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CERT Advisory 146
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============================================================================
CERT* Advisory CA-97.14
Original issue date: May 21, 1997
Last revised: May 23, 1997
Appendix A, BSDI - added information.
Topic: Vulnerability in metamail
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The CERT Coordination Center has received reports of a vulnerability in
metamail, a program that implements MIME. By exploiting the vulnerability, a
sender of a MIME-encoded electronic mail message can cause the receiver of the
message to execute an arbitrary command if the receiver processes the message
using the metamail package. If the attacker has an account on the target
user's local system or if the target user's system supports AFS or another
distributed filesystem, then the attacker can arrange for the arbitrary
command to be one the attacker created. This affects versions of metamail
through 2.7 (the current version).
The CERT/CC team recommends installing a vendor patch, if one is available,
patching metamail yourself, or disabling metamail (see Section III).
We will update this advisory as we receive additional information.
Please check our advisory files regularly for updates that relate to your site.
- -----------------------------------------------------------------------------
I. Description
Multipurpose Internet Mail Extensions (MIME) is a standard format
for extended Internet electronic mail. The MIME format permits email
to include enhanced text, graphics, and audio in a standardized and
interoperable manner. MIME is described in RFCs 2045 through 2049.
metamail is a package that implements MIME (note: metamail can be
obtained from ftp://ftp.funet.fi/pub/unix/mail/metamail/mm2.7.tar.Z).
Using a configurable "mailcap" file, metamail determines how to treat
blocks of electronic mail text based on the content as described by email
headers. Some popular packages for handling electronic mail have hooks
that allow metamail to be called automatically while a message is being
processed.
A condition exists in metamail in which there is insufficient variable
checking in some support scripts. By carefully crafting appropriate
message headers, a sender can cause the receiver of the message to
execute an arbitrary command if the receiver processes the message
using the metamail package.
II. Impact
A sender of a MIME encoded mail message can cause the receiver to
execute an arbitrary command. If the attacker has an account on the
target user's local system or if the target user's system supports
AFS or another distributed filesystem, then the attacker can arrange for
the arbitrary command to be one the attacker created.
III. Solution
If your vendor supplies metamail with its distribution, then install a
patch from your vendor (Solution A). If your vendor does not distribute
metamail with their products or does not have a patch available, use the
workaround in Solution B. An alternative for those with sufficient
expertise is to patch the metamail scripts as described in Solution C.
A. Install a patch from your vendor, if appropriate
The vendors we have heard from so far are listed below, with details
in Appendix A. We will update the appendix as we receive more
information.
Berkeley Software Design, Inc. (BSDI)
Cray Research - A Silicon Graphics Company
Digital Equipment Corporation
FreeBSD, Inc.
Hewlett-Packard Company
IBM Corporation
Linux
NEC Corporation
Silicon Graphics Inc.
Solbourne
Sun Microsystems, Inc.
B. Disable metamail scripts
To disable the metamail scripts, remove the execute permissions from
the scripts that are located in the mm2.7/src/bin directory of
metamail v2.7 (the latest version of metamail). Remember that,
depending on your installation of metamail, the scripts may be located
in other directories in your operating system.
C. Patch metamail yourself
Sites that need to use metamail and have the expertise may wish to
patch the scripts that are part of the metamail distribution. Note
that the guidance below is supplied as is, and you need to be sure
that you understand the impact (if any) that your modifications will
have on metamail functionality.
The scripts referred to in the following material are all located in
the mm2.7/src/bin directory of metamail v2.7 (the latest version of
metamail). They may be located in other directories in your operating
system.
1. Ensure that parameters supplied to the scripts do not contain any
white space. Using showexternal as an example, add the following code
before any argument processing:
# Check argument integrity. Don't trust mail headers
switch ("$1$2$3$4$5$6$7")
case "*[\t ]*":
echo "Illegal white space in arguments\!"
echo "Command was:"
echo "'$0' '$1' '$2' '$3' '$4' '$5' '$6' '$7'"
exit 2
endsw
Add this code to the showexternal script at the very least, prior to
any argument processing within that script. We encourage you to
add this code to other scripts in mm2.7/src/bin directory to ensure
that arguments in those scripts also exclude white space. You may need
to adapt the code for your particular system.
Note that this patch may affect functionality in cases (such as
filenames) where parameters may have legitimately included
white space.
This step addresses the problem referred to in this advisory. As
part of a more generally secure programming practice, please
also consider the following modifications.
2. Ensure that script parameter references are quoted. For
instance, in showexternal, change this line:
set name=$3
to
set name="$3"
This should be done for every reference to a command line argument
in each of the scripts.
Note that csh has a :q operator which is also intended for this
purpose. If you prefer, you can use this operator in each case
instead of quoting.
3. Any variables in these scripts that take their value (either directly
or indirectly) from a script parameter should also be quoted where
necessary. For instance, in the showexternal script, change the line
get $name $NEWNAME
to
get "$name" "$NEWNAME"
Also change the following line:
if ($NEWNAME != $name) then
to
if ("$NEWNAME" != "$name") then
Similarly, there will be other instances where $name specifically,
and other variables in general, should be quoted. The reason is that
these variables take their value from the script parameters (for
example, $name takes its value from $3, and $NEWNAME may take its
value from $name).
