Public Vulnerability Reports

Multiple Vendor X Server EVI and MIT-SHM Extensions Integer Overflow Vulnerabilities



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 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.

One vulnerability exists within the EVI extension. When processing a request, the server uses a 32-bit value provided by the client in an arithmetic operation that calculates the number of bytes to allocate for a dynamic buffer. This operation can overflow, which later leads to a potentially exploitable heap overflow.

Another vulnerability exists within the MIT-SHM extension. When allocating a pixmap, the server uses values from the request to verify that the requested size is not greater than the amount of allocated shared memory. The calculation can overflow, which leads to the overwriting of arbitrary addresses in memory that aren't part of the shared memory segment.


Exploitation allows an attacker to execute arbitrary code with root privileges. In order to exploit these vulnerabilities, 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 these vulnerabilities can be exploited remotely.


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


If the EVI or MIT-SHM extensions have not been built-in to the server, they 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 Extended-Visual-Information" Option "omit MIT-SHM" EndSubSection EndSection 

To check if an 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.


The X.Org team has addressed these vulnerabilities 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-2007-6429 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
11/29/2007 Initial vendor response
01/17/2008 Coordinated public disclosure


These vulnerabilities were reported to VeriSign iDefense by regenrecht.

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Copyright © 2008 Verisign, Inc.

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