Multiple Vendor X Server BDF Font Parsing Integer Overflow Vulnerability

iDefense Security Advisory 04.03.07
http://labs.idefense.com/intelligence/vulnerabilities/
Apr 03, 2007

I. BACKGROUND

The X Window System (or X11) is a graphical windowing system used on
Unix-like systems. It is based on a client/server model. More
information about about The X Window system is available at the
following URL.

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

II. DESCRIPTION

Local exploitation of an integer overflow vulnerability in multiple
vendors' implementations of the X Window System server BDF font parsing
component could allow execution of arbitrary commands with elevated
privileges.

The vulnerability specifically exists in the parsing of BDF fonts. When
the X server encounters a specially crafted BDF font, an integer
overflow occurs leading to a potentially exploitable heap overflow
condition.

III. ANALYSIS

Exploitation allows attackers to execute arbitrary code with elevated
privileges.

As the X11 server requires direct access to video hardware, it runs with
elevated privileges. A user compromising an X server would gain those
permissions.

In order to exploit this vulnerability, an attacker would need to be
able to cause the X server to use a maliciously constructed font. The
X11 server contains multiple methods for a user to define additional
paths to look for fonts. An exploit has been developed using the "-fp"
command line option to the X11 server to pass the location of the
attack to the server. It is also possible to use "xset" command with
the "fp" option to perform an attack on an already running server.

Some distributions allow users to start the X11 server only if they are
logged on at the console, while others will allow any user to start it.

Attempts at exploiting this vulnerability may put the console into an
unusable state. This will not prevent repeated exploitation attempts.

IV. DETECTION

iDefense has confirmed the existence of this vulnerability in X.Org
X11R7.1. Older versions are suspected to be vulnerable. Additionally,
it is reported that the freetype library is also vulnerable.

V. WORKAROUND

iDefense is currently unaware of any effective workaround for this
issue.

VI. VENDOR RESPONSE

The X.Org Foundation has addressed this vulnerability with source code
patches. More information can be found from their advisory at the
following URL.

http://lists.freedesktop.org/archives/xorg-announce/2007-april/0286.html

The freetype developers have committed a fix for this issue to their CVS
repository. Future releases will contain this fix.

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2007-1351 to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org/), which standardizes names for
security problems.

VIII. DISCLOSURE TIMELINE

02/21/2007  Initial vendor notification
03/28/2007  Initial vendor response
04/03/2007  Coordinated public disclosure

IX. CREDIT

This vulnerability was discovered by Greg MacManus of iDefense Labs.

Get paid for vulnerability research
http://labs.idefense.com/methodology/vulnerability/vcp.php

Free tools, research and upcoming events
http://labs.idefense.com/

X. LEGAL NOTICES

Copyright © 2007 iDefense, Inc.

Permission is granted for the redistribution of this alert
electronically. It may not be edited in any way without the express
written consent of iDefense. If you wish to reprint the whole or any
part of this alert in any other medium other than electronically,
please e-mail customerservice@idefense.com for permission.

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
author nor the publisher accepts any liability for any direct,
indirect, or consequential loss or damage arising from use of, or
reliance on, this information.