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</details>
## Basic Information
Mac OS binaries usually are compiled as **universal binaries**. A **universal binary** can **support multiple architectures in the same file**.
These binaries follows the **Mach-O structure** which is basically compased of:
* Header
* Load Commands
* Data
.png>)
## Fat Header
Search for the file with: `mdfind fat.h | grep -i mach-o | grep -E "fat.h$"`
uint32_t offset; /* file offset to this object file */
uint32_t size; /* size of this object file */
uint32_t align; /* alignment as a power of 2 */
};
</code></pre>
The header has the **magic** bytes followed by the **number** of **archs** the file **contains** (`nfat_arch`) and each arch will have a `fat_arch` struct.
As you may be thinking usually a universal binary compiled for 2 architectures **doubles the size** of one compiled for just 1 arch.
## **Mach-O Header**
The header contains basic information about the file, such as magic bytes to identify it as a Mach-O file and information about the target architecture. You can find it in: `mdfind loader.h | grep -i mach-o | grep -E "loader.h$"`
```c
#define MH_MAGIC 0xfeedface /* the mach magic number */
This specifies the **layout of the file in memory**. It contains the **location of the symbol table**, the main thread context at the beginning of execution, and which **shared libraries** are required.\
The commands basically instruct the dynamic loader **(dyld) how to load the binary in memory.**
Load commands all begin with a **load\_command** structure, defined in the previously mentioned **`loader.h`**:
```objectivec
struct load_command {
uint32_t cmd; /* type of load command */
uint32_t cmdsize; /* total size of command in bytes */
};
```
There are about **50 different types of load commands** that the system handles differently. The most common ones are: `LC_SEGMENT_64`, `LC_LOAD_DYLINKER`, `LC_MAIN`, `LC_LOAD_DYLIB`, and `LC_CODE_SIGNATURE`.
### **LC\_SEGMENT/LC\_SEGMENT\_64**
{% hint style="success" %}
Basically, this type of Load Command define **how to load the sections** that are stored in DATA when the binary is executed.
{% endhint %}
These commands **define segments** that are **mapped** into the **virtual memory space** of a process when it is executed.
There are **different types** of segments, such as the **\_\_TEXT** segment, which holds the executable code of a program, and the **\_\_DATA** segment, which contains data used by the process. These **segments are located in the data section** of the Mach-O file.
**Each segment** can be further **divided** into multiple **sections**. The **load command structure** contains **information** about **these sections** within the respective segment.
In the header first you find the **segment header**:
<preclass="language-c"><codeclass="lang-c">struct segment_command_64 { /* for 64-bit architectures */
uint32_t cmd; /* LC_SEGMENT_64 */
uint32_t cmdsize; /* includes sizeof section_64 structs */
char segname[16]; /* segment name */
uint64_t vmaddr; /* memory address of this segment */
uint64_t vmsize; /* memory size of this segment */
uint64_t fileoff; /* file offset of this segment */
uint64_t filesize; /* amount to map from the file */
int32_t maxprot; /* maximum VM protection */
int32_t initprot; /* initial VM protection */
<strong> uint32_t nsects; /* number of sections in segment */
It's also possible to get **headers information** from the **command line** with:
```bash
otool -lv /bin/ls
```
Common segments loaded by this cmd:
* **`__PAGEZERO`:** It instructs the kernel to **map** the **address zero** so it **cannot be read from, written to, or executed**. The maxprot and minprot variables in the structure are set to zero to indicate there are **no read-write-execute rights on this page**. 
* This allocation is important to **mitigate NULL pointer dereference vulnerabilities**.
* **`__TEXT`**: Contains **executable****code** and **data** that is **read-only.** Common sections of this segment:
*`__text`: Compiled binary code
*`__const`: Constant data
*`__cstring`: String constants
*`__stubs` and `__stubs_helper`: Involved during the dynamic library loading process
* **`__DATA`**: Contains data that is **writable.**
*`__data`: Global variables (that have been initialized)
*`__bss`: Static variables (that have not been initialized)
*`__objc_*` (\_\_objc\_classlist, \_\_objc\_protolist, etc): Information used by the Objective-C runtime
* **`__LINKEDIT`**: Contains information for the linker (dyld) such as, "symbol, string, and relocation table entries."
* **`__OBJC`**: Contains information used by the Objective-C runtime. Though this information might also be found in the \_\_DATA segment, within various in \_\_objc\_\* sections.
### **`LC_MAIN`**
Contains the entrypoint in the **entryoff attribute.** At load time, **dyld** simply **adds** this value to the (in-memory) **base of the binary**, then **jumps** to this instruction to start execution of the binary’s code.
### **LC\_CODE\_SIGNATURE**
Contains information about the **code signature of the Macho-O file**. It only contains an **offset** that **points** to the **signature blob**. This is typically at the very end of the file.
### **LC\_LOAD\_DYLINKER**
Contains the **path to the dynamic linker executable** that maps shared libraries into the process address space. The **value is always set to `/usr/lib/dyld`**. It’s important to note that in macOS, dylib mapping happens in **user mode**, not in kernel mode.
### **`LC_LOAD_DYLIB`**
This load command describes a **dynamic****library** dependency which **instructs** the **loader** (dyld) to **load and link said library**. There is a LC\_LOAD\_DYLIB load command **for each library** that the Mach-O binary requires.
* This load command is a structure of type **`dylib_command`** (which contains a struct dylib, describing the actual dependent dynamic library):
```objectivec
struct dylib_command {
uint32_t cmd; /* LC_LOAD_{,WEAK_}DYLIB */
uint32_t cmdsize; /* includes pathname string */
struct dylib dylib; /* the library identification */
};
struct dylib {
union lc_str name; /* library's path name */
uint32_t timestamp; /* library's build time stamp */
uint32_t current_version; /* library's current version number */
uint32_t compatibility_version; /* library's compatibility vers number*/
};
```
.png>)
You could also get this info from the cli with:
```bash
otool -L /bin/ls
/bin/ls:
/usr/lib/libutil.dylib (compatibility version 1.0.0, current version 1.0.0)
/usr/lib/libncurses.5.4.dylib (compatibility version 5.4.0, current version 5.4.0)
/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1319.0.0)
```
Some potential malware related libraries are:
* **DiskArbitration**: Monitoring USB drives
* **AVFoundation:** Capture audio and video
* **CoreWLAN**: Wifi scans.
{% hint style="info" %}
A Mach-O binary can contain one or **more****constructors**, that will be **executed****before** the address specified in **LC\_MAIN**.\
The offsets of any constructors are held in the **\_\_mod\_init\_func** section of the **\_\_DATA\_CONST** segment.
{% endhint %}
## **Mach-O Data**
The heart of the file is the final region, the data, which consists of a number of segments as laid out in the load-commands region. **Each segment can contain a number of data sections**. Each of these sections **contains code or data** of one particular type.
{% hint style="success" %}
The data is basically the part containing all the information loaded by the load commands LC\_SEGMENTS\_64
* Do you work in a **cybersecurity company**? Do you want to see your **company advertised in HackTricks**? or do you want to have access to the **latest version of the PEASS or download HackTricks in PDF**? Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
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* **Share your hacking tricks by submitting PRs to the** [**hacktricks repo**](https://github.com/carlospolop/hacktricks) **and** [**hacktricks-cloud repo**](https://github.com/carlospolop/hacktricks-cloud).
