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

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. More
information about about The X Window system is available at the
following URL.

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

II. DESCRIPTION

Local exploitation of a memory corruption vulnerability in the X.Org X
server, as included in various vendors' operating system distributions,
could allow an attacker to execute arbitrary code with the privileges of
the X server, typically root.


The vulnerable code exists within multiple functions in the XInput
extension, and occurs when swapping the byte order of client requests.
By claiming to be sending integer data in the opposite byte order of
the server, the client can cause the server to swap the byte order of a
request. The number of bytes swapped can be controlled by the client,
and is not properly validated by the server. This results in the
corruption of heap memory located after the client's request data.

The vulnerable code is shown below:

From Xi/chgkmap.c:SProcXChangeDeviceKeyMapping()

87  REQUEST_AT_LEAST_SIZE(xChangeDeviceKeyMappingReq);

88  p = (long *)&stuff[1];

89  count = stuff->keyCodes * stuff->keySymsPerKeyCode;

90  for (i = 0; i < count; i++) {

91   swapl(p, n);

92   p++;

93  }

On line 89, the count value is calculated by multiplying two values
taken directly from the client request. The for loop then proceeds to
swap the byte order of the request data, and eventually of other heap
memory as well.

Duplicate vulnerabilities are also found in the following functions:

Xi/chgprop.c, SProcXChangeDeviceDontPropagateList()

Xi/grabdev.c, SProcXGrabDevice()

Xi/grabdevb.c, SProcXGrabDeviceButton()

Xi/grabdevk.c, SProcXGrabDeviceKey()

Xi/selectev.c, SProcXSelectExtensionEvent()

Xi/sendexev.c, SProcXSendExtensionEvent()

III. ANALYSIS

Exploitation allows an attacker to execute arbitrary code with root
privileges. In order 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. This
would then allow an attacker to exploit this vulnerability and elevate
their privileges to root.

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 the vulnerability can be exploited remotely.

IV. DETECTION

iDefense has confirmed the existence of this vulnerability in X.org X11
version R7.3. Previous versions may also be affected.

V. WORKAROUND

iDefense is currently unaware of any workarounds for this issue. The
XInput extension is normally compiled into the X Server; as such, it's
not possible to disable it from being loaded in the configuration file.

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 reported to iDefense by regenrecht.

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X. LEGAL NOTICES

Copyright © 2007 iDefense, Inc.

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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.