CVE-2021-1732 is a 0-Day vulnerability exploited by the BITTER APT organization in one operation which was disclosed in February this year[1][2][3]. This vulnerability exploits a user mode callback opportunity in win32kfull module to break the normal execution flow and set the error flag of window object (tagWND) extra data, which results in kernel-space out-of-bounds memory access violation.

Root cause analysis

The root cause of CVE-2021-1732 is:
In the process of creating window (CreateWindowEx), when the window object tagWND has extra data (tagWND.cbwndExtra != 0), the function pointer of user32!_xxxClientAllocWindowClassExtraBytes saved in ntdll!_PEB.kernelCallbackTable (offset+0x58) in user mode will be called via the nt!KeUserModeCallback callback mechanism, and the system heap allocator (ntdll!RtlAllocateHeap) is used to allocate the extra data memory in user-space.
By hooking user32!_xxxClientAllocWindowClassExtraBytes function in user mode, and modifying the properties of the window object extra data in the hook function manually, the kernel mode atomic operation of allocating memory for extra data can be broken, then the out-of-bounds read/write ability based on the extra data memory is achieved finally.

The normal flow of the window object creation (CreateWindowEx) process is shown as follows (partial):

From the above figure, we can see that: when the window extra data size (tagWND.cbWndExtra) is not 0, win32kfull!xxxCreateWindowEx calls the user mode function user32!_xxxClientAllocWindowClassExtraBytes via the kernel callback mechanism, requests for the memory of the window extra data in user-space. After allocation, the pointer of allocated memory in user-space will be returned to the tagWND.pExtraBytes property:

Here are two modes of saving tagWND extra data address (tagWND.pExtraBytes):
[Mode 1] In user-space system heap
As the normal process shown in the figure above, the pointer of extra data memory allocated in user-space system heap is saved in tagWND.pExtraBytes directly.
One tagWND memory layout of Mode 1 is shown in the following figure:

[Mode 2] In kernel-space desktop heap
The function ntdll!NtUserConsoleControl allocates extra data memory in kernel-space desktop heap by function DesktopAlloc, calculates the offset of allocated extra data memory address to the kernel desktop heap base address, saves the offset to tagWND.pExtraBytes, and modifies tagWND.extraFlag |= 0x800:

One tagWND memory layout of Mode 2 is shown in the following figure: avatar

So we can hook the function user32!_xxxClientAllocWindowClassExtraBytes in user-space, call NtUserConsoleControl manually in hook function to modify the tagWND extra data storage mode from Mode 1 to Mode 2, call ntdll!NtCallbackReturn before the callback returns:

Then return the user mode controllable offset value to tagWND.pExtraBytes through ntdll!NtCallbackReturn, and realize the controllable offset out-of-bounds read/write ability based on the kernel-space desktop heap base address finally.

The modified process which can trigger the vulnerability is shown as follows:

According to the modified flowchart above, the key steps of triggering this vulnerability are explained as follows:

  1. Modify the user32!_xxxClientAllocWindowClassExtraBytes function pointer in PEB.kernelCallbackTable to a custom hook function.
  2. Create some normal window objects, and leak the user-space memory addresses of these tagWND kernel objects through user32!HMValidateHandle.
  3. Destroy part of the normal window objects created in step 2, and create one new window object named ‘hwndMagic’ with the specified tagWND.cbwndExtra. The hwndMagic can probably reuse the previously released window object memory. Therefore, by searching the previously leaked window object user-space memory addresses with the specified tagWND.cbwndExtra in the custom hook function, the hwndMagic can be found before CreateWindowEx returns.
  4. Call NtUserConsoleControl in the custom hook function to modify the tagWNDMagic.extraFlag with flag 0x800.
  5. Call NtCallbackReturn in the custom hook function to assign a fake offset to tagWNDMagic.pExtraBytes.
  6. Call SetWindowLong to write data to the address of kernel-space desktop heap base address + specified offset, which can result in out-of-bounds memory access violation.

