Disassembling Executables with Capstone

Disassembling executables is a critical task in reverse engineering and cybersecurity, which can be accomplished effectively with Capstone.

It is a lightweight, multi-platform, multi-architecture disassembly framework which has gained popularity over the years due to its robustness and flexibility.

Capstone

Capstone is designed to convert binary machine code into human-readable assembly language. It supports multiple architectures, including x86, ARM, MIPS, PowerPC, and more.

It focuses on providing a high-level API that is easy to integrate into various tools and applications, making it a preferred choice for developers and security researchers.

Before disassembling an executable, you need to set up Capstone on your system.

Installing Capstone

On Linux, open a terminal and execute the following commands.

sudo apt-get update
sudo apt-get install libcapstone-dev capstone-tool

To install on Windows, just download the precompiled binaries from the Capstone website. Then extract the files and add the Capstone directory to your system’s PATH environment variable.

In Python, run the following.

pip install capstone

Usage

Capstone provides a simple and intuitive API for disassembling code, for instance the following script initializes a Capstone disassembler for the x86 architecture in 64-bit mode, disassembles the given machine code, and prints the disassembled instructions.

from capstone import *

# Define the x86 machine code to be disassembled
CODE = b"\x55\x48\x8b\x05\xb8\x13\x00\x00"

# Initialize the disassembler for x86 architecture
md = Cs(CS_ARCH_X86, CS_MODE_64)

# Disassemble the machine code
for instruction in md.disasm(CODE, 0x1000):
    print("0x%x:\t%s\t%s" % (instruction.address, instruction.mnemonic, instruction.op_str))

To disassemble an entire executable file. Just open the executable file in binary mode and read its contents.

with open("path/to/executable", "rb") as f:
    binary_code = f.read()

Use a library like lief or pefile to parse the executable and identify the code sections.

import lief

pe = lief.parse("path/to/executable")
code_section = pe.get_section(".text")
code = code_section.content
code_start = pe.optional_header.imagebase + code_section.virtual_address

Then initialize Capstone and disassemble the identified code section.

md = Cs(CS_ARCH_X86, CS_MODE_64)
for instruction in md.disasm(bytes(code), code_start):
    print("0x%x:\t%s\t%s" % (instruction.address, instruction.mnemonic, instruction.op_str))

Capstone can also provide detailed information about each instruction, including operand types, register accesses, and more.

md.detail = True
for instruction in md.disasm(CODE, 0x1000):
    print("0x%x:\t%s\t%s" % (instruction.address, instruction.mnemonic, instruction.op_str))
    for i in instruction.operands:
        if i.type == CS_OP_REG:
            print("\t\treg: %s" % instruction.reg_name(i.reg))
        elif i.type == CS_OP_IMM:
            print("\t\timm: 0x%x" % i.imm)
        elif i.type == CS_OP_MEM:
            print("\t\tmem: base=%s, index=%s, scale=%s, disp=0x%x" % (
                instruction.reg_name(i.mem.base),
                instruction.reg_name(i.mem.index),
                i.mem.scale,
                i.mem.disp))

It also supports multiple architectures, and allows you to switch between them as follows.

# ARM architecture
md_arm = Cs(CS_ARCH_ARM, CS_MODE_ARM)

Lastly, Capstone also supports different disassembly modes for each architecture, such as 16-bit, 32-bit, and 64-bit modes for x86.

md_32 = Cs(CS_ARCH_X86, CS_MODE_32)
md_64 = Cs(CS_ARCH_X86, CS_MODE_64)

Summary

Capstone is a versatile and powerful disassembly framework that provides essential tools for reverse engineering and security analysis. It offers support for multiple architectures, detailed instruction information, and easy integration with other tools. Whether you are analyzing malware, debugging software, or conducting security assessments.