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 © 2008 iDefense, Inc.
Permission is granted for the redistribution of this alert electronically. It may not be edited in
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permission.
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.
