Public Vulnerability Reports

Multiple Vendor X Server Render Extension ProcRenderAddGlyphs Memory Corruption Vulnerability



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.


Local exploitation of a memory corruption vulnerability in the "ProcRenderAddGlyphs" function in the X.Org and XFree86 X server could allow an attacker to execute arbitrary code with privileges of the X server, typically root.

This vulnerability specifically lies within the Render extension. Insufficient input validation exists when allocating memory for glyph management data structures. By sending a specially crafted X protocol request to the Render extension, an attacker can cause an exploitable memory corruption condition.


Successful exploitation allows an attacker to execute arbitrary as the root user. In order to exploit this vulnerability an attacker would require the ability 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 would be 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.


iDefense has confirmed the existence of this vulnerability in the X.Org server version 7.1-1.1.0. Previous versions may also be affected.


Access to the vulnerable code can be prevented when the Render extension is not built into the X binary. This can be accomplished by removing the entry for the Render extension from your X server's configuration file, often stored in /etc/X11 and named xorg.conf or XF86Config-4. To do this, remove the following line from the 'Module' section:

Load "render"

This will prevent the Render extension from loading, which may affect the appearance or operation of some applications.


The X.Org foundation has addressed this vulnerability within version 7.2 RC3 of X.Org's X server. Additionally, patches have been made available for older releases.


The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2006-6101 to this issue. This is a candidate for inclusion in the CVE list (, which standardizes names for security problems.


12/04/2006 Initial vendor notification
12/05/2006 Initial vendor response
01/09/2007 Coordinated public disclosure


This vulnerability was discovered by Sean Larsson, iDefense Labs.

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Copyright © 2006 Verisign, Inc.

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