Multiple Vendor X Font Server Multiple Vulnerabilities

iDefense Security Advisory 10.02.07
http://labs.idefense.com/intelligence/vulnerabilities/
Oct 02, 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. The X Window
System font server (xfs) is used to render fonts for the X server. More
information can be found at the following URLs.

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

http://www.x.org/wiki/

II. DESCRIPTION

Remote exploitation of a multiple vulnerabilities in X.Org Foundation's
X Font Server, as included in various vendors' operating system
distributions, could allow an attacker to execute arbitrary code.

An integer overflow vulnerability exists within the handlers for the
QueryXBitmaps and QueryXExtents protocol requests. Both requests result
in a call to the build_range() function. This function takes a 32bit
integer from the request, and uses it in an arithmetic operation that
calculates the size of a dynamic buffer. This calculation can overflow,
which leads to an improperly sized memory allocation. This results in a
heap overflow.

Additionally, a heap corruption vulnerability exists within the handlers
for the QueryXBitmaps and QueryXExtents protocol requests. Both requests
result in a call to the swap_char2b() function. This function takes a
32bit integer from the request, and uses it as the number of bytes to
swap in the request buffer. This allows an attacker to swap an
arbitrary number of bytes on the heap.

III. ANALYSIS

Exploitation of these vulnerabilities could result in the execution of
arbitrary code with the privileges of the X Font Server, usually 'xfs'.

 On current versions of Solaris, these vulnerabilities are remotely
exploitable. The XFS service is turned on by default, and listens on
TCP port 7100. On modern Linux systems, these vulnerabilities are only
locally exploitable since the server is configured to listen on a UNIX
socket only.

IV. DETECTION

iDefense has confirmed the existence of these vulnerabilities in XFS
version X11R7.2-1.0.4. Previous versions may also be affected.

V. WORKAROUND

On Solaris, stop XFS from listening remotely by disabling it via the
service manager.

VI. VENDOR RESPONSE

The X.Org team has addressed these vulnerabilities with the release of
XFS version 1.0.5. Additionally, a patch for version 1.0.4 has been
made available. For more information, consult the X.Org advisory at the
following URL.

http://lists.freedesktop.org/archives/xorg-announce/2007-October/000416.html

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2007-4568 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

09/05/2007  Initial vendor notification
09/08/2007  Initial vendor response
10/02/2007  Public disclosure

IX. CREDIT

These vulnerabilities were discovered by Sean Larsson of VeriSign
iDefense Labs.

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X. LEGAL NOTICES

Copyright © 2007 iDefense, Inc.

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