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326 lines
15 KiB
Markdown
326 lines
15 KiB
Markdown
# macOS Universal binaries & Mach-O Format
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<details>
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<summary><strong>Learn AWS hacking from zero to hero with</strong> <a href="https://training.hacktricks.xyz/courses/arte"><strong>htARTE (HackTricks AWS Red Team Expert)</strong></a><strong>!</strong></summary>
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Other ways to support HackTricks:
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* If you want to see your **company advertised in HackTricks** or **download HackTricks in PDF** Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
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* Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com)
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* Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
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* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@carlospolopm**](https://twitter.com/hacktricks\_live)**.**
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* **Share your hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.
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</details>
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## Basic Information
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Mac OS binaries usually are compiled as **universal binaries**. A **universal binary** can **support multiple architectures in the same file**.
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These binaries follows the **Mach-O structure** which is basically compased of:
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* Header
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* Load Commands
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* Data
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.png>)
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## Fat Header
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Search for the file with: `mdfind fat.h | grep -i mach-o | grep -E "fat.h$"`
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<pre class="language-c"><code class="lang-c"><strong>#define FAT_MAGIC 0xcafebabe
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</strong><strong>#define FAT_CIGAM 0xbebafeca /* NXSwapLong(FAT_MAGIC) */
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</strong>
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struct fat_header {
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<strong> uint32_t magic; /* FAT_MAGIC or FAT_MAGIC_64 */
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</strong><strong> uint32_t nfat_arch; /* number of structs that follow */
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</strong>};
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struct fat_arch {
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cpu_type_t cputype; /* cpu specifier (int) */
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cpu_subtype_t cpusubtype; /* machine specifier (int) */
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uint32_t offset; /* file offset to this object file */
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uint32_t size; /* size of this object file */
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uint32_t align; /* alignment as a power of 2 */
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};
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</code></pre>
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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.
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Check it with:
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<pre class="language-shell-session"><code class="lang-shell-session">% file /bin/ls
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/bin/ls: Mach-O universal binary with 2 architectures: [x86_64:Mach-O 64-bit executable x86_64] [arm64e:Mach-O 64-bit executable arm64e]
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/bin/ls (for architecture x86_64): Mach-O 64-bit executable x86_64
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/bin/ls (for architecture arm64e): Mach-O 64-bit executable arm64e
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% otool -f -v /bin/ls
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Fat headers
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fat_magic FAT_MAGIC
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<strong>nfat_arch 2
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</strong><strong>architecture x86_64
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</strong> cputype CPU_TYPE_X86_64
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cpusubtype CPU_SUBTYPE_X86_64_ALL
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capabilities 0x0
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<strong> offset 16384
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</strong><strong> size 72896
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</strong> align 2^14 (16384)
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<strong>architecture arm64e
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</strong> cputype CPU_TYPE_ARM64
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cpusubtype CPU_SUBTYPE_ARM64E
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capabilities PTR_AUTH_VERSION USERSPACE 0
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<strong> offset 98304
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</strong><strong> size 88816
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</strong> align 2^14 (16384)
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</code></pre>
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or using the [Mach-O View](https://sourceforge.net/projects/machoview/) tool:
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<figure><img src="../../../.gitbook/assets/image (1091).png" alt=""><figcaption></figcaption></figure>
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As you may be thinking usually a universal binary compiled for 2 architectures **doubles the size** of one compiled for just 1 arch.
