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Multiple Vendor chmlib CHM File Handling Buffer Overflow Vulnerability

10.28.05

BACKGROUND

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

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.

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.

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

WORKAROUND

Do not open CHM files sent from an untrusted source.

VENDOR RESPONSE

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

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

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.

DISCLOSURE TIMELINE

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

CREDIT

The discoverer of this vulnerability wishes to remain anonymous.

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LEGAL NOTICES

Copyright © 2005 Verisign, Inc.

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at the time of publishing based on currently available information. Use
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