Function Hooking – Stealing Execution

Function hooking is a technique used in computer programming that allows a programmer to intercept and modify the behavior of a function in a third-party application or library. This is typically done by redirecting the function’s entry point to a custom function that the programmer has written, which can then perform any desired operations before or after calling the original function.

Function hooking is commonly used in a variety of contexts, including software debugging, reverse engineering, and malware analysis. In the case of debugging, hooking functions can allow a programmer to monitor the behavior of a program and identify potential problems or bugs. For example, a programmer might use function hooking to log the arguments passed to a function and the values it returns, in order to understand why a program is not behaving as expected. In reverse engineering, hooking can be used to understand the inner workings of a program and potentially modify its behavior. For example, a programmer might use function hooking to observe the internal state of a program and modify it in real-time, in order to understand how it works and potentially make changes to its behavior. And in the case of malware analysis, hooking can be used to monitor and analyze the behavior of malicious software in a controlled environment, in order to understand how it works and potentially identify ways to defend against it.

One of the key advantages of function hooking is that it allows a programmer to modify the behavior of a program without having access to its source code. This can be particularly useful when dealing with proprietary software or when a program’s source code is unavailable or difficult to work with. In many cases, the only way to understand or modify the behavior of such programs is to use function hooking to observe and potentially change their behavior at runtime. In addition, function hooking can be a powerful tool for understanding the behavior of complex systems, such as operating systems or network protocols, by providing a way to observe and modify their behavior in real-time. This can be useful for identifying bugs or security vulnerabilities, or for developing custom applications that interact with such systems in novel ways.

Virtual Function Table Hooking

Function hooking can be implemented in several ways, depending on the context and the programming language being used. In languages with strong type checking, such as C++, function hooking can be achieved using a technique called “virtual function table hooking,” which involves redirecting a function’s entry point in its virtual function table. This technique relies on the fact that in many object-oriented languages, functions are stored in a table of function pointers, called a virtual function table, that is associated with each object. By modifying the virtual function table, a programmer can redirect a function’s entry point to a custom function, which can then perform any desired operations before or after calling the original function.

Virtual Function Table Hooking (C++)
// This is the original function that we want to hook.
// It takes an integer argument and returns an integer result.
int original_function(int x) {
    // The original function simply returns the square of its argument.
    return x * x;
}

// This is the custom function that we will use to replace the original function.
// It has the same signature as the original function, so it can be called in its place.
int custom_function(int x) {
    // The custom function first logs the argument that was passed to the original function.
    std::cout << "Original function called with argument: " << x << std::endl;

    // Then it calls the original function to compute its result.
    int result = original_function(x);

    // Finally, it logs the result that was returned by the original function.
    std::cout << "Original function returned: " << result << std::endl;

    // The custom function returns the same result as the original function.
    return result;
}

int main() {
    // Here, we create an object that contains the original function.
    SomeObject obj;

    // We obtain a reference to the type information for this object.
    // This includes a pointer to its virtual function table.
    const std::type_info& type = typeid(obj);

    // We use the virtual function table pointer to obtain a pointer to the original function.
    int (SomeObject::*original_ptr) (int) = &SomeObject::original_function;

    // We create a pointer to the custom function, which will be used to replace the original function.
    int (SomeObject::*custom_ptr) (int) = &SomeObject::custom_function;

    // We obtain a pointer to the virtual function table itself, as an array of function pointers.
    void** vtable = *reinterpret_cast<void***>(&obj);

    // We iterate over the virtual function table, searching for the original function.
    for (std::size_t i = 0; i < type.GetVtableSize(); ++i) {
        // If we find the original function in the virtual function table...
        if (vtable[i] == *reinterpret_cast<void**>(&original_ptr)) {
            // ...we replace it with the custom function.
            vtable[i] = *reinterpret_cast<void**>(&custom_ptr);
            break;
        }
    }

    // Now, whenever the original function is called on the object,
    // it will be redirected to the custom function instead.
    // The custom function can then perform any desired operations
    // before or after calling the original function.
    obj.original_function(5);

    return 0;
}

Decorator Function Hooking

In languages with more flexible type systems, such as Python or JavaScript, function hooking can be implemented using higher-level abstractions, such as decorators or higher-order functions. Decorators are a language feature that allows a programmer to define a function that wraps another function, modifying its behavior in some way. By using a decorator to wrap a function, a programmer can implement function hooking in a more abstract and flexible way, without needing to manipulate the function’s entry point directly. Similarly, higher-order functions are functions that take other functions as arguments or return them as results. By using higher-order functions, a programmer can implement function hooking in a more modular and reusable way, making it easier to write and maintain complex programs that use function hooking.

Decorator Function Hooking (JavaScript)
// Define a decorator that adds a hook to the given function
function hook(func) {
  // Return a new function that wraps the original one
  return function(...args) {
    // Use the original function to get the result
    const result = func(...args);

    // Modify the result and return it
    return result + 1;
  }
}

// Define a function that we want to hook
function add(a, b) {
  return a + b;
}

// Use the decorator to add a hook to the function
add = hook(add);

// Test the hooked function
console.log(add(1, 2));  // Output: 4

Hooks can be applied to kernel module exports

Kernel Module Export Function Hook (C++)
#include <ntddk.h>

// Declare the original MmCopyMemory function.
NTSTATUS (*MmCopyMemory_Original)(
  VOID UNALIGNED *Destination,
  CONST VOID UNALIGNED *Source,
  SIZE_T Length
);

// Define the new MmCopyMemory function that will act as the hook.
NTSTATUS MmCopyMemory_Hook(
  VOID UNALIGNED *Destination,
  CONST VOID UNALIGNED *Source,
  SIZE_T Length
) {
  // Perform any actions you want the hook to do here.

  // Call the original MmCopyMemory function to do the actual memory copy.
  return MmCopyMemory_Original(Destination, Source, Length);
}

// Define the driver's entry point.
extern "C" NTSTATUS DriverEntry(PDRIVER_OBJECT DriverObject, PUNICODE_STRING RegistryPath) {
  // Hook MmCopyMemory by replacing it with our own function.
  MmCopyMemory_Original = (NTSTATUS (*)(VOID UNALIGNED *, CONST VOID UNALIGNED *, SIZE_T)) MmCopyVirtualMemory;
  MmCopyVirtualMemory = MmCopyMemory_Hook;

  // Return success.
  return STATUS_SUCCESS;
}

This function is patch-guard protected, so it will cause a BSOD at some point if tampered with in this manner.

Hooks via Inserted JMP Instructions

It is also possible to overwrite the bytes of a function with shellcode that will jmp to an arbitrary location.

Conclusion

Despite its advantages, function hooking is not without its challenges and drawbacks. For one, it can be difficult to implement, especially in languages with strong type checking or when dealing with low-level code. In many cases,

implementing function hooking requires a deep understanding of the underlying program, the programming language being used, and the hardware and operating system on which the program is running. Additionally, hooking functions can potentially interfere with the normal operation of a program, leading to instability or even crashing the program altogether. For example, if a custom function that has been hooked to a function does not properly handle the arguments or return values of the original function, it can cause the program to behave in unexpected ways or even crash. As a result, function hooking should be used with caution and only when necessary.

In conclusion, function hooking is a valuable technique that can provide a powerful way to modify and analyze the behavior of programs. While it can be difficult to implement and comes with its own set of risks, it can be a valuable tool for a wide range of applications, from software debugging to malware analysis. By providing a way to observe and potentially modify the behavior of a program at runtime, function hooking can help programmers understand and improve the behavior of complex systems, and can provide a powerful tool for creating custom applications that interact with such systems in novel ways.

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