Intel Content Protection HECI Service - Type Confusion Privilege Escalation

EDB-ID: 43373
Author: Google Security Research
Published: 2017-12-19
CVE: CVE-2017-5717
Type: Dos
Platform: Windows
Aliases: N/A
Advisory/Source: Link
Tags: Type Confusion
Vulnerable App: N/A

Intel Content Protection HECI Service Type Confusion EoP
Platform: Tested on Windows 10, service version
Class: Elevation of Privilege

The Intel Content Protection HECI Service exposes a DCOM object to all users and most sandboxes (such as Edge LPAC and Chrome GPU). It has a type confusion vulnerability which can be used to elevate to SYSTEM privileges.


The Intel Content Protection HECI Service runs as LocalSystem and hosts a DCOM service. The main function is StartIo which takes an input variant and returns a variant. Based on what it’s trying to do I’d assume the input variant is supposed to be a byte array, however it contains a bug.

The vulnerable code is roughly:

CComSafeArray<char> array;
// Work with array

The issue here is that the In variant is used as a SAFEARRAY without checking that the VARIANT contains a SAFEARRAY. This leads to type confusion, for example a caller could pass VT_UI4 integer with any value they like and this code would interpret that integer as a pointer to a SAFEARRAY structure. This might seem to be only an arbitrary read issue, however the copy of the safe array can be made to execute arbitrary memory. If you point the type confused pointer at a block of memory which looks like a IUnknown array then when copying the array it will try and add a reference to each COM object in the array. This causes a VTable dispatch to AddRef which if carefully crafted should get arbitrary code execution.

The call to CopyFrom does verify that the variant type is VT_UI1 (a byte array) however you can set some feature flags such as FADF_UNKNOWN which will force a call to IUnknown::AddRef on the elements of the array without changing the supposed variant type. Also you don’t need to guess the allocation address for the fake safearray as you can use a byte length BSTR which contains arbitrary data. The BSTR length field and the SAFEARRAY variant field lines up so as long as the lower 16 bits of the length is set to 17 (which is VT_UI1) it passes the checks and reads out the arbitrary contents from the allocated BSTR.

The really bad thing about this service is not only is it intentionally designed to be accessible from even a heavily restrictive sandbox such as Edge LPAC but it runs with full LocalSystem privileges. While on Win10 CFG might make it harder to exploit, on Win7 you don’t have any such protection. Also the call is done inside an exception handler so even if the wrong address is chosen the service won’t crash (except for fast fail such as CFG).

The following is an example crash when sending a fake safe array to the service (with just a dummy address of 0x18181818 as the IUnknown memory location).

