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Multiple Vendor X Server XFree86-Misc Extension Invalid Array Index Vulnerability

17.01.08

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

DESCRIPTION

Local exploitation of an invalid array index 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 exists within the XFree86-Misc extension. When processing a request, a 32-bit value from the client's request is used as an index into an array of structures. This structure contains an array of function pointers, one of which is used later in the request handling. By supplying a large array index, an arbitrary function pointer can be dereferenced. This results in the execution of arbitrary code.

ANALYSIS

Exploitation allows an attacker to execute arbitrary code with root privileges. 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.

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 the vulnerability can be exploited remotely.

DETECTION

iDefense has confirmed the existence of this vulnerability in X.org X11 version R7.3. Previous versions may also be affected.

WORKAROUND

If the XFree86-Misc extension has not been built-in to the server, then it 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 XFree86-Misc" EndSubSection EndSection 

To check if the 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.

VENDOR RESPONSE

The X.Org team has addressed this vulnerability 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

CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2007-5760 to this issue. This is a candidate for inclusion in the CVE list (http://cve.mitre.org/), which standardizes names for security problems.

DISCLOSURE TIMELINE

11/29/2007 Initial vendor response
11/30/2007 Initial vendor notification
01/17/2008 Coordinated public disclosure

CREDIT

This vulnerability was reported to VeriSign iDefense by regenrecht.

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LEGAL NOTICES

Copyright © 2008 Verisign, Inc.

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