CVE-2022-21974: Unitialized pointer free in RMSRoamingSecurity Remote Code Execution

Root-cause

The RMSRoamingSecurity!ATL::CComCreator<ATL::CComObject<CRmsRoamingSecurity> >::CreateInstance constructor allocates and initializes a CRmsRoamingSecurity object (0x3470 bytes) but it fails to initialize completely its state.

For example, at offset 0x3458 there is an uninitialized pointer that gets free'd when the destructor is invoked (in CRmsRoamingSecurity::_Cleanup):

0:000> 
RMSRoamingSecurity!CRmsRoamingSecurity::_Cleanup+0x29:
00007ffd`719767a5 488b8b58340000  mov     rcx,qword ptr [rbx+3458h] ds:000001c6`612f9fe8=c0c0c0c0c0c0c0c0

0:000> p
RMSRoamingSecurity!CRmsRoamingSecurity::_Cleanup+0x30:
00007ffd`719767ac 4885c9          test    rcx,rcx

0:000> 
RMSRoamingSecurity!CRmsRoamingSecurity::_Cleanup+0x33:
00007ffd`719767af 7414            je      RMSRoamingSecurity!CRmsRoamingSecurity::_Cleanup+0x49 (00007ffd`719767c5) [br=0]

0:000> 
RMSRoamingSecurity!CRmsRoamingSecurity::_Cleanup+0x35:
00007ffd`719767b1 48ff15e0b90000  call    qword ptr [RMSRoamingSecurity!_imp_CoTaskMemFree (00007ffd`71982198)] ds:00007ffd`71982198={combase!CoTaskMemFree (00007ffd`ce8454d0)}

This is the function using the uninitialized data, see [0] and [1]:

void __fastcall CRmsRoamingSecurity::_Cleanup(struct_v7 *this)
{
  void *v2; // rcx
  void *qword3460; // rcx
  void *qword33E8; // rcx

  if ( this->qword33E0 )
  {
    IpcFreeMemory();
    this->qword33E0 = 0i64;
  }
  v2 = *(void **)&this->gap33F0[104];      // <--- [0] uninitialized
  if ( v2 )
  {
    CoTaskMemFree(v2);                     // <--- [1] free
    *(_QWORD *)&this->gap33F0[104] = 0i64;
  }
  //...
}

Repro

1. Turn on PageHeap via Gflags on winword.exe
2. Start Word, attach a debugger to it
3. Open cve-2022-21974.rtf in Word

(12f0.2f64): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
verifier!AVrfpDphFindBusyMemoryNoCheck+0x8a:
00007ffd`7a4c4742 817ac0bbbbcdab  cmp     dword ptr [rdx-40h],0ABCDBBBBh ds:c0c0c0c0`c0c0c080=????????

0:000> kc
 # Call Site
00 verifier!AVrfpDphFindBusyMemoryNoCheck
01 verifier!AVrfpDphFindBusyMemory
02 verifier!AVrfpDphFindBusyMemoryAndRemoveFromBusyList
03 verifier!AVrfDebugPageHeapFree
04 ntdll!RtlDebugFreeHeap
05 ntdll!RtlpFreeHeap
06 ntdll!RtlpFreeHeapInternal
07 ntdll!RtlFreeHeap
08 RMSRoamingSecurity!CRmsRoamingSecurity::_Cleanup
09 RMSRoamingSecurity!ATL::CComObject<CRmsRoamingSecurity>::~CComObject<CRmsRoamingSecurity>
0a RMSRoamingSecurity!ATL::CComObject<CRmsRoamingSecurity>::`vector deleting destructor'
0b RMSRoamingSecurity!ATL::CComCreator<ATL::CComObject<CRmsRoamingSecurity> >::CreateInstance
0c RMSRoamingSecurity!ATL::CComClassFactory::CreateInstance
0d combase!CServerContextActivator::CreateInstance
0e combase!ActivationPropertiesIn::DelegateCreateInstance
0f combase!CApartmentActivator::CreateInstance
10 combase!CProcessActivator::CCICallback
11 combase!CProcessActivator::AttemptActivation
12 combase!CProcessActivator::ActivateByContext
13 combase!CProcessActivator::CreateInstance
14 combase!ActivationPropertiesIn::DelegateCreateInstance
15 combase!CClientContextActivator::CreateInstance
16 combase!ActivationPropertiesIn::DelegateCreateInstance
17 combase!ICoCreateInstanceEx
18 combase!CComActivator::DoCreateInstance
19 combase!CoCreateInstanceEx
1a combase!CoCreateInstance
1b ole32!wCreateObject
1c ole32!OleLoadWithoutBinding
1d ole32!OleLoad

I've reproduced this on a Windows 10 x64 VM and on Windows 11 x64 with both Wordpad (but requires one click) and Office Word 2019.