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. The Render extension is used to provide
Porter-Duff image compositing for the X server. It is built into many X
servers by default and loaded as a default module when it is not. For
more information, see the vendor's site found at the following link.

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

II. DESCRIPTION

Local exploitation of an integer overflow 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 occurs when parsing a client request for one of the
following functions:

  SProcRenderCreateLinearGradient
  SProcRenderCreateRadialGradient
  SProcRenderCreateConicalGradient

In each case, values are taken from the client request and used to
calculate the number of bytes to swap in the client request data. The
calculations attempt to verify that the byte swap range if valid, but
they are incorrect, which can lead to heap memory being corrupted.

III. ANALYSIS

Exploitation allows an attacker to execute arbitrary code with the
privileges of the X server, typically root. 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. One method of gaining the
required access is to remotely exploit a vulnerability in, for example,
a graphical Web browser. This would then allow an attacker to exploit
this vulnerability and elevate their privileges to root.

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 this vulnerability in X.org X11
version R7.3, 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 Render extension. However, doing so may
seriously impair the functionality of the server. Adding the following
lines to the X configuration file will disable the Render extension:

  Section "Extensions"
    Option "RENDER" "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-2008-2362 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
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