---
description: >-
  This project dives into the fascinating world of low-level programming and
  binary manipulation, challenging you to understand and modify Mach-O files
  (the executable format used by macOS).
---

# Dynamic Mach-O ARM64 Packer

{% @github-files/github-code-block url="https://github.com/jdecorte-be/42-WoodyWoodpacker" %}

## Understanding Mach-O

Welcome to the world of Mach-O files! Before we can dive into code injection or create something as cool as your own packer, we need to get our hands dirty and understand the intricate structure of these binaries. Think of this as your blueprint for understanding how macOS executables work under the hood.

By the end of this guide, you'll not only feel comfortable navigating a Mach-O file, but also confident in modifying one. Let’s jump in.

## What's a Mach-O File, Really?

Imagine a Mach-O file as a well-organized toolbox. Inside, you’ve got everything macOS needs to:

1. Load the program into memory.
2. Resolve any external dependencies (like dynamic libraries).
3. Protect the program with security features.
4. Execute the program starting at the right place.

It’s a modular format, which means every piece has a job, and the system knows exactly where to find it. This precision makes Mach-O files powerful but also a little tricky to work with—mess up one part, and the whole thing might crash. That’s why understanding its structure is so crucial.

<div data-full-width="false"><figure><img src="https://3970632862-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FQ00laJf1SeNghDje6snG%2Fuploads%2Fg0Pw2dJ6nFuQ5zUJ1958%2Fmacho_structure.webp?alt=media&#x26;token=95a801b1-4f05-4c93-a15c-161b2e614005" alt=""><figcaption></figcaption></figure></div>

### **Peeking Inside a Mach-O File**

To understand a Mach-O file, let’s break it down into its major components. Each part serves a specific purpose, like gears in a machine.

#### **1. The Header: The File's Introduction**

The header is the very first thing you’ll find in a Mach-O file. It’s like the front page of a book—it tells the system what kind of file it’s dealing with. Here are some important details stored in the header:

* **Magic Number**: Identifies the file as a Mach-O binary (e.g., `MH_MAGIC_64` for 64-bit files).
* **CPU Type and Subtype**: Specifies the architecture (like ARM64 for Apple Silicon).
* **File Type**: Indicates whether it’s an executable, library, or object file.
* **Load Command Info**: Includes how many load commands follow and their total size.

The header is small but mighty. It’s the first thing the system reads, so if it’s corrupted, the program won’t even start.

**How to Inspect It:** Want to see the header of a Mach-O file? Use `otool` like this:

<figure><img src="https://3970632862-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FQ00laJf1SeNghDje6snG%2Fuploads%2FSPmZdP7r96rA4ZAzlYLy%2FScreenshot%202024-11-20%20at%2023.47.05.png?alt=media&#x26;token=04aef845-bdb8-43a7-a9f2-e53237d8b94b" alt=""><figcaption></figcaption></figure>

#### **2. Load Commands: The Brain of the Binary**

Right after the header, you’ll find the load commands. These are the real instructions for the operating system. They tell it how to:

* Map the binary into memory.
* Handle external libraries.
* Set up the entry point for execution.

Some of the most common load commands include:

* **LC\_SEGMENT\_64**: Describes segments in the binary, such as code or data.
* **LC\_LOAD\_DYLIB**: Points to dynamic libraries the binary depends on.
* **LC\_MAIN**: Tells the system where to start execution.
* **LC\_CODE\_SIGNATURE**: Holds the code signature to verify the binary’s integrity.

**Why Load Commands Matter:** When you’re injecting code or adding functionality, these commands will likely be the first thing you modify. For instance, adding a new library involves inserting an `LC_LOAD_DYLIB` command. Want to redirect execution? You’ll tweak the `LC_MAIN` command.

**Toolbox Tip:** Run this command to list all load commands in a Mach-O file:

![](https://3970632862-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FQ00laJf1SeNghDje6snG%2Fuploads%2FdJzMzu3gbCKez6u28CUI%2FScreenshot%202024-11-20%20at%2023.48.46.png?alt=media\&token=d550167f-2b15-4cec-a7ca-2e07a3af6b2e)

#### **3. Segments and Sections: The Binary's Organization**

Segments are like the big rooms in a house, and sections are the furniture inside them. Segments divide the binary into logical regions, such as:

* **`__TEXT` Segment**: Where the program’s code lives. This segment is read-only and executable.
  * `__text Section`: The actual machine instructions.
  * `__stubs Section`: Data for dynamically linked functions.
* **`__DATA` Segment**: Holds writable data like global variables.
  * `__data Section`: Stores initialized global variables.
  * `__bss Section`: Contains uninitialized variables.
* **`__LINKEDIT` Segment**: Stores metadata for linking, like symbol tables.

{% hint style="info" %}
**Why They Matter:** If you’re injecting code, you’ll likely add it to the `__TEXT` segment or create a new segment altogether. Segments also dictate permissions (read, write, execute), so you’ll need to ensure your modifications don’t break these rules.
{% endhint %}

**4. Entry Point: Where the Magic Starts**

The entry point is where the system starts executing your program. It’s defined by the `LC_MAIN` load command, which provides the offset to the starting function. This is the heart of the binary, and modifying it is common in code injection.

Here’s what you’ll do when injecting:

1. Redirect the entry point to point to your custom loader.
2. Have your loader do its thing (decrypting, decompressing, etc.).
3. Jump back to the original entry point to resume normal execution.

