Detection Systems: Can detect and block malicious network activity associated with shellcode execution.
5. Regular Security Updates: Patch software vulnerabilities promptly to reduce the attack surface.

Resources

• Shell-Storm: https://shellstorm.net/ (Shellcode database)
• Metasploit Framework: A powerful penetration testing tool that includes shellcode generation and exploitation capabilities.