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Multiple Vendor X Server CID-keyed Fonts 'CIDAFM()' 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
'CIDAFM()' 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 'CIDAFM()' function of the
code responsible for handling AFM (Adobe Font Metrics) files. The number
of character metrics is obtained from the "StartCharMetrics" line of an
AFM file and that value is then multiplied by the size of a single
character metric record in order to calculate the space required to
store the metrics. If the result of the multiplication is larger than
the largest value that can be held in an integer, the amount actually
allocated will be much smaller. Following this, the function attempts to
read as many metric records as were specified on the line into that
memory. As the contents of the file can be specified by a local user,
and as the function will stop reading if an error is detected in the
input, a controlled heap overflow may occur which may allow the
execution of arbitrary code.

ANALYSIS

Successful local exploitation allows an attacker to execute arbitrary
code 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-2006-3739 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
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