An implementation of the hook function is demonstrated as follows:

void* WINAPI MyxxxClientAllocWindowClassExtraBytes(ULONG* size) {

	do {
		if (MAGIC_CBWNDEXTRA  == *size) {
			HWND hwndMagic = NULL;
			//search from freed NormalClass window mapping desktop heap
			for (int i = 2; i < 50; ++i) {
				ULONG_PTR cbWndExtra = *(ULONG_PTR*)(g_pWnd[i] + _WND_CBWNDEXTRA_OFFSET);
				if (MAGIC_CBWNDEXTRA == cbWndExtra) {
					hwndMagic = (HWND)*(ULONG_PTR*)(g_pWnd[i]);
					printf("[+] bingo! find &hwndMagic = 0x%llx in callback :) \n", g_pWnd[i]);
			if (!hwndMagic) {
				printf("[-] Not found hwndMagic, memory layout unsuccessfully :( \n");

			// 1. set hwndMagic extraFlag |= 0x800
			CONSOLEWINDOWOWNER consoleOwner = { 0 };
			consoleOwner.hwnd = hwndMagic;
			consoleOwner.ProcessId = 1;
			consoleOwner.ThreadId = 2;
			NtUserConsoleControl(6, &consoleOwner, sizeof(consoleOwner));

			// 2. set hwndMagic pExtraBytes fake offset
			struct {
				ULONG_PTR retvalue;
				ULONG_PTR unused1;
				ULONG_PTR unused2;
			} result = { 0 };		
			//offset = 0xffffff00, access memory = heap base + 0xffffff00, trigger BSOD	
			result.retvalue = 0xffffff00;			
			NtCallbackReturn(&result, sizeof(result), 0);
	} while (false);

	return _xxxClientAllocWindowClassExtraBytes(size);

BSOD snapshot:

Exploit analysis

From Root cause anaysis, we can see that:
“An opportunity to read/write data in the address which calculated by the kernel-space desktop heap base address + specified offset” can be obtained via this vulnerability.

For the kernel mode exploitation, the attack target is to obtain system token generally. A common method is shown as follows:

  1. Exploit the vulnerability to obtain a arbitrary memory read/write primitive in kernel-space.
  2. Leak the address of some kernel object, find the system process through the EPROCESS chain.
  3. Copy the system process token to the attack process token to complete the privilege escalation job.

The obstacle is step 1: How to exploit “An opportunity to read/write data in the address which calculated by the kernel-space desktop heap base address + specified offset” to obtain the arbitrary memory read/write primitive in kernel-space.

One solution is shown in the following figure:

  1. The offset of tagWNDMagic extra data (wndMagic_extra_bytes) is controllable via the vulnerability, so we can use SetWindowLong to modify the data in specified address calculated by desktop heap base address + controllable offset.
  2. Use the vulnerability ability to modify tagWNDMagic.pExtraBytes to the offset of tagWND0 (the offset of tagWND0 is obtained by tagWND0+0x8), call SetWindowLong to modify tagWND0.cbWndExtra = 0x0fffffff to obtain a tampered tagWND0.pExtraBytes which can achieve read/write out-of-bounds.
  3. Calculate the offset from tagWND0.pExtraBytes to tagWND1, call SetWindowLongPtr to replace the spMenu of tagWND1 with a fake spMenu by the tampered tagWND0.pExtraBytes, realize the arbitrary memory read ability with the help of fake spMenu and function GetMenuBarInfo.
    The logic of GetMenuBarInfo to read the data in specified address is shown as follows, the 16 bytes data is stored into MENUBARINFO.rcBar structure: avatar

  4. Use the tampered tagWND0.pExtraBytes to modify tagWND1.pExtraBytes with specified address, and use the SetWindowLongPtr of tagWND1 to obtain the arbitrary memory write ability.
  5. After obtaining the arbitrary memory read/write primitive, we need to leak a kernel object address in desktop heap to find EPROCESS. Fortunately, when setting the fake spMenu for tagWND1 in step 3, the return value of SetWindowLongPtr is the kernel address of original spMenu, which can be used directly.
  6. Finally, find the system process by traversing the EPROCESS chain, and copy the system process token to the attack process to complete the privilege escalation job. This method is relatively common, so will not be described in detail.

The final privilege escalation demonstration:

Patch analysis