### **The Mach-O Packer !**

Now that you have a solid understanding of the Mach-O file structure, let’s take it up a notch and explore advanced techniques. These are the methods that transform basic binary modifications into powerful, functional injections. By mastering these tricks, you’ll not only be able to manipulate Mach-O files but also handle the challenges that come with making those changes while keeping the binary operational.

## **Advanced Mach-O Tricks: Dynamic Injection**

In this section, we focus on the art of dynamic injection—adding a dynamic library to an existing Mach-O binary. This is the core of the **WoodyWoodpacker** project and an essential skill for modifying macOS executables. Dynamic library injection allows you to extend the functionality of a program without rewriting its original code, making it a powerful tool for customization, testing, or, in our case, creating a dynamic injection system.

Let’s break down the techniques, challenges, and best practices to seamlessly inject a library into a Mach-O binary.

### **What is Dynamic Library Injection?**

Dynamic library injection involves modifying a Mach-O binary to include a new `LC_LOAD_DYLIB` load command. This command instructs macOS to load a specified dynamic library at runtime. Once loaded, the library’s functions become accessible to the binary, allowing you to introduce new behavior or augment existing functionality.

In essence, you’re embedding a new dependency into the binary. The operating system will treat this library as if it were always part of the program.

***

### **Injecting a Dynamic Library: The Process**

Dynamic injection involves a few key steps:

1. **Extend the Load Commands**: Add a new `LC_LOAD_DYLIB` command to reference your library.
2. **Adjust the Mach-O Header**: Update the number and size of load commands in the header.
3. **Ensure Space for the Library Path**: Write the path of the library into the binary, ensuring alignment and padding.
4. **Test and Debug**: Validate that the binary successfully loads your library at runtime.

***

**Step 1: Extending the Load Commands**

The first step in dynamic injection is adding an `LC_LOAD_DYLIB` load command. This command contains the library path and metadata, such as the library's compatibility and current version.

A typical `LC_LOAD_DYLIB` command structure looks like this:

```c
struct dylib_command {
    uint32_t cmd;       // LC_LOAD_DYLIB
    uint32_t cmdsize;   // Size of this command
    struct dylib {
        uint32_t name;  // Offset to the library path
        uint32_t timestamp;
        uint32_t current_version;
        uint32_t compatibility_version;
    } dylib;
};
```

To add this command, locate the end of the existing load commands, append the `LC_LOAD_DYLIB` structure, and write the library path. Ensure the `cmdsize` includes the size of the structure and the library path, padded to alignment.

***

#### **Step 2: Adjusting the Mach-O Header**

The Mach-O header specifies the total number and size of load commands. After adding the new load command, update these fields to reflect the changes:

* Increment the `ncmds` field to include the new command.
* Add the size of the new load command to `sizeofcmds`.

Example:

```c
struct mach_header_64 header;
fread(&header, sizeof(header), 1, binary_file);
header.ncmds += 1;
header.sizeofcmds += new_command_size;
fseek(binary_file, 0, SEEK_SET);
fwrite(&header, sizeof(header), 1, binary_file);
```

***

#### **Step 3: Writing the Library Path**

The `LC_LOAD_DYLIB` command includes an offset to the library path, which must be appended to the binary. The path must be null-terminated and padded to maintain alignment.

For instance, if you’re injecting a library located at `/usr/local/lib/my_library.dylib`, ensure the path fits within the allocated space. If not, you may need to extend the binary.

Here’s how you might append a library path:

```c
char library_path[] = "/usr/local/lib/my_library.dylib";
size_t path_size = strlen(library_path) + 1;  // Include null terminator
fwrite(library_path, path_size, 1, binary_file);
```

***

#### **Step 4: Testing the Injection**

Dynamic library injection isn’t complete until you test your changes. Load the modified binary in a debugger or simply run it to ensure that:

* The library loads without errors.
* The program executes as expected.
* The new functionality introduced by the library is active.

Using `otool`, you can confirm that your library was successfully added:

```bash
otool -L modified_binary
```

You should see your injected library listed alongside the binary’s original dependencies.

***

### **Challenges and Considerations**

#### **Alignment and Padding**

Mach-O binaries are highly sensitive to alignment. Ensure that the library path and load command are properly padded to align with memory boundaries. Failure to do so can corrupt the binary.

#### **File Size Limits**

Extending the binary may require resizing its segments. If there isn’t enough space for your library path, you might need to move or realign existing sections.

#### **Code Signing**

Modifying a Mach-O binary invalidates its code signature. To bypass this, you can remove the `LC_CODE_SIGNATURE` command. However, this disables macOS security checks and should only be used in controlled environments.

***

### **Putting It All Together: A Dynamic Injection Workflow**

Here’s an example workflow for injecting a dynamic library into a Mach-O binary:

1. **Locate the Load Commands**: Use a tool like `otool` to inspect the existing load commands and determine where to append your `LC_LOAD_DYLIB` command.
2. **Append the Load Command**: Write the `LC_LOAD_DYLIB` structure to the binary, ensuring proper alignment.
3. **Add the Library Path**: Append the library path to the binary, padding it as needed.
4. **Update the Header**: Increment the `ncmds` and adjust `sizeofcmds` in the Mach-O header.
5. **Test the Modified Binary**: Run the binary and confirm that the library loads and functions as expected.
6. **Debug Any Issues**: Use `lldb` to debug runtime behavior and ensure your modifications didn’t introduce errors.

{% hint style="danger" %}
The information you gather from this blog, all the techniques, proofs-of-concept code, or whatever else you may possibly find here, are _strictly_ for educational purposes. I do not condone the usage of anything you might gather from this blog for malicious purposes. I've made this blog therein, to consolidate my learning by teaching it to the world.
{% endhint %}

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