iDefense Security Advisory 01.17.08
http://labs.idefense.com/intelligence/vulnerabilities/ 
Jan 17, 2008

I. BACKGROUND

The X Window System (or X11) is a graphical windowing system used on
Unix-like systems. It is based on a client/server model. More
information about about The X Window system is available at the
following URL.

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

II. DESCRIPTION

Local exploitation of an invalid array index vulnerability in the X.Org
X server, as included in various vendors' operating system
distributions, could allow an attacker to execute arbitrary code with
the privileges of the X server, typically root.

The vulnerability exists within the XFree86-Misc extension. When
processing a request, a 32-bit value from the client's request is used
as an index into an array of structures. This structure contains an
array of function pointers, one of which is used later in the request
handling. By supplying a large array index, an arbitrary function
pointer can be dereferenced. This results in the execution of arbitrary
code.

III. ANALYSIS

Exploitation allows an attacker to execute arbitrary code with root
privileges. In order to exploit this vulnerability, an attacker must be
able to send commands to an affected X server. This typically requires
access to the console or access to the same account as a user who is on
the console.

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.org X11
version R7.3. Previous versions may also be affected.

V. WORKAROUND

If the XFree86-Misc extension has not been built-in to the server, then
it can be prevented from loading by inserting the following into the X
configuration file (usually in /etc/X11/xorg.conf).

  Section "Module"
    SubSection "extmod"
      Option "omit XFree86-Misc"
    EndSubSection
  EndSection

To check if the extension is built-in to the server, grep the output of
the X Server log file.

  grep built-in /var/log/Xorg.0.log

The result will list all built in extensions. The location of the log
file may need to be changed.

VI. VENDOR RESPONSE

The X.Org team has addressed this vulnerability with the release of
Xserver version 1.4.1. Additionally, patches for versions 1.4 and 1.2
have been made available. For more information, consult the X.Org
advisory at the following URL.

http://lists.freedesktop.org/archives/xorg/2008-January/031918.html 

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2007-5760 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

11/29/2007  Initial vendor response
11/30/2007  Initial vendor notification
01/17/2008  Coordinated public disclosure

IX. CREDIT

This vulnerability was reported to VeriSign 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
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