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

09.12.06

BACKGROUND

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:

http://en.wikipedia.org/wiki/X_Window_System

DESCRIPTION

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
region.

ANALYSIS

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.

DETECTION

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
affected.

WORKAROUND

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.

VENDOR RESPONSE

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

"
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.
"

CVE INFORMATION

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.

DISCLOSURE TIMELINE

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

CREDIT

The discoverer of this vulnerability wishes to remain anonymous.

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LEGAL NOTICES

Copyright © 2006 Verisign, Inc.

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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
of the information constitutes acceptance for use in an AS IS condition.
There are no warranties with regard to this information. Neither the
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