AIX crontab Vulnerability

Original issue date: May 26, 1992
Last revised: September 19, 1997
Attached copyright statement 

A complete revision history is at the end of this file.

The Computer Emergency Response Team/Coordination Center (CERT/CC) has received information concerning a vulnerability in crontab(1) in version 3.2 of IBM's AIX operating system.

IBM is aware of this problem and a fix is available as apar number "ix26997" for AIX version 3.2.  The version information for the patched /usr/bin/crontab is shown in the following what(1) output:

% what /usr/bin/crontab
04 1.23 com/cmd/cntl/cron/crontab.c, cmdcntl, bos320, 9218320f 4/8/92 11:50:42
07 1.8  com/cmd/cntl/cron/permit.c, bos, bos320 4/25/91 17:16:59
11 1.15  com/cmd/cntl/cron/cronsub.c, bos, bos320 8/18/91 20:42:32
06 1.9  com/cmd/cntl/cron/funcs.c, bos, bos320 6/8/91 21:22:40
If your crontab contains older modules than the above output indicates, we suggest that you install the fix.

I. Description

The distributed version of /usr/bin/crontab contains a security vulnerability.

II. Impact

Local users can gain unauthorized root access to the system.

III. Solution

The CERT/CC suggests that sites install the fix that IBM has made available.  As an interim step, we suggests that sites prevent all non-root users from running /usr/bin/crontab by removing (or renaming) the /var/adm/cron/cron.allow and /var/adm/cron/cron.deny files.
  • Obtain the fix from IBM Support.
    1. To order from IBM call 1-800-237-5511 and ask that the fix be shipped.  Patches may be obtained               outside the U.S. by contacting your local IBM
    2. If you are on the Internet, use anonymous ftp to obtain the fix from (

      Patch           Filename                Checksum
      AIX 3.2         pub/aix3/cronta.tar.Z   02324   154
      The patch must be retrieved using binary mode.
  • Install the fix following the instructions in the README file.

The CERT/CC would like to thank Fuat Baran of Advanced Network & Services, Inc. for bringing this security vulnerability to our attention and IBM for their quick response to this problem.

This document is available from:

CERT/CC Contact Information

Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
Postal address:
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890

CERT/CC personnel answer the hotline 08:00-17:00 EST(GMT-5) / EDT(GMT-4) Monday through Friday; they are on call for emergencies during other hours, on U.S. holidays, and on weekends.

Using encryption

We strongly urge you to encrypt sensitive information sent by email.  Our public PGP key is available from

If you prefer to use DES, please call the CERT hotline for more information.

Getting security information

CERT publications and other security information are available from our web site

* "CERT" and "CERT Coordination Center" are registered in the U.S. Patent and Trademark Office.

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Conditions for use, disclaimers, and sponsorship information

Copyright 1992 Carnegie Mellon University.

Revision History
September 19,1997  Attached copyright statement