Attachment 185373 Details for Bug 345131

pub_Multiple Vendor X Server MIT-SHM Extension Integer Overflow Vulnerability.txt

pub_Multiple Vendor X Server MIT-SHM Extension Integer Overflow Vulnerability.txt (text/plain), 4.66 KB, created by Thomas Biege on 2007-11-30 08:13:44 UTC

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Creator: Thomas Biege

Created: 2007-11-30 08:13:44 UTC

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>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 an integer overflow 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 vulnerability exists within the code responsible for creating a
>pixmap in shared memory. When allocating the pixmap, the server uses
>values from the request to verify that the requested size is not
>greater than the allocated shared memory. The calculation can overflow,
>which leads to the overwriting of arbitrary addresses in memory that
>aren't part of the shared memory segment.
>
>The vulnerable code is shown below:
>
>148  #define VERIFY_SHMSIZE(shmdesc,offset,len,client) \
>
>149  { \
>
>150   if ((offset + len) > shmdesc->size) \
>
>151   { \
>
>152     return BadAccess; \
>
>153   } \
>
>154  }
>
>Same file, ProcShmCreatePixmap():
>
>1070  VERIFY_SHMSIZE(shmdesc, stuff->offset,
>
>1071   PixmapBytePad(stuff->width, stuff->depth) * stuff->height,
>
>1072   client);
>
>1073  pMap = (*shmFuncs[pDraw->pScreen->myNum]->CreatePixmap)(
>
>1074   pDraw->pScreen, stuff->width,
>
>1075   stuff->height, stuff->depth,
>
>1076   shmdesc->addr + stuff->offset);
>
>The calculcation in the VERIFY_SHMSIZE() macro can overflow if len +
>offset > 0xffffffff. These values are then used later to create the
>pixmap in the CreatePixmap() function. The result is the creation of
>zero sized pixmap, and the ability to overwrite arbitrary values in
>memory by indexing beyond the end of the shared memory segment.
>
>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
>
>If the MIT-SHM extension has not been built-in to the server, then it
>can be prevented from loading by inserting the following into the X
>configuration file (usually in /etc/X11/xorg.conf):
>
>Section "Module"
>
>        SubSection "extmod"
>
>                Option "omit MIT-SHM"
>
>        EndSubSection
>
>EndSection
>
>To check if the extension is built-in to the server, grep the output of
>the X Server log file:
>
>grep built-in /var/log/Xorg.0.log
>
>The result will list all built in extensions. The location of the log
>file may need to be changed.
>
>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.
>
>Get paid for vulnerability research
>http://labs.idefense.com/methodology/vulnerability/vcp.php
>
>Free tools, research and upcoming events
>http://labs.idefense.com/
>
>X. LEGAL NOTICES
>
>Copyright © 2007 iDefense, Inc.
>
>Permission is granted for the redistribution of this alert
>electronically. It may not be edited in any way without the express
>written consent of iDefense. If you wish to reprint the whole or any
>part of this alert in any other medium other than electronically,
>please e-mail customerservice@idefense.com for permission.
>
>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.

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