Multiple Vendor X Server fonts.dir File Parsing Integer Overflow Vulnerability
iDefense Security Advisory 04.03.07 http://labs.idefense.com/intelligence/vulnerabilities/ Apr 03, 2007
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.
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.
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.
iDefense has confirmed the existence of this vulnerability in X.Org X11R7.1. Older versions are suspected to be vulnerable.
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.
VII. 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.
VIII. DISCLOSURE TIMELINE
02/21/2007 Initial vendor notification 02/21/2007 Initial vendor response 04/03/2007 Coordinated public disclosure
This vulnerability was discovered by Greg MacManus of iDefense Labs.
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X. LEGAL NOTICES
Copyright © 2007 iDefense, Inc.
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