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Re: Internet Explorer Remote Null Pointer Crash(mshtml.dll)
From: "Berend-Jan Wever" <skylined () edup tudelft nl>
Date: Wed, 28 Jul 2004 17:19:57 +0200

Here's a detailed description of what's going wrong with the <a href=::%7b> right click bug.
(yeah, my reply is a bit late but I was busy and went on a holliday)

Right click on a link with href="file://::XXXX" and IE will try to download
it, fail and try to report an error. While creating this errormessage
FormatMessageW calls RtlFormatMessage with parts of our url in the

- RtlFormatMessage's arguments include a pointer to the formatstring and a
pointer to a pointer to a pointer to an Array of arguments (Array1). All
pointers, the formatstring and Array1 are on the stack.
- Array1 should contain the normal printf-type arguments (except the
formatstring ofcourse) but since none are expected to be there, the pointers
to Array1 exist but Array1 itself is totally empty (doesn't exist). The
pointer points to the end of a stack frame.
- The formatstring for RtlFormatMessage looks like this:
"%index!conversion!", eg. "%1!x!" will display Array1[0] as heX, "%2!s!"
will display Array1[1] as string.
- The actual conversion is done with _snwprintf. RtlFormatMessage passes the
destination string, the length, the formatstring, and Array1[index] as first
argument to convert. We control the formatstring and Array1[index]: %10!x%x!
will give _snwprintf %x%x as formatstring and Array1[9] as first argument.

- The href gets converted to "[file] from [server]" (XXXX from ::), which is
used as formatstring in RtlFormatMessage.
- The total href can not be more then 97 bytes or _snwprintf won't be
- [file] cannot be more then 28 bytes or _snwprintf won't be called.
- "file://::XXXX/A" converts to "A from ::XXXX". Use this to bypass the
restriction on the [file]-length.
- Each conversion specifier passed to RtlFormatMessage (%index!conversion!)
cannot be more then 34 bytes or snwprintf won't be called.
- RtlFormatMessage limits to total length of the message to 10000 bytes,
each call to _snwprintf gets a proper max length accordingly.

Here's a little stack layout, I hope it doesn't get to mangled:

Address    Stack      Procedure / arguments                 Called from
xxxxEB3C   77F8F40C   ntdll._snwprintf                      ntdll.77F8F407
xxxxEB40   yyyyyyyy     destination = yyyyyyyy
xxxxEB44   00010000     count = 10000 (10000-allready printed chars)
xxxxEB48   xxxxEE84     format = "%x" (conversion)
xxxxEB4C   xxxxxxxx       = 1  Array1[index]
xxxxEB50   00000000       = 0
xxxxEB54   00000000       = 0
xxxxEB58   00000500       = 0x500
xxxxEB5C   00000000       = 0
xxxxEB60   00000000       = 0
xxxxEB64   xxxxFE6C       = &Array1[1]
xxxxEEDC   7C57F221   ntdll.RtlFormatMessage
KERNEL32.7C57F21B             xxxxEF74
xxxxEEE0   04246F30     Source = UNICODE "A from ::%1!x!"
xxxxEEE4   00000000
xxxxEEE8   00000000
xxxxEEEC   00000000
xxxxEEF0   00000000
xxxxEEF4   xxxxFE24     Arguments = xxxxFE24 [&&&Array of arguments]
xxxxEEF8   yyyy0000     Destination = freshly allocated heap.
xxxxEEFC   00010000     Length = 10000 (updated by RtlFormatMessage and
passed to _snwprintf)
xxxxEF64   xxxxFFDC   [Pointer to next SEH record]
xxxxEF68   xxxxxxxx   [SE handler]
xxxxEF78   7C57F2CB   KERNEL32.7C57F102
KERNEL32.7C57F2C6             xxxxEF74
xxxxEFA0   70A9D0CD   KERNEL32.FormatMessageW               SHLWAPI.70A9D0C7
xxxxEFA4   00000500     Flags = ALLOCATE_BUFFER|FROM_STRIN
xxxxEFA8   04246F30     pSource = 04246F30
xxxxEFAC   00000000     MessageId = 0
xxxxEFB0   00000000     LanguageId = 0 (LANG_NEUTRAL)
xxxxEFB4   xxxxFE28     Buffer = xxxxFE28
xxxxEFB8   00000000     BufSize = 0
xxxxEFBC   xxxxFE24     Arguments = xxxxFE24 [&&&Array of arguments]
xxxxF3F8   7174B9DD   SHLWAPI.#68                           SHDOCVW.7174B9D7
xxxxFC38   UNICODE "::%1!x!/A"
xxxxFE24   xxxxFE44   [&&Array of arguments]
xxxxFE30   7175E158   SHDOCVW.7174B97C                      SHDOCVW.7175E153
xxxxFE44   xxxxFE6C   [&Array of arguments]
xxxxFE48   717610C6   SHDOCVW.7175E122                      SHDOCVW.717610C1
xxxxFE6C   xxxxFEA4   [arg 1 -> arg 15]
xxxxFE70   7176111D   SHDOCVW.7176103E              SHDOCVW.7176111D
xxxxFE74   00199020
xxxxFE78   001D4358
xxxxFE94   001D5F74
xxxxFEA4   xxxxFEEC   [arg 15 -> arg 33 ]
xxxxFEA8   71761331   RETURN to SHDOCVW.71761331 from SHDOCVW.717610EC
xxxxFEAC   00199020
xxxxFEB0   001D4358
xxxxFED0   001D5F74
xxxxFEE4   001D5F50
xxxxFEE8   00199020
xxxxFEEC   xxxxFF50   [ arg 33 -> ... ]
xxxxFEF0   717604AB   RETURN to SHDOCVW.717604AB from SHDOCVW.717612E2
xxxxFEF4   00199C80   (arg 35) UNICODE "file://:|%1!x!/A"
xxxxFE70   7176111D   SHDOCVW.7176103E                      SHDOCVW.71761118
xxxxFEA8   71761331   SHDOCVW.717610EC                      SHDOCVW.7176132C
xxxxFEF0   717604AB   SHDOCVW.717612E2                      SHDOCVW.717604A6
xxxxFF50   040BFFB4   [ ... -> ... ]
xxxxFF54   70AAC487   Includes SHDOCVW.717604AB             SHLWAPI.70AAC484
xxxxFFB8   7C57438B   Includes SHLWAPI.70AAC487
KERNEL32.7C574388             xxxxFFB4
xxxxFFDC   FFFFFFFF   [Pointer to next SEH record]
xxxxFFE0   xxxxxxxx   [SE handler]

