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iDEFENSE Security Advisory 07.09.04: wvWare Library Buffer Overflow Vulnerability
From: idlabs-advisories () idefense com
Date: Fri, 9 Jul 2004 10:39:48 -0400

wvWare Library Buffer Overflow Vulnerability

iDEFENSE Security Advisory 07.09.04
www.idefense.com/application/poi/display?id=115&type=vulnerabilities
July 09, 2004

I. BACKGROUND

Caolán McNamara and Dom Lachowicz's wvWare is a library used to load and
parse Microsoft Word files on unix-based systems. wvWare is used in some
third-party programs to view and convert Microsoft Word documents to
other formats.

II. DESCRIPTION

Caolán McNamara and Dom Lachowicz's wv library has been found to contain
a buffer overflow condition that can be exploited through a specially
crafted document.

The issue lies in the handling of the DateTime field of a document as 
can be seen from the following lines of code taken from field.c and the
function wvHandleDateTimePicture():

...
            default:
                temp[0] = *token;
                temp[1] = '\0';
                strcat (timestr, temp);
                break;
            }
...

The above section of code can be reached if the wv library is presented 
with a token that it does not recognize. The utilization of the insecure
function call strcat() without appropriate bounds checking leads to a 
classic and exploitable buffer overflow.

The following is a walkthrough of a sample exploitation of the buffer 
overflow that will execute the command "id > /tmp/wv_exploit" upon 
success.

$ id
luid=501(farmer) gid=501(farmer) groups=501(farmer)

$ ls /tmp/wv_exploit
ls: /tmp/wv_exploit: No such file or directory

$ dd if=wv_exploit.doc of=a3.doc ibs=1 count=42945
42945+0 records in
83+1 records out

$ perl wv_exploit.pl >> a3.doc 

$ wvHtml wv_exploit.doc exploit.html

$ cat /tmp/wv_exploit 
uid=501(farmer) gid=501(farmer) groups=501(farmer)

The final exploit document size must be a multiple of 4096 bytes to be 
valid. Because of some input filtering the shellcode and return address 
can only contain ASCII characters (00-7f) not including any of the 
following: 0x22, 0x60, 0x48, 0x68, 0x41, 0x61, 0x4d, 0x6d, 0x53, 0x73, 
0x44, 0x64, 0x59, 0x79.

III. ANALYSIS

If an attacker can convince a user to open an exploit document in HTML 
mode using an application that builds upon the wv library, it is 
possible for the attacker to execute arbitrary code under the privileges 
of that user.

IV. DETECTION

iDEFENSE has confirmed the existence of this vulnerability in version 
0.7.4, and a slight variant of this vulnerability in versions 0.7.5, 
0.7.6 and 1.0.0.

V. WORKAROUND

Users should be careful to open documents from only trusted sources. 
When opening Microsoft Word documents with applications utilizing the wv 
library ensure that HTML view is not enabled. Careful low-level scrutiny 
of the document in question can also reveal whether or not the document 
is valid or not.

VI. VENDOR RESPONSE

Dom Lachowicz has posted the following patch details:

http://www.abisource.com/bonsai/cvsview2.cgi?diff_mode=context&whitespac
e_mode=show&root=/cvsroot&subdir=wv&command=DIFF_FRAMESET&root=/cvsroot&
file=field.c&rev1=1.19&rev2=1.20

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CAN-2004-0645 to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org), which standardizes names for
security problems.

VIII. DISCLOSURE TIMELINE

06/29/2004   Initial vendor contact
07/06/2004   Vendor response
07/09/2004   Public disclosure

IX. CREDIT

Karol Weisek is credited with discovering this vulnerability.

Get paid for vulnerability research
http://www.idefense.com/poi/teams/vcp.jsp

X. LEGAL NOTICES

Copyright (c) 2004 iDEFENSE, Inc.

Permission is granted for the redistribution of this alert
electronically. It may not be edited in any way without the express
written consent of iDEFENSE. If you wish to reprint the whole or any
part of this alert in any other medium other than electronically, please
email customerservice () idefense com for permission.

Disclaimer: The information in the advisory is believed to be accurate
at the time of publishing based on currently available information. Use
of the information constitutes acceptance for use in an AS IS condition.
There are no warranties with regard to this information. Neither the
author nor the publisher accepts any liability for any direct, indirect,
or consequential loss or damage arising from use of, or reliance on,
this information.




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