iDefense Security Advisory XX.XX.XX
http://labs.idefense.com/intelligence/vulnerabilities/
MMM DD, YYYY

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 links.

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.

 Multiple vulnerabilities are present in the Render extension in the
code responsible for reading in client requests. The following
functions contain vulnerabilities:

AllocateGlyph() ProcRenderCreateCursor()
SProcRenderCreateLinearGradient()

In each case, values are taken from the client request and used in
arithmetic operations that calculate the size of dynamic buffers. These
calculations can overflow, which results in undersized buffers being
allocated. The allocated buffers are then overflowed with data from the
client request.

III. ANALYSIS

Exploitation allows an attacker to execute arbitrary code with the
privileges of the X server, typically root. 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. 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 these vulnerabilities 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

iDefense is currently working with the vendor to address these issues.
Since there is no vendor fix at this time, please handle this
information with sensitivity.

VII. CVE INFORMATION

A Mitre Corp. Common Vulnerabilities and Exposures (CVE) number has not
been assigned yet.

VIII. DISCLOSURE TIMELINE

XX/XX/XXXX  Initial vendor notification

IX. CREDIT

This vulnerability was reported to iDefense by regenrecht.

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X. LEGAL NOTICES

Copyright © 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.