Public Vulnerability Reports

Multiple Vendor X Server CID-keyed Fonts 'scan_cidfont()' Integer Overflow 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 link:


Local exploitation of an integer overflow vulnerability in the
'scan_cidfont()' 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.

The vulnerability specifically exists in the handling of 'CMap' and
'CIDFont' font data. When parsing this information no checks are made
that the count of items for the 'begincodespacerange', 'cidrange' and
'notdefrange' sections.

In addition to a 'standard' integer overflow, the implementation of
'vm_alloc()' makes it possible to overwrite memory before the allocated


Successful local 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
server version 6.8.2. Analysis of the source code for the current
versions of the and XFree86 servers indicates that current
versions of both are vulnerable. Previous versions may also be


Access to the vulnerable code can be prevented by removing the entry
for the Type1 font module from your Xservers 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 "type1"

This will prevent Type 1 fonts from loading, which may affect the
appearance or operation of some applications.


The X.Org foundation has addressed this vulnerability with libXfont
version 1.2.1. Additionally, patches have been made available for older

The XFree86 Project Inc. is making available a source patch available at that, in part,
addresses this vulnerability.


The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2006-3740 to this issue. This is a candidate for inclusion in
the CVE list (, which standardizes names for
security problems.


08/25/2006 Initial vendor notification
08/25/2005 Initial vendor response (X.Org)
09/10/2006 Initial vendor response (XFree86)
09/12/2006 Coordinated public disclosure


The discoverer of this vulnerability wishes to remain anonymous.

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Copyright © 2006 Verisign, Inc.

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