Multiple Vendor X Font Server build_range() Server Integer Overflow Vulnerability

iDefense Security Advisory XX.XX.XX
http://labs.idefense.com/intelligence/vulnerabilities/
MMM DD, YYYY

I. BACKGROUND

Xfs is the X Window System font server. It is used to render fonts for
the X server. More information can be found on the vendor's website.

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

II. DESCRIPTION

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

The vulnerability exists within the handlers for the QueryXBitmaps and
QueryXExtents protocol requests. Both of these 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.

III. ANALYSIS

Successful exploitation of this vulnerability results in the execution
of arbitrary code with the privileges of the X Font Server, usually
'xfs'. Exploitation of this vulnerability is difficult due to the
nature of the overflow. A large number of bytes are copied into a small
buffer, which usually results in a segmentation fault when unmapped
memory is written to. However, the attacker has significant control
over the heap, and it may be possible to create a memory layout that
makes exploitation possible.

On modern Linux systems, this vulnerability is only locally exploitable.
XFS no longer listens on tcp port 7100 on current distributions. On
current versions of Solaris, this vulnerability is remotely
exploitable. The XFS service is turned on by default, and listens on
tcp port 7100.

IV. DETECTION

iDefense has confirmed the existence of this vulnerability 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

iDefense is currently working with the vendor to address this issue.
Since there is no vendor fix at this time, please handle this
information with sensitivity.

VII. CVE INFORMATION

A Mitre Corp. Common Vulnerabilities and Exposures (CVE) number has not
been assigned yet.

VIII. DISCLOSURE TIMELINE

XX/XX/XXXX  Initial vendor notification

IX. CREDIT

This vulnerability was discovered by Sean Larsson, iDefense Labs.

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X. LEGAL NOTICES

Copyright © 2007 iDefense, Inc.

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at the time of publishing based on currently available information. Use
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