(1110.1188): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
eax=18181818 ebx=001e6290 ecx=18181818 edx=00209390 esi=11d41024 edi=18181818
eip=18181818 esp=0126efc4 ebp=0126efec iopl=0 nv up ei pl nz na pe nc
cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010206
18181818 1818 sbb byte ptr [eax],bl ds:002b:18181818=18
0:003> k
# ChildEBP RetAddr
WARNING: Frame IP not in any known module. Following frames may be wrong.
00 0126efc0 74f740fb 0x18181818
01 0126efec 74f73e42 OLEAUT32!SafeArrayCopyData+0x21b
02 0126f018 010335d3 OLEAUT32!SafeArrayCopy+0x182
03 0126f030 01034e1b IntelCpHeciSvc+0x135d3
04 0126f118 750326e0 IntelCpHeciSvc+0x14e1b
05 0126f144 74ff4fc2 RPCRT4!Invoke+0x34
06 0126f598 7525555e RPCRT4!NdrStubCall2+0x452
07 0126f5e4 74f70706 combase!CStdStubBuffer_Invoke+0xde [onecore\com\combase\ndr\ndrole\stub.cxx @ 1449]
08 0126f614 75300c48 OLEAUT32!CUnivStubWrapper::Invoke+0x136
09 (Inline) -------- combase!InvokeStubWithExceptionPolicyAndTracing::__l6::<lambda_1ba7c1521bf8e7d0ebd8f0b3c0295667>::operator()+0x4e [onecore\com\combase\dcomrem\channelb.cxx @ 1824]
0a 0126f668 75303621 combase!ObjectMethodExceptionHandlingAction<<lambda_1ba7c1521bf8e7d0ebd8f0b3c0295667> >+0xa8 [onecore\com\combase\dcomrem\excepn.hxx @ 91]
0b (Inline) -------- combase!InvokeStubWithExceptionPolicyAndTracing+0x8e [onecore\com\combase\dcomrem\channelb.cxx @ 1822]
0c 0126f78c 75307330 combase!DefaultStubInvoke+0x221 [onecore\com\combase\dcomrem\channelb.cxx @ 1891]
0d (Inline) -------- combase!SyncStubCall::Invoke+0x22 [onecore\com\combase\dcomrem\channelb.cxx @ 1948]
0e (Inline) -------- combase!SyncServerCall::StubInvoke+0x22 [onecore\com\combase\dcomrem\servercall.hpp @ 779]
0f (Inline) -------- combase!StubInvoke+0x287 [onecore\com\combase\dcomrem\channelb.cxx @ 2173]
10 0126f90c 7530009b combase!ServerCall::ContextInvoke+0x440 [onecore\com\combase\dcomrem\ctxchnl.cxx @ 1541]
11 (Inline) -------- combase!CServerChannel::ContextInvoke+0x669 [onecore\com\combase\dcomrem\ctxchnl.cxx @ 1437]
12 (Inline) -------- combase!DefaultInvokeInApartment+0x669 [onecore\com\combase\dcomrem\callctrl.cxx @ 3532]
13 (Inline) -------- combase!ClassicSTAInvokeInApartment+0x669 [onecore\com\combase\dcomrem\callctrl.cxx @ 3296]
14 0126f9ac 75302b39 combase!AppInvoke+0x8bb [onecore\com\combase\dcomrem\channelb.cxx @ 1604]
15 0126fb3c 7530ff85 combase!ComInvokeWithLockAndIPID+0x599 [onecore\com\combase\dcomrem\channelb.cxx @ 2722]
16 0126fb98 7531056b combase!ComInvoke+0x1c5 [onecore\com\combase\dcomrem\channelb.cxx @ 2242]
17 (Inline) -------- combase!ThreadDispatch+0x83 [onecore\com\combase\dcomrem\chancont.cxx @ 421]
18 0126fbd8 76b12b5b combase!ThreadWndProc+0x21b [onecore\com\combase\dcomrem\chancont.cxx @ 741]
19 0126fc04 76b050f3 USER32!_InternalCallWinProc+0x2b
1a 0126fcec 76b04a82 USER32!UserCallWinProcCheckWow+0x2d3
1b 0126fd60 76b04850 USER32!DispatchMessageWorker+0x222
1c 0126fd6c 010364e1 USER32!DispatchMessageW+0x10
1d 0126fda0 01037039 IntelCpHeciSvc+0x164e1
1e 0126fda8 0103e562 IntelCpHeciSvc+0x17039
1f 0126fde0 0103e5ec IntelCpHeciSvc+0x1e562
20 0126fdec 76928744 IntelCpHeciSvc+0x1e5ec
21 0126fe00 770a582d KERNEL32!BaseThreadInitThunk+0x24
22 0126fe48 770a57fd ntdll!__RtlUserThreadStart+0x2f
23 0126fe58 00000000 ntdll!_RtlUserThreadStart+0x1b

Proof of Concept:

I’ve provided a PoC as a VS project which you can run which will cause the service to access invalid memory. Note that you’ll need a debugger attached to IntelCpHeciSvc.exe as the RPC/DCOM dispatch will swallow the exception, it doesn’t crash the service. The Poc builds a fake SAFEARRAY structure and passes it as a BSTR to the service which gets interpreted as a pointer to a SAFEARRAY. Ultimately it tries to copy the array and will call AddRef on elements of the array.

1) Attach a debugger to IntelCpHeciSvc.exe
2) Compile and run the provided poc.

Expected Result:
Sending the fake SAFEARRAY should fail.

Observed Result:
The service tries to execute invalid memory at 0x18181818 (or at least crashes on an invalid memory location).

Proof of Concept:

Related Posts