Cyber Journal

Imagine you’re trying to understand how a complex clock works, but you can’t actually wind it up or let its gears turn. Instead, you have to examine its components, blueprints, and all the tiny inscriptions on its parts. This is very much like static analysis and reverse engineering in the world of cybersecurity and software analysis. It’s the art of examining software without ever running it. This approach is not just fascinating; it’s a critical skill, especially when dealing with potentially dangerous software like malware.

The “No Execution” Rule: Why We Analyze Binaries Statically

At its heart, static analysis means we scrutinize a program file’s disassembled code, graphical images, printable strings, and other on-disk resources. We’re primarily interested in what the malware (or any binary) looks like in its file form. This allows us to investigate the executable’s format or its overall container to identify anomalous attributes, which can be crucial for further investigation.

Why avoid execution? Running a suspicious program, especially malware, even in a contained environment, always carries a degree of risk. Static analysis provides a safer alternative, allowing analysts to gather significant information directly from a binary’s structure and contents. This method is particularly useful for new, unknown, or highly obfuscated samples, where execution might trigger anti-analysis techniques designed to hide the program’s true behavior.

Binaries are self-contained files that store both machine instructions (code) and data (like variables and constants). They are produced through compilation, where high-level source code (like C or C++) is translated into machine code that a processor can execute.

Deep Dive into Disassembly

A cornerstone of static analysis is disassembly. This process involves translating a binary’s machine code into human-readable x86 assembly language. Even though malware is often written in high-level languages like C or C++, it gets compiled into x86 binary code. Therefore, becoming proficient in reading disassembled x86 code is essential for binary analysts.

The goal of a static disassembler is to convert all code within a binary into a format that a human can read or a machine can further process. However, this is easier said than done. Perfect disassembly, especially when facing deliberate obfuscation, remains an unsolved problem in computer science. Malware authors employ various tricks like self-modifying code, packing, resource obfuscation, and anti-disassembly techniques to thwart reverse engineers. Additionally, stripped binaries, which lack crucial symbolic information like function names, present a much greater challenge for analysts.

There are two primary approaches to static disassembly:

1. Linear Disassembly: This conceptually simpler approach iterates through all code segments in a binary, decoding all bytes consecutively and parsing them into a list of instructions. Tools like objdump often use this method. While it can provide complete disassembly for benign ELF binaries without inline data, it’s prone to errors if it misinterprets data as code, leading to invalid or bogus instructions.

2. Recursive Disassembly: This method is sensitive to the program’s control flow. It starts from known entry points (like the main function or exported symbols) and then follows control flow instructions (jumps, calls). This makes it less susceptible to errors caused by data mixed with code. However, it might miss instructions reachable only via indirect control flows that cannot be statically resolved. IDA Pro is a widely recognized industry-standard recursive disassembler, known for its extensive graphical interface, but it comes with a cost.

Several tools assist in disassembly:

• objdump: A simple, user-friendly disassembler found on most Linux systems, great for a quick overview of a binary’s code and data.

• Capstone: A powerful, free, and open-source disassembly framework providing a simple, lightweight API for building custom disassemblers across multiple architectures (x86/x86-64, ARM, MIPS). It supports a “detailed disassembly mode” that provides crucial control flow information, essential for recursive disassembly.

• pefile and capstone (Python libraries): These open-source Python libraries can be used together to dissect PE files and disassemble their x86 binary code, which is particularly useful in malware analysis.

Understanding Fundamental Concepts for Static Code Analysis

Knowing the tools is just the beginning; a solid grasp of underlying concepts is crucial for effective static analysis:

• Binary Anatomy & Formats: You need to understand how binaries are structured, including formats like ELF (Executable and Linkable Format, common on Linux) and PE (Portable Executable, used on Windows). Binaries are typically divided into sections, each serving a different purpose. For example, the .text section usually contains executable code, .rodata stores read-only data, and the .idata (or imports) section lists dynamically linked libraries and their functions. Inspecting the Import Address Table (IAT) in the .idata section can reveal a program’s high-level purpose by showing the library calls it makes.

