[NT] Microsoft Word 6.0/95 Document Converter Buffer Overflow (MS04-041)

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Date: 15 Dec 2004 19:26:31 +0200

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  Microsoft Word 6.0/95 Document Converter Buffer Overflow (MS04-041)
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SUMMARY

WordPad is "a word processing application that uses the MFC rich edit
control classes. It is installed by default on most Windows platforms, and
contains filters for converting from other filetypes into RTF (Rich Text
Format)".

Remote exploitation of a buffer overflow vulnerability in Microsoft
Corp.'s Word 6.0/95 Document Converter could allow attackers to exploit
arbitrary code under the privileges of the target user.

DETAILS

The Microsoft Word 6.0/95 Document Converter (MSWRD632.WPC) is a module
that is utilized by WordPad and potentially other applications to convert
Microsoft Word format files into the Rich Text Format natively handled by
WordPad. The module is installed by default in:

C:\Program Files\Common Files\Microsoft Shared\TextConv

The problem specifically exists when a specially crafted file is opened by
WordPad or another application that utilizes the vulnerable library and
results in a buffer overflow. The overflow is caused by copying a length
tagged segment of a file into a fixed length stack buffer of smaller size.
The following instruction sequence is found within ConvertForeignToRtf():

0150eba6 8bd1 mov edx, ecx
0150eba8 83e203 and edx, 0x3
0150ebab c1e902 shr ecx, 0x2
0150ebae f3a5 rep movsd edi, esi

This instruction sequence will copy bytes from the memory region pointed
to by ESI into the memory region pointed to by EDI. Due to a lack of
bounds checking, an overflow occurs directly overwriting the stored return
address and frame pointer on the stack and allowing for the eventual
execution of arbitrary code.

Analysis:
Successful exploitation allows remote attackers to execute arbitrary code
under the privileges of the target user that opened the malicious
document. WordPad, a vulnerable application, is installed by default and
will open WRI and large TXT files. If Microsoft Word is not installed,
WordPad will also be the default application for opening DOC and RTF
files.

In order for this vulnerability to be exploited, a user would need to open
an attacker-supplied file with a vulnerable application.

Detection:
The following operating systems appear to be impacted by this
vulnerability in their default configuration:

  Windows XP
  Windows 2000
  Windows 2003
  Windows NT 4.0
  Windows ME
  Windows 98

iDEFENSE Labs has confirmed that MSWRD632.WPC, file version 1999.8.7.0 is
vulnerable. Any application that utilizes this module to convert Word
documents may be considered vulnerable. This includes wordpad.exe, which
is the default application for opening files with the .wri extension, and
doc and .rtf files if Microsoft Word is not installed.

It does not seem to be possible to exploit Microsoft Word itself with this
vulnerability, as it does not appear to use this library.

As this module comes with Windows by default, even if you have Word
installed, WordPad is still vulnerable to exploitation from files with the
 .wri extension, or by opening an affected file from within WordPad.

Workaround:
User awareness is the best defense against this class of attack. Users
should be aware of the existence of such attacks and proceed with caution
when following links or opening attachments from suspicious and/or
unsolicited e-mail.

Alternatively, concerned users can remove the affected converter module,
MSWRD632.WPC. This will prevent the user from opening Word for Windows
files, but will still allow other supported file types to be opened such
as .txt or .rtf. However, the error will be handled gracefully and the
described vulnerability will no longer be exploitable.

Vendor Response:
This vulnerability is addressed in Microsoft Security Bulletin MS04-041
available at:
http://www.microsoft.com/technet/security/Bulletin/MS04-041.mspx

CVE Information:
 <http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2004-0901>
CAN-2004-0901

Disclosure Timeline:
09/22/2004 - Initial vendor notification
09/23/2004 - Initial vendor response
12/14/2004 - Coordinated public disclosure

ADDITIONAL INFORMATION

The information has been provided by
<mailto:idlabs-advisories@idefense.com> iDEFENSE.
The original article can be found at:
<http://www.idefense.com/application/poi/display?id=162&type=vulnerabilities> http://www.idefense.com/application/poi/display?id=162&type=vulnerabilities

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