Is this exploitable ? Probably not...
1. I can't reach the SEH. RtlFormatMessage doesn't allow enough room in the
formatstring to have _snwprintf do stackpopping and overwrite it.
2. I can't overwrite anything with a proper value: I can print 0x10000 bytes
max, so I can only overwrite with values 00000000-00010000. %hn is nice, but
then I'd have to have two pointers that point to the high and low word of
something usefull. And I can't use integer overflow with %hn to write a big value followed by a small value.



----- Original Message ----- 
From: "Rafel Ivgi, The-Insider" <theinsider () 012 net il>
To: "vulnwatch" <vulnwatch () vulnwatch org>
Sent: Monday, June 14, 2004 23:20
Subject: [Full-disclosure] Internet Explorer Remote Null Pointer Crash(mshtml.dll)


Application:      Internet Explorer
Vendors:           http://www.microsoft.com
Versions:          6.0.2800.1106.xpclnt_qfe.021108-2107
Patched With:  SP1;Q832894;Q330994;Q837009;Q831167;
ModName:       mshtml.dll
ModVer:           6.0.2734.1600
Platforms:        Windows
Bug:                  Remote/Local Null Pointer Crash
Exploitation:    Remote with browser
Date:                14 Jun 2004
Author:             Rafel Ivgi, The-Insider
e-mail:              the_insider () mail com
web:                 http://theinsider.deep-ice.com


1) Introduction
2) Bugs
3) The Code


1) Introduction

Internet Explorer is currently the most common internet browser in the
It comes by default with every windows operating system. Therefore any
concerning it is an highly important issue.


2) Bug

Upon clicking "Save As" on a link with double colon --> "::"
a left curly bracket --> "{"
Internet Explorer Will Crash.

AppName: iexplore.exe  AppVer: 6.0.2600.0  ModName: ntdll.dll
ModVer: 5.1.2600.114  Offset: 00056074


3) The Code

Paste into an htm/html file:
<center><a href=::%7b>Right  Click aOn Me And Click "Save Target As"</a>


Rafel Ivgi, The-Insider

"Scripts and Codes will make me D.O.S , but they will never HACK me."

Full-Disclosure - We believe in it.
Charter: http://lists.netsys.com/full-disclosure-charter.html

Full-Disclosure - We believe in it.
Charter: http://lists.netsys.com/full-disclosure-charter.html

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