iDefense Security Advisory 05.20.09: IBM AIX libc MALLOCDEBUG File Overwrite Vulnerability

iDefense Security Advisory 05.19.09
http://labs.idefense.com/intelligence/vulnerabilities/
May 19, 2009

I. BACKGROUND

IBM's AIX is a Unix operating system based on System V, which runs on
the PowerPC (PPC) architecture. For more information, visit the product
web site at the following URL.

http://www.ibm.com/systems/power/software/aix/index.html

II. DESCRIPTION

Local exploitation of a file overwrite vulnerability in IBM Corp.'s
Advanced Interactive eXecutive (AIX) could allow an attacker to
overwrite arbitrary files and execute arbitrary code.

The AIX libc implementation of malloc includes a debugging mechanism
that is initiated by setting the MALLOCTYPE and MALLOCDEBUG environment
variables. This debugging feature writes to a user-specified log file
under certain conditions. There is a gap in time between the checks to
see if the file is a symbolic link and the process of opening the file.
If an attacker can change the file to be a symbolic link to another file
within this time frame, it is possible to cause a set-uid binary to
write to files owned by privileged users.

III. ANALYSIS

Exploitation of this issue allows an attacker to overwrite arbitrary
files and execute arbitrary code as root.

IV. DETECTION

iDefense confirmed the existence of this vulnerability in IBM Corp.'s
AIX version 5.3. Other versions may also be affected.

V. WORKAROUND

iDefense is currently unaware of any workaround for this issue.

VI. VENDOR RESPONSE

IBM has released a patch which addresses this issue. For more
information, consult their advisory at the following URL:

http://aix.software.ibm.com/aix/efixes/security/libc_advisory.asc

VII. CVE INFORMATION

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

VIII. DISCLOSURE TIMELINE

01/05/2008 - PoC Requested
12/16/2008 - Initial Response
12/16/2008 - Initial notification
01/06/2009 - PoC Sent
05/19/2009 - Coordinated public Disclosure

IX. CREDIT

The discoverer of this vulnerability wishes to remain anonymous.

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X. LEGAL NOTICES

Copyright © 2009 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.