Public Vulnerability Reports

Multiple Vendor X Server TOG-CUP Extension Information Disclosure Vulnerability



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.


Local exploitation of an information disclosure vulnerability in the X.Org X server, as included in various vendors' operating system distributions, could allow an attacker to gain access to sensitive information stored in server memory.

The vulnerable code exists within the TOG-CUP extension. A 32-bit client supplied value is taken directly from the request, and then used as an index into an array. The value located at this index is then stored into a buffer which is later sent to the client. This allows a client to read memory from arbitrary locations in server memory.


Exploitation allows an attacker to read arbitrary memory within the X Server's address space.

By itself, the impact of this vulnerability is minimal. However, when coupled with a code execution vulnerability, this vulnerability can be used to greatly increase the reliability of an exploit.

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.


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


If the TOG-CUP 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 TOG-CUP" EndSubSection EndSection 

To check if the extension is built-in to the server, grep the output of the X Server log file as shown below.

 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.


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.


The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2007-6428 to this issue. This is a candidate for inclusion in the CVE list (, which standardizes names for security problems.


11/29/2007 Initial vendor notification
12/13/2007 Initial vendor response
01/17/2008 Coordinated public disclosure


This vulnerability was reported to VeriSign iDefense by regenrecht.

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Copyright © 2008 Verisign, Inc.

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