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 create a denial of service (DoS) condition on
the affected X server.

The vulnerability exists within the ProcRenderCreateCursor() function.
When parsing a client request, values are taken from the request and
used in an arithmetic operation that calculates the size of a dynamic
buffer. This calculation can overflow, which results in an undersized
buffer being allocated. This leads to an invalid memory access, which
crashes the X server.

III. ANALYSIS

Exploitation allows an attacker to crash the Xserver; code execution is
not possible. 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.

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 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-2008-2361 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.
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