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             AUSCERT External Security Bulletin Redistribution

                       ESB-2006.0662 -- [UNIX/Linux]
         Multiple Vendor X Server CID-keyed Fonts 'scan_cidfont()'
                      Integer Overflow Vulnerability
                             14 September 2006


        AusCERT Security Bulletin Summary

Product:              XFree86
Publisher:            iDEFENSE
Operating System:     UNIX variants (UNIX, Linux, OSX)
Impact:               Root Compromise
                      Execute Arbitrary Code/Commands
Access:               Existing Account
CVE Names:            CVE-2006-3740

Original Bulletin:    

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

iDefense Security Advisory 09.12.06
Sep 12, 2006


The X Window System is a graphical windowing system based on a
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 X.org
server version 6.8.2. Analysis of the source code for the current
versions of the X.org 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
ftp://xfree86.org/pub/XFree86/4.6.0/fixes/fix-01 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 (http://cve.mitre.org), 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 iDefense, Inc.

Permission is granted for the redistribution of this alert
electronically. It may not be edited in any way without the express
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Disclaimer: The information in the advisory is believed to be accurate
at the time of publishing based on currently available information. Use
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There are no warranties with regard to this information. Neither the
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or consequential loss or damage arising from use of, or reliance on,
this information.

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