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

From: SecuriTeam (support_at_securiteam.com)
Date: 12/15/04

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    To: list@securiteam.com
    Date: 15 Dec 2004 19:26:31 +0200

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


    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

    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.


    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.

    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

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

    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.

    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:

    CVE Information:

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


    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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