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 multiple integer overflow vulnerabilities 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.

One vulnerability exists within the EVI extension. When processing a
request, the server uses a 32-bit value provided by the client in an
arithmetic operation that calculates the number of bytes to allocate
for a dynamic buffer. This operation can overflow, which later leads to
a potentially exploitable heap overflow.

Another vulnerability exists within the MIT-SHM extension. When
allocating a pixmap, the server uses values from the request to verify
that the requested size is not greater than the amount of allocated
shared memory. The calculation can overflow, which leads to the
overwriting of arbitrary addresses in memory that aren't part of the
shared memory segment.

III. ANALYSIS

Exploitation allows an attacker to execute arbitrary code with root
privileges. In order to exploit these vulnerabilities, 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 these vulnerabilities can be exploited remotely.

IV. DETECTION

iDefense has confirmed the existence of these vulnerabilities in X.Org
X11 version R7.3. Previous versions may also be affected.

V. WORKAROUND

If the EVI or MIT-SHM extensions have not been built-in to the server,
they 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 Extended-Visual-Information"
      Option "omit MIT-SHM"
    EndSubSection
  EndSection

To check if an 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 these vulnerabilities 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-2007-6429 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 notification
11/29/2007  Initial vendor response
01/17/2008  Coordinated public disclosure

IX. CREDIT

These vulnerabilities were 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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reliance on, this information.