Public Vulnerability Reports

Multiple Vendor X Server fonts.dir File Parsing Integer Overflow Vulnerability

04.03.07

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

DESCRIPTION

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

The vulnerability specifically exists in the parsing of the "fonts.dir" font information file. When the element count on the first line of the file specifies it contains more than 1,073,741,824 (2 to the power of 30) elements, a potentially exploitable heap overflow condition occurs.

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.

DETECTION

iDefense has confirmed the existence of this vulnerability in X.Org X11R7.1. Older versions are suspected to be vulnerable.

WORKAROUND

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

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

CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2007-1352 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

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

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/

LEGAL NOTICES

Copyright © 2007 Verisign, 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 customer service 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.