IBM AIX 5.2 crontab BSS Buffer Overflow Vulnerability

iDefense Security Advisory 10.30.07
http://labs.idefense.com/intelligence/vulnerabilities/
Oct 30, 2007

I. BACKGROUND

The crontab program is a user utility that enables users to create,
remove, and edit cron jobs. The cron jobs will then later be executed,
on behalf of the user, at the specified time. Under AIX, the crontab
program is installed by default and is set-uid root. More information
can be found at the URL shown.

http://publib.boulder.ibm.com/infocenter/pseries/v5r3/topic/com.ibm.aix.cmds/doc/aixcmds1/crontab.htm

II. DESCRIPTION

Local exploitation of a buffer overflow vulnerability in the crontab
program of IBM Corp.'s AIX 5.2 operating system allows attackers to
execute arbitrary code with root privileges.

The problem specifically exists within the main function. While
processing command line arguments, the crontab program will copy a
user-supplied argument to a fixed size BSS (data segment) buffer. Since
no bounds checking is performed, it's possible to overwrite a large
portion of the data stored in the BSS memory area.

III. ANALYSIS

Exploitation allows an attacker to execute arbitrary code with root
privileges. Local access is required to execute the crontab program.

IV. DETECTION

iDefense has confirmed the existence of this vulnerability within AIX
version 5.2. Previous versions are suspected to be vulnerable. AIX 5.3
does not appear to be vulnerable.

V. WORKAROUND

Removing the set-uid bit from the crontab program will protect against
exploitation. However, doing so will render the program unusable.

VI. VENDOR RESPONSE

IBM Corp. has addressed this vulnerability by releasing interim fixes.
More information can be found via the Bulletins tab of IBM's
Subscription Service for UNIX and Linux servers. You can reach this
service by clicking the URL shown below.

http://www14.software.ibm.com/webapp/set2/subscriptions/pqvcmjd?mode=1

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2007-4621 to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org/), which standardizes names for
security problems.

VIII. DISCLOSURE TIMELINE

08/29/2007  Initial vendor notification
09/12/2007  Initial vendor response
10/30/2007  Coordinated public disclosure

IX. CREDIT

The discoverer of this vulnerability wishes to remain anonymous.

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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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