iDefense Security Advisory 06.11.08
http://labs.idefense.com/intelligence/vulnerabilities/ 
Jun 11, 2008

I. BACKGROUND

The X Window System is a graphical windowing system based on a
client/server model. More information about about The X Window system
is available at the following link.

http://en.wikipedia.org/wiki/X_Window_System 

II. DESCRIPTION

Local exploitation of an information disclosure vulnerability in the
X.Org X server, as included in various vendors' operating system
distributions, could allow an attacker to gain access to sensitive
information stored in server memory.

The vulnerability exists when creating a Pixmap in the fbShmPutImage()
function. The width and height of the Pixmap, which are controlled by
the user, are not properly validated to ensure that the Pixmap they
define are within the bounds of the shared memory segment. This allows
an attacker to read arbitrary areas of memory in the X server process.

III. ANALYSIS

Exploitation allows an attacker to read arbitrary memory within the X
Server's address space. By itself, the impact of this vulnerability is
minimal. However, when coupled with a code execution vulnerability,
this vulnerability can be used to greatly increase the reliability of
an exploit. Additionally, this vulnerability can be used to crash the
server. If the server automatically restarts, this can be useful since
it resets the state of the server to a known state.

If an X Server is configured to listen for TCP based client connections,
and a client is granted access to create sessions (via the xhosts file),
then the vulnerability can be exploited remotely.

IV. DETECTION

iDefense has confirmed the existence of this vulnerability in X server
1.4 included with X.org X11R7.3, with all patches as of 03/01/08
applied. Previous versions may also be affected.

V. WORKAROUND

Access to the vulnerable code can be prevented by preventing the X
server from loading the MIT-SHM extension. However, doing so may impair
the functionality of the server. Adding the following lines to the X
configuration file will disable the MIT-SHM extension:

  Section "Extensions"
    Option "MIT-SHM" "disable"
  EndSection

VI. VENDOR RESPONSE

The X.Org team has addressed this vulnerability by releasing patches for
version 1.4 of the X server. For more information, consult the X.Org
advisory at the following URL.

http://lists.freedesktop.org/archives/xorg/2008-June/036026.html 

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2008-1379 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

03/26/2008  Initial vendor notification
03/26/2008  Initial vendor response
06/11/2008  Coordinated public disclosure

IX. CREDIT

This vulnerability was reported to iDefense by regenrecht.

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X. LEGAL NOTICES

Copyright =A9 2008 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
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.