Apple Mac OS X passwd Arbitrary Binary File Creation/Modification

iDefense Security Advisory 03.02.06
http://www.idefense.com/application/poi/display?type=vulnerabilities
March 02, 2006

I. BACKGROUND

Mac OS X is an operating system for the Apple family of microcomputers.
More information is available at the following link:

    http://www.apple.com/macosx/

II. DESCRIPTION

Local exploitation of a design error in version 10.3.9 of Apple Computer
Inc.'s Mac OS X could allow arbitrary files to be overwritten with user
supplied contents.

The /usr/bin/passwd binary is a setuid application which allows users to
change their password. There are two related vulnerabilities.

The first vulnerability occurs because the Mac OS X version of the
passwd utility accepts options specifying which password database to
operate on. The passwd binary does not check that the user has
permissions to create a file in the location specified and does not set
the created file permissions. By setting the file creation mask to 0 a
user can create arbitrary files owned by root, with permissions which
allow any user to change the contents.

The second vulnerability exists in the insecure creation of temporary
files with predictable names. The temporary filename created by the
process is in the form /tmp/.pwtmp.<pid> where <pid> is the process id
of the passwd process. By creating a symbolic link to the target file,
and then changing the password, it is possible to put controllable
contents into the target file.

III. ANALYSIS

Successful exploitation of either of these vulnerabilities would allow a
local attacker to gain elevated privileges in a number of ways.

In the case of the first vulnerability, a new file could be created in
the  /etc directory, such as etc/rc.local_tuning, which is sourced if it
exists during the system start up process as the root user.

The second vulnerability would allow an attacker overwrite a file with
user controlled contents. This can be leveraged to provide privilege
escalation by, for example, creating a new /etc/sudoers file.

IV. DETECTION

iDefense has confirmed the existence of this vulnerability in Mac OS X
Version 10.3.9. In addition, the following versions been confirmed by
the vendor to  be vulnerable:

 * Mac OS X Server Version 10.3.9
 * Mac OS X Version 10.4.5
 * Mac OS X Server Version 10.4.5
 
It is suspected that all prior releases are vulnerable.

V. WORKAROUND

Remove the setuid bit from the /usr/bin/passwd binary by executing the
following command as root:

  chmod -s /usr/bin/passwd

This workaround will prevent non-root users from being able to change
their password.

VI. VENDOR RESPONSE

Apple have released an update for this vulnerability, details of which
are available at the following location:

    http://docs.info.apple.com/article.html?artnum=61798

Apple security updates are available via the Software Update mechanism:

    http://docs.info.apple.com/article.html?artnum=106704
   
Apple security updates are also available for manual download:

    http://www.apple.com/support/downloads

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
following names to these issues:

   CVE-2005-2713 - passwd file creation and permissions

   CVE-2005-2714 - temporary file symlink problem

VIII. DISCLOSURE TIMELINE

08/23/2005  Initial vendor notification
08/27/2005  Initial vendor response
03/02/2006  Coordinated public disclosure

IX. CREDIT

Discovery of these vulnerabilities are credited to vade79.

Get paid for vulnerability research
http://www.idefense.com/poi/teams/vcp.jsp

Free tools, research and upcoming events
http://labs.idefense.com

X. LEGAL NOTICES

Copyright (c) 2006 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
email 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.