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Multiple Vendor X Server Render Extension ProcRenderCreateCursor() Integer Overflow Vulnerability

11.06.08

BACKGROUND

The X Window System is a graphical windowing system based on a client/server model. The Render extension is used to provide Porter-Duff image compositing for the X server. It is built into many X servers by default, and loaded as a default module when it is not. For more information, see the vendor's site found at the following link.

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

DESCRIPTION

Local exploitation of an integer overflow vulnerability in the X.Org X server, as included in various vendors' operating system distributions, could allow an attacker to create a denial of service (DoS) condition on the affected X server.

The vulnerability exists within the ProcRenderCreateCursor() function. When parsing a client request, values are taken from the request and used in an arithmetic operation that calculates the size of a dynamic buffer. This calculation can overflow, which results in an undersized buffer being allocated. This leads to an invalid memory access, which crashes the X server.

ANALYSIS

Exploitation allows an attacker to crash the Xserver; code execution is not possible. To exploit this vulnerability, an attacker must be able 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 is to remotely exploit a vulnerability in, for example, a graphical Web browser.

If an X Server is configured to listen for TCP-based client connections, and a client is granted access to create sessions (via the xhosts file), then these vulnerabilities can be exploited remotely.

DETECTION

iDefense has confirmed the existence of these this vulnerability in X.org X11 version R7.3, with all patches as of 03/01/08 applied. Previous versions may also be affected.

WORKAROUND

Access to the vulnerable code can be prevented by preventing the X server from loading the Render extension. However, doing so may seriously impair the functionality of the server. Adding the following lines to the X configuration file will disable the Render extension:

 Section "Extensions" Option "RENDER" "disable" EndSection 

VENDOR RESPONSE

The X.Org team has addressed this vulnerability by releasing patches for version 1.4 of the X server. For more information, consult the X.Org advisory at the following URL.

http://lists.freedesktop.org/archives/xorg/2008-June/036026.html

CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2008-2361 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

03/26/2008 Initial vendor notification
03/26/2008 Initial vendor response
06/11/2008 Coordinated public disclosure

CREDIT

This vulnerability was reported to iDefense by regenrecht.

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LEGAL NOTICES

Copyright © 2008 Verisign, Inc.

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