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CERT* Advisory CA-97.18
Original issue date: June 12, 1997
Last revised: --
Topic: Vulnerability in the at(1) program
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The CERT Coordination Center has received reports of a buffer overflow
condition in some versions of the at(1) program. By carefully specifying the
data that overflows this buffer, any user can execute arbitrary commands as
root.
The CERT/CC team recommends installing a vendor patch if one is available
(see Section III.A). Until you can do so, we recommend disabling at(1) (see
Section III.B).
We will update this advisory as we receive additional information. Please
check advisory files regularly for updates that relate to your site.
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I. Description
The at(1) program can be used by local users to schedule commands to be
executed at a later time. When those commands are run, they are run as
the user who originally ran at(1). That user will be referred to as the
scheduling user.
As a precaution, the scheduling user's list of commands is stored in a
file in a directory that is not writable by other users. The file's
ownership is changed to that of the scheduling user, and that
information is used to define the identity of the process that runs the
commands when the appointed time arrives. These measures are intended
to prevent other users from changing the scheduling user's list of
commands or creating new lists to be executed as another user. To
achieve this additional level of security, the at(1) program runs as
set-user-id root.
Some versions of at(1) contain a programming defect that can result in a
buffer local to at(1) being overflowed. Through the careful specification
of the data that overflows this buffer, arbitrary commands can be executed
with the identity of at(1) process, root in this case.
II. Impact
Any user with an account on a system that contains a defective version
of at(1) can execute programs as root.
III. Solution
A. Install a patch from your vendor
Below is a list of vendors who have provided information about
at. Details are in Appendix A of this advisory; we will update
the appendix as we receive more information. If your vendor's
name is not on this list, the CERT/CC did not hear from that
vendor. Please contact your vendor directly.
Cray Research - A Silicon Graphics Company
Hewlett-Packard Company
IBM Corporation
Santa Cruz Operation, Inc. (SCO)
Silicon Graphics, Inc.
Sun Microsystems, Inc.
B. Until you are able to install the appropriate patch, we recommend
the following workaround:
Turn off at(1) by setting its mode to 0. Do the following as
root:
# chmod 0 /usr/bin/at
Note that the location of at(1) varies from system to system.
Consult your system's documentation for the correct location.
After you turn off the at(1) command, users will not be able to use
it. As an alternative to at(1), consider using the crontab(1)
command if your system provides it.
...........................................................................
Appendix A - Vendor Information
Below is a list of the vendors who have provided information for this
advisory. We will update this appendix as we receive additional information.
If you do not see your vendor's name, the CERT/CC did not hear from that
vendor. Please contact the vendor directly.
Cray Research - A Silicon Graphics Company
==========================================
Neither Unicos nor Unicos/mk is believed to be vulnerable.
Hewlett-Packard Company
=======================
Hewlett Packard is currently investigating the problem. We will update this
advisory through the CERT/CC when the investigation is complete.
IBM Corporation
===============
See the appropriate release below to determine your action.
AIX 3.2
-------
Apply the following fixes to your system:
PTF - U443452 U443486 U444191 U444206 U444213 U444243
APAR - IX60796
To determine if you have these PTFs on your system, run the following
commands:
lslpp -lB U443452 U443486 U444191 U444206 U444213 U444243
AIX 4.1
-------
Apply the following fixes to your system:
APAR - IX60894
APAR - IX60890
To determine if you have this APAR on your system, run the following
commands:
instfix -ik IX60894
instfix -ik IX60890
Or run the following commands:
lslpp -h bos.rte.cron
lslpp -h bos.rte.libc
Your version of bos.rte.cron should be 4.1.4.8 or later.
Your version of bos.rte.libc should be 4.1.4.18 or later.
AIX 4.2
-------
Apply the following fixes to your system:
APAR - IX60892
APAR - IX61125
To determine if you have this APAR on your system, run the following
commands:
instfix -ik IX60892
instfix -ik IX61125
Or run the following commands:
lslpp -h bos.rte.cron
lslpp -h bos.rte.libc
Your version of bos.rte.cron should be 4.2.0.1 or later.
Your version of bos.rte.libc should be 4.2.0.5 or later.
To Order
--------
APARs may be ordered using Electronic Fix Distribution (via FixDist)
or from the IBM Support Center. For more information on FixDist,
reference URL:
http://service.software.ibm.com/aixsupport/
or send e-mail to This email address is being protected from spambots. You need JavaScript enabled to view it. with a subject of "FixDist".
IBM and AIX are registered trademarks of International Business Machines
Corporation.
Santa Cruz Operation, Inc. (SCO)
================================
All SCO operating systems are vulnerable. SCO has made an interim fix
available for anonymous ftp:
ftp://ftp.sco.com/SSE/sse007.ltr.Z - cover letter
ftp://ftp.sco.com/SSE/sse007.tar.Z - replacement binaries
The fix includes binaries for the following SCO operating systems:
- SCO CMW+ 3.0
- SCO Open Desktop/Open Server 3.0, SCO UNIX 3.2v4
- SCO OpenServer 5.0
- SCO UnixWare 2.1
Silicon Graphics, Inc.
======================
At this time, Silicon Graphics does not have any public information
for the at(1) issue. Silicon Graphics has communicated with CERT
and other external security parties and is actively investigating
this issue. When more Silicon Graphics information (including any
possible patches) is available for release, that information will
be released via the SGI security mailing list, wiretap.
For subscribing to the wiretap mailing list and other SGI security
related information, please refer to the Silicon Graphics Security
Headquarters website located at:
http://www.sgi.com/Support/Secur/security.html
Sun Microsystems, Inc.
======================
Sun will be producing patches.
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Technical information for this advisory was drawn in part from a posting by
Don Farmer to the bugtraq mailing list. Thanks to Wolfgang Ley of DFN-CERT for
his help in developing this advisory.
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If you believe that your system has been compromised, contact the CERT
Coordination Center or your representative in the Forum of Incident Response
and Security Teams (see http://www.first.org/team-info).
CERT/CC Contact Information
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Email This email address is being protected from spambots. You need JavaScript enabled to view it.
Phone +1 412-268-7090 (24-hour hotline)
CERT personnel answer 8:30-5:00 p.m. EST(GMT-5) / EDT(GMT-4)
and are on call for emergencies during other hours.
Fax +1 412-268-6989
Postal address
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890
USA
Using encryption
We strongly urge you to encrypt sensitive information sent by email. We can
support a shared DES key or PGP. Contact the CERT/CC for more information.
Location of CERT PGP key
ftp://info.cert.org/pub/CERT_PGP.key
Getting security information
CERT publications and other security information are available from
http://www.cert.org/
ftp://info.cert.org/pub/
CERT advisories and bulletins are also posted on the USENET newsgroup
comp.security.announce
To be added to our mailing list for advisories and bulletins, send
email to
This email address is being protected from spambots. You need JavaScript enabled to view it.
In the subject line, type
SUBSCRIBE your-email-address
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Copyright 1997 Carnegie Mellon University
This material may be reproduced and distributed without permission provided
it is used for non-commercial purposes and the copyright statement is
included.
* Registered U.S. Patent and Trademark Office.
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This file: ftp://info.cert.org/pub/cert_advisories/CA-97.18.at
http://www.cert.org
click on "CERT Advisories"
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Revision history
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