hacktricks/macos-hardening/macos-security-and-privilege-escalation/mac-os-architecture/macos-function-hooking.md

# macOS Function Hooking

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## Function Interposing

Create a **dylib** with an **`__interpose`** section (or a section flagged with **`S_INTERPOSING`**) containing tuples of **function pointers** that refer to the **original** and the **replacement** functions.

Then, **inject** the dylib with **`DYLD_INSERT_LIBRARIES`** (the interposing needs occur before the main app lodas). Obviously this restriction has the **restrictions** applied to the use of DYLD\_INSERT\_LIBRARIES.&#x20;

### Interpose printf

{% tabs %}
{% tab title="interpose.c" %}
{% code title="interpose.c" %}
```c
// gcc -dynamiclib interpose.c -o interpose.dylib
#include <stdio.h>
#include <stdarg.h>

int my_printf(const char *format, ...) {
    //va_list args;
    //va_start(args, format);
    //int ret = vprintf(format, args);
    //va_end(args);

    int ret = printf("[+] Hello from interpose\n");
    return ret;
}

__attribute__((used)) static struct { const void *replacement; const void *replacee; } _interpose_printf
__attribute__ ((section ("__DATA,__interpose"))) = { (const void *)(unsigned long)&my_printf, (const void *)(unsigned long)&printf };
```
{% endcode %}
{% endtab %}

{% tab title="hello.c" %}
```c
//gcc hello.c -o hello
#include <stdio.h>

int main() {
    printf("Hello, World!\n");
    return 0;
}
```
{% endtab %}
{% endtabs %}

```bash
DYLD_INSERT_LIBRARIES=./interpose.dylib ./hello
[+] Hello from interpose
```

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