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## **Mach-O Header**
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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$"`
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```c
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#define MH_MAGIC 0xfeedface /* the mach magic number */
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#define MH_CIGAM 0xcefaedfe /* NXSwapInt(MH_MAGIC) */
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struct mach_header {
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uint32_t magic; /* mach magic number identifier */
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cpu_type_t cputype; /* cpu specifier (e.g. I386) */
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cpu_subtype_t cpusubtype; /* machine specifier */
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uint32_t filetype; /* type of file (usage and alignment for the file) */
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uint32_t ncmds; /* number of load commands */
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uint32_t sizeofcmds; /* the size of all the load commands */
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uint32_t flags; /* flags */
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};
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#define MH_MAGIC_64 0xfeedfacf /* the 64-bit mach magic number */
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#define MH_CIGAM_64 0xcffaedfe /* NXSwapInt(MH_MAGIC_64) */
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struct mach_header_64 {
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uint32_t magic; /* mach magic number identifier */
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int32_t cputype; /* cpu specifier */
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int32_t cpusubtype; /* machine specifier */
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uint32_t filetype; /* type of file */
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uint32_t ncmds; /* number of load commands */
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uint32_t sizeofcmds; /* the size of all the load commands */
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uint32_t flags; /* flags */
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uint32_t reserved; /* reserved */
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};
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```
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**Filetypes**:
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* MH\_EXECUTE (0x2): Standard Mach-O executable
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* MH\_DYLIB (0x6): A Mach-O dynamic linked library (i.e. .dylib)
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* MH\_BUNDLE (0x8): A Mach-O bundle (i.e. .bundle)
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```bash
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# Checking the mac header of a binary
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otool -arch arm64e -hv /bin/ls
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Mach header
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magic cputype cpusubtype caps filetype ncmds sizeofcmds flags
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MH_MAGIC_64 ARM64 E USR00 EXECUTE 19 1728 NOUNDEFS DYLDLINK TWOLEVEL PIE
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```
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Or using [Mach-O View](https://sourceforge.net/projects/machoview/):
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<figure><img src="../../../.gitbook/assets/image (1130).png" alt=""><figcaption></figcaption></figure>
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## **Mach-O Load commands**
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The **file's layout in memory** is specified here, detailing the **symbol table's location**, the context of the main thread at execution start, and the required **shared libraries**. Instructions are provided to the dynamic loader **(dyld)** on the binary's loading process into memory.
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The uses the **load\_command** structure, defined in the mentioned **`loader.h`**:
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```objectivec
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struct load_command {
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uint32_t cmd; /* type of load command */
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uint32_t cmdsize; /* total size of command in bytes */
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};
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```
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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`.
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### **LC\_SEGMENT/LC\_SEGMENT\_64**
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{% hint style="success" %}
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Basically, this type of Load Command define **how to load the \_\_TEXT** (executable code) **and \_\_DATA** (data for the process) **segments** according to the **offsets indicated in the Data section** when the binary is executed.
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{% endhint %}
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These commands **define segments** that are **mapped** into the **virtual memory space** of a process when it is executed.
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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.
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**Each segment** can be further **divided** into multiple **sections**. The **load command structure** contains **information** about **these sections** within the respective segment.
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In the header first you find the **segment header**:
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<pre class="language-c"><code class="lang-c">struct segment_command_64 { /* for 64-bit architectures */
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uint32_t cmd; /* LC_SEGMENT_64 */
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uint32_t cmdsize; /* includes sizeof section_64 structs */
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char segname[16]; /* segment name */
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uint64_t vmaddr; /* memory address of this segment */
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uint64_t vmsize; /* memory size of this segment */
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uint64_t fileoff; /* file offset of this segment */
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uint64_t filesize; /* amount to map from the file */
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int32_t maxprot; /* maximum VM protection */
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int32_t initprot; /* initial VM protection */
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<strong> uint32_t nsects; /* number of sections in segment */
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</strong> uint32_t flags; /* flags */
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};
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</code></pre>
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Example of segment header:
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<figure><img src="../../../.gitbook/assets/image (1123).png" alt=""><figcaption></figcaption></figure>
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This header defines the **number of sections whose headers appear after** it:
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```c
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struct section_64 { /* for 64-bit architectures */
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char sectname[16]; /* name of this section */
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char segname[16]; /* segment this section goes in */
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uint64_t addr; /* memory address of this section */
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uint64_t size; /* size in bytes of this section */
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uint32_t offset; /* file offset of this section */
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uint32_t align; /* section alignment (power of 2) */
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uint32_t reloff; /* file offset of relocation entries */
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uint32_t nreloc; /* number of relocation entries */
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uint32_t flags; /* flags (section type and attributes)*/
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uint32_t reserved1; /* reserved (for offset or index) */
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uint32_t reserved2; /* reserved (for count or sizeof) */
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uint32_t reserved3; /* reserved */
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};
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```
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Example of **section header**:
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<figure><img src="../../../.gitbook/assets/image (1105).png" alt=""><figcaption></figcaption></figure>
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If you **add** the **section offset** (0x37DC) + the **offset** where the **arch starts**, in this case `0x18000` --> `0x37DC + 0x18000 = 0x1B7DC`
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<figure><img src="../../../.gitbook/assets/image (698).png" alt=""><figcaption></figcaption></figure>
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It's also possible to get **headers information** from the **command line** with:
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```bash
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otool -lv /bin/ls
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```
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Common segments loaded by this cmd:
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* **`__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**.
