Hook Injection

Created the Monday 18 March 2019. Updated 2 months ago.

Hook injection is a technique used by malware to alter the behavior of internal functions in an operating system or application. This is typically achieved by inserting malicious code into existing function calls, allowing the malware to intercept and manipulate the normal flow of execution.

In the case of Windows, the SetWindowsHookEx function can be used by programs to install hooks that monitor events such as mouse clicks and keyboard key presses. This can enable malware to monitor user actions and steal sensitive information or perform other malicious actions. By using hook injection, malware can evade detection and continue to operate stealthily within a system.

Technique Identifiers

U1227 E1055.m01

Technique Tags

Hook injection malware internal functions operating system application malicious code existing function calls intercept manipulate normal flow of execution SetWindowsHookEx function

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

Description

In this code, the HookFunction is injected as a hook using the SetWindowsHookEx function. This function will be called whenever a low-level mouse event occurs, allowing the malware to monitor and manipulate user actions. The CallNextHookEx function is used to call the next hook in the chain, allowing the normal flow of execution to continue.

#include <windows.h>

// The function to be injected as a hook
LRESULT CALLBACK HookFunction(int code, WPARAM wParam, LPARAM lParam) {
  // Perform malicious actions here
  
  // Return the result of the next hook in the chain
  return CallNextHookEx(NULL, code, wParam, lParam);
}

int main() {
  // Install the hook
  HHOOK hHook = SetWindowsHookEx(WH_MOUSE_LL, HookFunction, NULL, 0);
  
  // Run the message loop to receive events
  MSG msg;
  while (GetMessage(&msg, NULL, 0, 0)) {
    TranslateMessage(&msg);
    DispatchMessage(&msg);
  }
  
  // Uninstall the hook
  UnhookWindowsHookEx(hHook);
  
  return 0;
}

Description

The HookFunction is injected as a hook using the SetWindowsHookEx function, and it still calls the CallNextHookEx function to continue the normal flow of execution. The main function installs the hook, runs the message loop to receive events, and then uninstalls the hook when finished.

import ctypes
from ctypes.wintypes import HHOOK, LPARAM, LRESULT, WPARAM


# The function to be injected as a hook
def HookFunction(code: int, wParam: WPARAM, lParam: LPARAM) -> LRESULT:
    # Perform malicious actions here

    # Return the result of the next hook in the chain
    return ctypes.windll.user32.CallNextHookEx(None, code, wParam, lParam)


def main():
    # Install the hook
    hHook = ctypes.windll.user32.SetWindowsHookEx(
        ctypes.c_int(14), HookFunction, None, 0
    )

    # Run the message loop to receive events
    msg = ctypes.wintypes.MSG()
    while ctypes.windll.user32.GetMessageW(ctypes.byref(msg), None, 0, 0):
        ctypes.windll.user32.TranslateMessageW(msg)
        ctypes.windll.user32.DispatchMessageW(msg)

    # Uninstall the hook
    ctypes.windll.user32.UnhookWindowsHookEx(hHook)


if __name__ == "__main__":
    main()

Detection Rules

                                    rule HookInjection {
  condition:
    (
      // SetWindowsHookEx is often used to install hooks
      (uint32(0) == 0x00EC8B55 and (pe.exports("SetWindowsHookExA") or pe.exports("SetWindowsHookExW")))
      
      // UnhookWindowsHookEx is often used to remove hooks
      or (uint32(0) == 0x00EC8B55 and (pe.exports("UnhookWindowsHookEx")))
      
      // A hook function often calls CallNextHookEx
      or (uint32(0) == 0x00EC8B55 and (pe.exports("CallNextHookEx")))
    )
}

                                    title: Hook Injection Detection
description: Detects instances of hook injection in Windows
author: Unprotect
references:
- https://en.wikipedia.org/wiki/Hooking
- https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-setwindowhookexe
- https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-unhookwindowshookex
- https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-callnexthookex
tags:
- attack.persistence
- attack.t1179
- malware.generic

# Check for the presence of the SetWindowsHookEx function, which is often used to install hooks
- 'SetWindowsHookExA'
- 'SetWindowsHookExW'

# Check for the presence of the UnhookWindowsHookEx function, which is often used to remove hooks
- 'UnhookWindowsHookEx'

# Check for the presence of the CallNextHookEx function, which is often used in hook functions
- 'CallNextHookEx'
This rule uses string matching to look for the presence of specific functions that are commonly used in hook injection. If any of these functions are found in a scanned file, the rule will match and the code will be detected as potentially using hook injection. As with the YARA rule, this is just an example and more advanced rules may be needed for more robust detection.

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.