• x86 Assembly Language: A foundational understanding of x86/x64 assembly is vital. This includes familiarity with CPU registers, arithmetic and data movement instructions, and the overall structure of assembly programs, which comprise instructions, directives, labels, and comments. Understanding number systems, especially hexadecimal, and data size representations is also a prerequisite.

• Control Flow: Disassemblers help structure code into functions and basic blocks. A basic block is a contiguous sequence of instructions where the first instruction is the sole entry point, and the last is the sole exit point. These basic blocks are connected by branch edges to form a Control Flow Graph (CFG), which visually represents how control flows through the code, making it easier to understand its behavior.

• Symbolic Information: This includes function and variable names. While often stripped from production binaries, symbolic information is incredibly useful as it makes disassembly easier by providing clear starting points for functions and helps human reverse engineers compartmentalize and understand the code. Tools like readelf can parse these symbols.

• Other Static Analysis Techniques:

◦ Strings Analysis: Extracting printable strings from a binary can reveal hints about its purpose, the compiler used, or even embedded scripts.

◦ Hex Editing: For small, precise modifications, directly editing the binary’s hexadecimal bytes using a hex editor is possible.

◦ Slicing: A data-flow analysis technique that identifies all instructions contributing to the value of a specific variable at a particular point in the program. It’s useful for debugging and reverse engineering.

◦ Symbolic Execution: An advanced technique that tracks metadata about the program state, allowing analysts to reason about how a program’s state came to be and to find inputs that lead to different program paths. Tools like Triton facilitate this.

◦ Taint Analysis (DTA): This technique tracks the influence of specific input data (taint sources) on other parts of the program (taint sinks). It’s valuable for vulnerability detection by showing how tainted data flows through a program. You can define taint sources (program locations where data is tracked), taint sinks (where tainted data should not go), and track taint propagation (how taint spreads). Libdft is an open-source library for dynamic taint analysis.

Resources to Start Your Journey

If you’re eager to dive into static analysis and reverse engineering, here are some excellent starting points from the provided materials:

• Set up your Lab: The VirtualBox Ubuntu instance provided by malwaredatascience.com comes preloaded with data, code, and necessary open-source libraries, making it an ideal isolated environment for experimentation. You can also set up a Kali Linux VM.

• Learn x86 Assembly: A fundamental understanding of x86/x64 assembly is crucial. The Intel programmer’s manual is a recommended resource.

• Explore Binary Formats: Read Chapters 2 and 3 of “Practical Binary Analysis” to understand ELF and PE file formats. You can use pefile (Python library) to dissect PE files.

• Start with Simple Disassembly: Use objdump for quick overviews. Then move to Capstone for custom disassemblers; Chapter 8 of “Practical Binary Analysis” is entirely devoted to this. You can install pefile and capstone using pip.

• Understand Symbolic Information: Use readelf –syms to view symbols in ELF binaries. Chapter 5 of “Practical Binary Analysis” provides more details on readelf.

• Dive into Taint Analysis: Chapters 10 and 11 of “Practical Binary Analysis” introduce the principles of Dynamic Taint Analysis (DTA) and guide you on building tools with libdft.

• Explore Symbolic Execution: Chapters 12 and 13 of “Practical Binary Analysis” cover the principles of symbolic execution and demonstrate building tools with Triton.

• Practice with Hands-on Examples: Many chapters provide code samples and exercises in accompanying directories (e.g., /ch1 in Malware Data Science, code in “The Art of Mac Malware, Volume 2” repository, code in “Practical Binary Analysis” VM).

• Utilize Linux Command-Line Tools: Tools like file and xxd (for hex dumps) are essential for initial binary inspection.

Static analysis is an indispensable skill for anyone in cybersecurity. It empowers you to understand the inner workings of programs, detect malicious intent, and analyze vulnerabilities without the immediate risks of execution.

Would you like me to elaborate on any specific tool or concept, perhaps by walking through a detailed example of how a tool like capstone is used, or by discussing the anatomy of a PE header in more detail? I can also quiz you on this material if you’d like to test your understanding of these fundamental concepts.