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* This allocation is important to **mitigate NULL pointer dereference vulnerabilities**.
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* **`__TEXT`**: Contains **executable** **code** with **read** and **execute** permissions (no writable)**.** Common sections of this segment:
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* `__text`: Compiled binary code
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* `__const`: Constant data
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* `__cstring`: String constants
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* `__stubs` and `__stubs_helper`: Involved during the dynamic library loading process
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* **`__DATA`**: Contains data that is **readable** and **writable** (no executable)**.**
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* `__data`: Global variables (that have been initialized)
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* `__bss`: Static variables (that have not been initialized)
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* `__objc_*` (\_\_objc\_classlist, \_\_objc\_protolist, etc): Information used by the Objective-C runtime
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* **`__LINKEDIT`**: Contains information for the linker (dyld) such as, "symbol, string, and relocation table entries."
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* **`__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.
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### **`LC_MAIN`**
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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.
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### **LC\_CODE\_SIGNATURE**
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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.\
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However, you can find some information about this section in [**this blog post**](https://davedelong.com/blog/2018/01/10/reading-your-own-entitlements/) and this [**gists**](https://gist.github.com/carlospolop/ef26f8eb9fafd4bc22e69e1a32b81da4).
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### **LC\_LOAD\_DYLINKER**
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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.
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### **`LC_LOAD_DYLIB`**
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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.
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* This load command is a structure of type **`dylib_command`** (which contains a struct dylib, describing the actual dependent dynamic library):
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```objectivec
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struct dylib_command {
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uint32_t cmd; /* LC_LOAD_{,WEAK_}DYLIB */
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uint32_t cmdsize; /* includes pathname string */
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struct dylib dylib; /* the library identification */
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};
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struct dylib {
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union lc_str name; /* library's path name */
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uint32_t timestamp; /* library's build time stamp */
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uint32_t current_version; /* library's current version number */
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uint32_t compatibility_version; /* library's compatibility vers number*/
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};
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```
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.png>)
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You could also get this info from the cli with:
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```bash
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otool -L /bin/ls
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/bin/ls:
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/usr/lib/libutil.dylib (compatibility version 1.0.0, current version 1.0.0)
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/usr/lib/libncurses.5.4.dylib (compatibility version 5.4.0, current version 5.4.0)
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/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1319.0.0)
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```
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Some potential malware related libraries are:
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* **DiskArbitration**: Monitoring USB drives
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* **AVFoundation:** Capture audio and video
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* **CoreWLAN**: Wifi scans.
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{% hint style="info" %}
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A Mach-O binary can contain one or **more** **constructors**, that will be **executed** **before** the address specified in **LC\_MAIN**.\
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The offsets of any constructors are held in the **\_\_mod\_init\_func** section of the **\_\_DATA\_CONST** segment.
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{% endhint %}
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## **Mach-O Data**
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At the core of the file lies the data region, which is composed of several segments as defined in the load-commands region. **A variety of data sections can be housed within each segment**, with each section **holding code or data** specific to a type.
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{% hint style="success" %}
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The data is basically the part containing all the **information** that is loaded by the load commands **LC\_SEGMENTS\_64**
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{% endhint %}
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 (3).png>)
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This includes:
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* **Function table:** Which holds information about the program functions.
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* **Symbol table**: Which contains information about the external function used by the binary
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* It could also contain internal function, variable names as well and more.
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To check it you could use the [**Mach-O View**](https://sourceforge.net/projects/machoview/) tool:
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<figure><img src="../../../.gitbook/assets/image (1117).png" alt=""><figcaption></figcaption></figure>
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Or from the cli:
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```bash
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size -m /bin/ls
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```
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<details>
|
||
|
||
<summary><strong>Learn AWS hacking from zero to hero with</strong> <a href="https://training.hacktricks.xyz/courses/arte"><strong>htARTE (HackTricks AWS Red Team Expert)</strong></a><strong>!</strong></summary>
|
||
|
||
Other ways to support HackTricks:
|
||
|
||
* If you want to see your **company advertised in HackTricks** or **download HackTricks in PDF** Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
|
||
* Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com)
|
||
* Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
|
||
* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@carlospolopm**](https://twitter.com/hacktricks\_live)**.**
|
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* **Share your hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.
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</details>
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