Delving Inside Programs: A Guide to Executable Analysis
Executable files, like those with the .exe extension on Windows, are the packaged instructions that tell your computer what to do. But what if you want to understand how a program works under the hood? Or examine potentially malicious code without risking your system? This tutorial will guide you through the core techniques of executable analysis: disassembly, debugging, and decompilation.
From Source Code to Execution
Before diving into analysis, it’s important to understand how programs are created. The process starts with human-readable source code (like C, C++, or Java). This code is then compiled or interpreted into machine code – a series of numerical instructions directly understood by the processor. The machine code is what’s ultimately stored in the executable file.
Each instruction in machine code corresponds to a specific operation the processor can perform (e.g., adding two numbers, reading data from memory). These instructions, when combined, make up the logic of the program.
Disassembly: Reconstructing the Assembly Language
Disassembly is the process of converting machine code back into a more human-readable form called assembly language. Assembly language is a low-level representation of machine code, where each instruction corresponds to a single operation. While still complex, it’s far easier to understand than raw machine code.
A disassembler is a tool that performs this conversion. It scans the executable file and translates the machine code into corresponding assembly instructions. Keep in mind that disassembly loses information present in the original source code, such as variable names and comments.
Tools for Disassembly:
- IDA Pro: A powerful (though commercial) disassembler, widely used by professionals for reverse engineering and malware analysis. Older versions are now available for free.
- W32Dasm: A free, though somewhat dated, disassembler for 32-bit Windows executables.
- Ghidra: A free and open-source reverse engineering suite developed by the NSA, offering powerful disassembly and decompilation capabilities.
Debugging: Stepping Through Execution
While disassembly provides a static view of the code, debugging allows you to observe the program as it runs. A debugger lets you:
- Set breakpoints: Pause execution at specific points in the code.
- Step through instructions: Execute the program one instruction at a time.
- Inspect variables: Examine the values of variables during execution.
- Trace execution flow: Understand the order in which instructions are executed.
Tools for Debugging:
- OllyDbg: A popular, free debugger for 32-bit Windows programs.
- WinDbg: A powerful debugger from Microsoft, especially useful for understanding Windows internals.
- x64dbg: An open-source, actively maintained debugger for both 32-bit and 64-bit Windows programs.
Decompilation: Reconstructing Higher-Level Code
Decompilation goes a step further than disassembly. It attempts to reconstruct the original source code from the executable. This is not always perfect – decompilers may not be able to fully recover all of the original logic or variable names – but it can provide a much more understandable representation of the program’s functionality.
Tools for Decompilation:
- Hex-Rays: A commercial decompiler that operates as a plugin for IDA Pro. It produces high-quality decompiled code but is expensive.
- Ghidra: Offers excellent decompilation capabilities as part of its reverse engineering suite.
- .NET Reflector/dotPeek: For .NET applications, these tools can decompile C# and other .NET languages.
Practical Considerations and Tools
- Virtual Machines: When analyzing potentially malicious code, always run it inside a virtual machine (like VMware or VirtualBox). This isolates the malware from your main system.
- Resource Editors: Tools like Resource Hacker allow you to view and modify resources embedded within the executable (icons, images, strings).
- Hex Editors: Tools like Hex Workshop allow you to view and edit the raw bytes of the executable file.
A Word of Caution
Reverse engineering and disassembling software may violate the terms of the End User License Agreement (EULA) for that software. Always respect intellectual property rights and use these techniques responsibly.