__________________________________________________________
The U.S. Department of Energy
Computer Incident Advisory Capability
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\___ __|__ / \ \___
__________________________________________________________
INFORMATION BULLETIN
cron/crontab Buffer Overrun Vulnerabilities
December 18, 1996 23:00 GMT Number H-17
______________________________________________________________________________
PROBLEM: Problems have been identified in cron(8) and crontab(1)
programs.
PLATFORM: All platforms running FreeBSD 1.0, 1.1, 2.1.0, 2.1.5, 2.1.6,
2.1.6.1 and 2.2.
All platforms running RedHat Linux version 4.0.
All releases of NetBSD up to and including 1.2.
DAMAGE: Local users may gain root privileges.
SOLUTION: Install the proper patches and/or use the workarounds provided
below.
______________________________________________________________________________
VULNERABILITY Exploit information involving this vulnerabilities has been
ASSESSMENT: made publicly available.
______________________________________________________________________________
[ Start AUSCERT Advisory ]
===========================================================================
AA-96.21 AUSCERT Advisory
cron/crontab Buffer Overrun Vulnerabilities
18 December 1996
Last Revised: --
----------------------------------------------------------------------------
AUSCERT has received information that vulnerabilities exist in the
cron(8) and crontab(1) programs found in the Unix cron package.
These vulnerabilities may allow local users to gain root privileges.
Exploit information involving these vulnerabilities has been made publicly
available.
The vulnerabilities in the cron package affect numerous vendors and
platforms. AUSCERT recommends that sites take the steps outlined in
section 3 as soon as possible.
This advisory will be updated as more information becomes available.
----------------------------------------------------------------------------
1. Description
AUSCERT has received information that vulnerabilities exist in the
cron(8) and crontab(1) programs. cron(8) executes commands at
specified times according to instructions placed in user crontab files.
crontab(1) is used to install, remove or list the tables used to
drive the cron daemon. Both of these programs are installed by
default.
Two unrelated vulnerabilities are known to exist in some versions
of the cron package.
1. Command line buffer overrun
Due to insufficient bounds checking on arguments which are supplied
by users, it is possible to overwrite the internal stack space of
the crontab program while it is executing. By supplying a
carefully designed argument to the crontab program, intruders may
be able to force crontab to execute arbitrary commands. As crontab
is setuid root, this may allow intruders to run arbitrary commands
with root privileges.
2. Reading environment buffer overrun
A similar vulnerability exists in the library routine used to
load environment variables. This vulnerable routine is used
in both cron and crontab. Due to insufficient bounds checking,
it may be possible for intruders to manipulate cron or crontab
into executing arbitrary commands with root privileges.
Both of these vulnerabilities are known to be present in the Vixie
cron package, up to and including version 3.0. This package is
installed by default under some versions of Unix. The Vixie cron
package may have also been installed as third party software by sites.
The following command may be used to indicate whether a version
of cron based on Vixie cron is installed:
# strings /usr/bin/crontab | grep -i vix
Sites which have versions based on Vixie cron should consider themselves
vulnerable unless they have specific information from their vendors
which suggests otherwise.
Other versions of the cron package supplied by vendors may also be
vulnerable (Section 3).
Exploit information involving these vulnerabilities has been made
publicly available.
2. Impact
Local users may gain root privileges.
3. Workarounds/Solution
AUSCERT recommends that sites limit the possible exploitation of these
vulnerabilities by immediately removing the setuid permissions on
crontab(1) and checking the contents of crontab files as stated
in Section 3.1.
Vendor information about the vulnerabilities described in this
advisory is provided in Section 3.2.
If the cron functionality is required for non privileged users, and
no vendor information or patches are available (Section 3.2), AUSCERT
recommends that access be restricted to a trusted set of users as
given in Section 3.3.
3.1 Remove setuid and non-root execute permissions and check crontab files
AUSCERT recommends that the setuid permissions be removed from the
crontab program immediately. As the crontab program will no longer
work for non-root users, it is recommended that the execute permissions
also be removed.
