Multiple Vendor chmlib CHM File Handling Buffer Overflow Vulnerability

iDefense Security Advisory 10.28.05
www.idefense.com/application/poi/display?id=332&type=vulnerabilities
October 28, 2005

I. BACKGROUND

chmlib is a library for dealing with Microsoft ITSS/CHM format files.

II. DESCRIPTION

Remote exploitation of a stack overflow vulnerability in chmlib as 
included in various Linux distributions allows attackers to execute 
arbitrary code.

The vulnerability specifically exists due to an unchecked memory copy 
while processing a CHM file. The vulnerability exists in the following 
code, which is found in chm_lib.c: 

static UChar *_chm_find_in_PMGL(UChar *page_buf,
                                UInt32 
block_len,
                                const 
char *objPath)
{
[...]
  char buffer[CHM_MAX_PATHLEN+1];    
    /* figure out where to start and end */
    cur = page_buf;
    hremain = _CHM_PMGL_LEN;
    if (! _unmarshal_pmgl_header(&cur, &hremain, &header))
        return NULL;
    end = page_buf + block_len - (header.free_space);
    
    /* now, scan progressively */
    while (cur < end)
    {
        /* grab the name */
        temp = cur;
        strLen = _chm_parse_cword(&cur);
        if (! _chm_parse_UTF8(&cur, strLen, buffer))
            return NULL;
[..] 

_chm_parse_cword can be forced to return a value larger than 
CHM_MAX_PATHLEN. This value is then used while copying user controlled 
data into a CHM_MAX_PATHLEN sized stack buffer. This allows the attacker

full control over execution flow by overwriting the saved return address

on the stack.

III. ANALYSIS

Exploitation could allow attackers to execute arbitrary code with the 
privileges of the user processing the CHM file. Remote exploitation can 
be achieved by sending a malicious file in an e-mail message to the 
target user.

IV. DETECTION

iDefense has confirmed the existence of this vulnerability in chmlib 
0.35. It is suspected that all versions of chmlib are vulnerable. 

The following vendors distribute susceptible chmlib packages within 
their respective operating system distributions:

	FreeBSD Project:
	  FreeBSD 5.4 and prior
	Gentoo Foundation Inc.:
	  Gentoo Linux 1.1a, 1.2, 1.4, 2004.0, 2004.1 and 2004.2 

V. WORKAROUND

Do not open CHM files sent from an untrusted source.

VI. VENDOR RESPONSE

chmlib-0.36 addresses this vulnerability and is available for download
at:

   http://freshmeat.net/projects/chmlib/

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CAN-2005-2930 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

09/08/2005  Initial vendor notification
09/09/2005  Initial vendor response
10/28/2005  Public disclosure

IX. CREDIT

The discoverer of this vulnerability wishes to remain anonymous.

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X. LEGAL NOTICES

Copyright (c) 2005 iDefense, Inc.

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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
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