Inline Hooking

Created the Saturday 23 March 2019. Updated 2 months ago.

Inline hooking is a technique used to intercept calls to target functions. It is commonly used by antiviruses, sandboxes, and malware to perform a variety of tasks, such as checking parameters, shimming, logging, spoofing returned data, and filtering calls.

The process of inline hooking involves directly modifying the code within the target function, usually by overwriting the first few bytes with a jump instruction to redirect execution to a different function. This allows processing to be performed before or after the target function executes. Rootkits often use hooks to modify data returned from system calls to hide their presence, while security software uses them to prevent or monitor potentially malicious operations.

The hooks are placed by directly modifying code within the target function (inline modification), usually by overwriting the first few bytes with a jump; this allows execution to be redirected before the function does any processing. Most hooking engines use a 32-bit relative jump (opcode 0xE9), which takes up five bytes of space.

Here is a general outline of the process of implementing an inline hook:

Determine the address of the target function and the code that needs to be replaced.
Create a trampoline function that will be used to execute the original code from the target function after the hook has been applied.
Calculate the relative address of the trampoline function and store it in a jump instruction.
Overwrite the first few bytes of the target function with the jump instruction.
Modify the hook function to transfer control back to the trampoline function after performing the desired processing.

Technique Identifiers

U1211 F0015.002

Technique Tags

Inline hooking Intercept calls Target functions Check parameters Shim Log Spoof returned data Filter calls Rootkits

Featured Windows API's

Below, you will find a list of the most commonly used Windows API's that are currently utilized by malware authors for current evasion technique. This list is meant to provide an overview of the API's that are commonly used for this purpose. If there are any API's that you feel should be included on this list, please do not hesitate to contact us. We will be happy to update the list and provide any additional information or documentation that may be helpful.

Code Snippets

#include <stdio.h>
#include <windows.h>

// Typedef for the target function
typedef int (*target_func_t)(int);

// Trampoline function that will be used to execute the original code from the target function
int trampoline(int arg) {
  printf("Trampoline function called with argument: %d\n", arg);
  return 0;
}

// Hook function that will be used to intercept calls to the target function
int hook(int arg) {
  printf("Hook function called with argument: %d\n", arg);

  // Perform desired processing

  // Transfer control back to the trampoline function
  return trampoline(arg);
}

int main() {
  // Determine the address of the target function and the code that needs to be replaced
  target_func_t target_func = (target_func_t)GetProcAddress(GetModuleHandle(NULL), "target_func");
  unsigned char* target_code = (unsigned char*)target_func;

  // Calculate the relative address of the trampoline function and store it in a jump instruction
  unsigned char jump_instruction[] = {0xE9, 0x00, 0x00, 0x00, 0x00};
  int trampoline_offset = (int)trampoline - (int)target_func - 5;
  *(int*)(jump_instruction + 1) = trampoline_offset;

  // Overwrite the first few bytes of the target function with the jump instruction
  DWORD old_protect;
  VirtualProtect(target_code, 5, PAGE_EXECUTE_READWRITE, &old_protect);
  memcpy(target_code, jump_instruction, 5);
  VirtualProtect(target_code, 5, old_protect, &old_protect);
}

Additional Resources

External Links

The resources provided below are associated links that will give you even more detailed information and research on current evasion technique. It is important to note that, while these resources may be helpful, it is important to exercise caution when following external links. As always, be careful when clicking on links from unknown sources, as they may lead to malicious content.

http://www.binaryguard.com/bgc/malware/sandbox/2015/11/09/dissecting_inline_hooks.html