For example:
# ls -l /usr/bin/crontab
-r-sr-xr-x 1 root bin 20480 Jun 10 1996 /usr/bin/crontab
# chmod 500 /usr/bin/crontab
# ls -l /usr/bin/crontab
-r-x------ 1 root bin 20480 Jun 10 1996 /usr/bin/crontab
Note that this will remove the ability for any non-root user to run the
crontab program.
This will prevent further exploitation of the crontab vulnerabilities
described in this advisory.
In addition, to ensure that cron can not be exploited through existing
user crontab files, sites should check the contents of all existing
crontab files for unusual contents. Unusual contents may include very
long lines or lines containing non-ASCII characters. If strange
environment settings or other unusual entries are found, it may
indicate a possible attack. User crontab files are usually located
in either /var/cron/tabs or /var/spool/cron.
3.2 Vendor information
Below is a list of vendors which are known to be affected by the
crontab vulnerabilities described in this advisory:
RedHat Linux
FreeBSD, Inc
NetBSD Project
The following vendors have informed AUSCERT that they are not
vulnerable to these vulnerabilities:
Hewlett Packard
IBM Corporation
The OpenBSD project
If your vendor's name is not listed above, please contact your
vendor directly.
For more specific vendor information, see Appendix A.
3.3 Restrict crontab access
If the cron functionality is required by a small set of trusted users,
sites may wish to restrict the execution of crontab to that group
of users. For example, if the Unix group "trusted" exists and contains
only those users allowed to use the cron functionality, the following
commands will restrict its use:
# chgrp trusted /usr/bin/crontab
# chmod 4750 /usr/bin/crontab
# ls -l /usr/bin/crontab
-rwsr-x--- 1 root trusted 20480 Jun 10 1996 /usr/bin/crontab
Access to any account in the "trusted" group will allow vulnerable
versions of the cron package to be exploited.
It should be noted that the use of cron allow/deny files (see
crontab(1)) will not prevent the exploitation of the command line
buffer overrun vulnerability.
-----------------------------------------------------------------------------
Appendix A Vendor information
This appendix will be updated as we receive additional information. If
your vendor is not listed below, or you require further vendor information,
please contact the vendor directly.
FreeBSD, Inc.
=============
FreeBSD versions 1.0, 1.1, 2.1.0, 2.1.5, 2.1.6, 2.1.6.1 and
2.2 -stable (prior to 16 Dec 1996) and 2.2 -current (prior
to 16 Dec 1996) are all affected by the crontab vulnerabilities
described in this advisory.
The FreeBSD Security Team have released an advisory and patch
information for the crontab vulnerabilities. This advisory
(FreeBSD-SA-96:20.stack-overflow) is available from:
ftp://freebsd.org/pub/CERT/advisories/FreeBSD-SA-96:20.stack-overflow.asc
Patches are available from:
ftp://freebsd.org/pub/CERT/patches/SA-96:20/
Hewlett Packard
===============
The version of crontab shipped with all current versions of
HP-UX 9.x and 10.x is not vulnerable to the buffer overflow
problems described in this advisory.
IBM Corporation
===============
The version of crontab shipped with AIX is not vulnerable to the
buffer overflow conditions listed in this advisory.
IBM and AIX are registered trademarks of International Business
Machines Corporation.
Linux (RedHat)
==============
RedHat Linux version 4.0 is known to be vulnerable. Earlier
versions may also be vulnerable.
The Net Project
===============
All releases of NetBSD up to and including 1.2 appear to be
vulnerable. These problems will be fixed in 1.3 and in the
upcoming bug fix release for 1.2.
The OpenBSD Project
===================
OpenBSD 2.0 is not susceptible to the vulnerabilities described
in this advisory.
----------------------------------------------------------------------------
AUSCERT thanks Theo de Raadt of the OpenBSD project for his assistance
in the preparation of this advisory. Thanks also to the vendors who
provided specific product information.
----------------------------------------------------------------------------
[ End AUSCERT Advisory ]
_______________________________________________________________________________
CIAC wishes to acknowledge the contributions of AUSCERT for the
information contained in this bulletin.
_______________________________________________________________________________
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