Mach-O file format for red team professionals - Part 1

A bird's eye view of the Mach-O file format used in operating systems developed by Apple (macOS, iOS, ipadOS etc.)

During a red team engagement, a lot of tradecraft is dependent on crafting or altering operating system executable files in a manner that allows execution of the malicious payload while avoiding detection. Therefore, it is important to understand the structure of these executable files and how they are laid out in memory.

Most operating systems support some or the other kind of executable file. Windows has Portable Executable (PE) files, Linux based systems have Executable and Linkable Format (ELF) files and macOS has Mach Object (Mach-O) files.

In this series of posts, I discuss the Mach-O file format, starting from a high level view and gradually diving deep.

The Mach-O file format was developed by Apple to store executable code, libraries, and object files. It replaced the older a.out format and is designed for efficiency and flexibility in modern macOS systems. A Mach-O file is made of following three parts:

• Header - The header contains basic information about the file (magic number, supported architectures, number of load commands, size of load commands etc.)

• Load Commands - Load commands contain information that is required to link and map the file into memory. They are placed immediately after the header. There are multiple types of load commands such as:

  • LC_SEGMENT - contains information about various segments and sections within the file.

  • LC_MAIN - contains information about the entry point of the file.

  • LC_LOAD_DYLIB - contains information about dynamic libraries that are required by the file and need to mapped at runtime.

• Data - Data portion contains the largest chunk of the executable file. The data in this section is organized into segments as described the the LC_SEGMENT load command. Each segment is usually divided into one or more sections which contain data or code of the same type.

Below is a visual representation of the high-level structure of a Mach-O file.

Red Team Notes
- Mach-O file format was developed by Apple to store executable code, libraries, and object files. It is made up of three parts, header, load commands and Data.

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In subsequent posts, I will be using otool (available in macOS) and MachOView to analyze the Mach-O file structure.