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libtiff Directory Entry Count Integer Overflow Vulnerability

12.21.04

BACKGROUND

This software provides support for the Tag Image File Format (TIFF), a
widely used format for storing image data. Included in this software
distribution is a library, libtiff, for reading and writing TIFF, a
small collection of tools for doing simple manipulations of TIFF images
on UNIX systems, and documentation on the library and tools. A small
assortment of TIFF-related software for UNIX that has been contributed
by others is also included.

More information is available at the following site:
   http://www.remotesensing.org/libtiff/

DESCRIPTION

Remote exploitation of a heap-based buffer overflow vulnerability
within the LibTIFF package could allow attackers to execute arbitrary
code.

The vulnerability specifically exists due to insufficient validation of
user-supplied data when calculating the size of a directory entry. A
TIFF file includes a number of directory entry header fields that
describe the data in the file. Included in these entries is an entry
count and offset value that are calculated to determine the size and
location of the data for that entry. If the directory entry type is
TIFF_ASCII or TIFF_UNDEFINED, the a call to CheckMalloc is made in
order to allocate space for the entry contents as shown below from
tif_dirread.c:

cp = CheckMalloc(tif, dp->tdir_count+1, 1, mesg); 
if( (ok = (cp && TIFFFetchString(tif, dp, cp))) != 0 )
    cp[dp->tdir_count] = '';  /* XXX */

If the tdir_count is set to 0xffffffff, the increment will cause the
function call to allocate 0 bytes of memory and later memcpy calls will
cause the buffer to overflow. An attacker may generate a malicious TIFF
file which takes advantage of this unchecked calculation to execute
arbitrary code with permissions of the user opening the TIFF file.

ANALYSIS

Remote attackers may be able to execute arbitrary code with permissions
of the user opening the malformed TIFF file. The exposure to this
vulnerability is mitigated by the fact that user interaction is
required and that the user must view the malicious TIFF file in an
application that is linked to a vulnerable version of libtiff.

DETECTION

iDEFENSE has confirmed the existence of this vulnerability in LibTIFF
3.5.7 and 3.7.0.

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

    - Debian Linux
    - Gentoo Linux
    - Red Hat Linux
    - SuSE Linux
  

WORKAROUND

Only open TIFF files from trusted users or in an application not linked
against vulnerable versions of libtiff.

VENDOR RESPONSE

The following patch will address this vulnerability:

--- tif_dirread.c.bak 2004-12-19 21:04:00.000000000 +0300
+++ tif_dirread.c 2004-12-19 21:04:03.000000000 +0300
@@ -71,7 +71,7 @@
  /*
   * XXX: Check for integer overflow.
   */
- if (elem_size && bytes / elem_size == nmemb)
+ if (nmemb && elem_size && bytes / elem_size == nmemb)
   cp = (char*)_TIFFmalloc(bytes);
 
  if (cp == NULL)

(the same applies for tif_fax3.c)

This patch has been incorporated into libtiff 3.7.1.

CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
names CAN-2004-1308 to these issues. This is a candidate for inclusion
in the CVE list (http://cve.mitre.org), which standardizes names for
security problems.

DISCLOSURE TIMELINE

12/17/2004   Initial vendor notification
12/17/2004   iDEFENSE clients notified
12/21/2004   Coordinated public disclosure

CREDIT

infamous41md[at]hotpop.com is credited with this discovery.

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

LEGAL NOTICES

Copyright © 2004 Verisign, Inc.

Permission is granted for the redistribution of this alert
electronically. It may not be edited in any way without the express
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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
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