As before, the :q operator can be used in each case.
Note that in doing this step, some care will be required.
4. Make sure that users have an appropriate umask set for directory
and file creation. Although the value is subject to local
restrictions, you may want to use a default value of 027
(depending upon the local environment).
5. Make sure that users have an appropriate value set for the
environment variable METAMAIL_TMPDIR. This environment variable tells
metamail where to create the temporary files it needs while
processing. If the variable is not set in the user's environment, the
default value is /tmp. Since /tmp is accessible by all users, it is
possible that use of this value will allow exploitation of race
conditions. We recommend setting the value to a protected directory
belonging to the user.
6. To ensure that the METAMAIL_TMPDIR is used properly and in a
secure manner, consider modifications along the following lines,
using the showexternal scripts as an example. These modifications
should reflect and reinforce the suggestions outlined in the
previous two items, namely that the temporary directory metamail
uses should be protected and accessible only by the user.
Note that the following code fragments are for example only, and
sites should adapt this code according to local requirements.
Change these lines:
if (! $?METAMAIL_TMPDIR) then
set METAMAIL_TMPDIR=/tmp
endif
to
# Set a sensible value for the temporary directory, if its not
# already set. If TMPDIR is set previously, then we will
# assume it is adequately protected.
if (! $?METAMAIL_TMPDIR) then
if ($?TMPDIR) then
set METAMAIL_TMPDIR="$TMPDIR"
else
set METAMAIL_TMPDIR=~/metamail_tmp
endif
endif
# Set a sensible umask value
umask 077
# Make sure that the temporary directory is available
if (! -d "$METAMAIL_TMPDIR") then
if (! -e "$METAMAIL_TMPDIR") then
mkdir "$METAMAIL_TMPDIR"
else
echo "$METAMAIL_TMPDIR exists, but is not a directory"
exit 2
endif
if ( $status != 0 || ! -d "$METAMAIL_TMPDIR" ) then
echo "Error creating $METAMAIL_TMPDIR"
exit 2
endif
endif
...........................................................................
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, please contact the vendor directly or
use the workaround in Section III.
Berkeley Software Design, Inc. (BSDI)
=====================================
BSDI ships metamail and is vulnerable to the attack. Patches are in progress.
Cray Research - A Silicon Graphics Company
==========================================
Cray Research does not ship metamail as part of either Unicos or Unicos/mk.
Digital Equipment Corporation
=============================
SOURCE: 19.MAY.1997
Digital Equipment Corporation
Software Security Response Team
Copyright (c) Digital Equipment Corporation 1997. All rights reserved.
This reported problem is not present for Digital's ULTRIX or
Digital UNIX Operating Systems Software.
- DIGITAL EQUIPMENT CORPORATION
FreeBSD, Inc.
=============
If you installed the metamail package or port then you are
vulnerable. All released versions of FreeBSD including 2.2.2R have
this flaw in them. The port was corrected as of May 21, 1997. Either
update your system from a more recent port, or apply the patches
contained in this advisory to those files affected.
Hewlett-Packard Company
=======================
HP-UX is vulnerable; patches are in progress.
IBM Corporation
===============
Not vulnerable, metamail is not shipped as part of AIX.
IBM and AIX are registered trademarks of International Business Machines
Corporation.
Linux
=====
Debian:
Debian uses its own bourne shell based metamail scripts not the standard
ones.
Red Hat:
All versions of Red Hat are vulnerable. A replacement RPM is available
at
ftp://ftp.redhat.com/pub/redhat/updates/4.1/i386/metamail-2.7-7.i386.rpm
NEC Corporation
===============
UX/4800 Not vulnerable for all versions.
EWS-UX/V(Rel4.2MP) Not vulnerable for all versions.
EWS-UX/V(Rel4.2) Not vulnerable for all versions.
UP-UX/V(Rel4.2MP) Not vulnerable for all versions.
EWS-UX/V(Rel4.0) Not vulnerable for all versions.
UP-UX/V Not vulnerable for all versions.
Silicon Graphics Inc.
=====================
At this time, Silicon Graphics does not have any public information
for the metamail 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
Solbourne
=========
We do not ship the utility.
We do not anticipate providing a patch, since we do not ship
the utility.
Sun Microsystems, Inc.
======================
Sun does not ship metamail with any of our platforms.
Sun has no plans to produce patches.
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The CERT Coordination Center staff thanks Olaf Kirch for contributing code to
the solution section and thanks BSDI and FreeBSD for their input on the
solution.
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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
- ----------------------------
Email cert@cert.org
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
cert-advisory-request@cert.org
In the subject line, type
SUBSCRIBE your-email-address
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* Registered U.S. Patent and Trademark Office.
Copyright 1997 Carnegie Mellon University
This material may be reproduced and distributed without permission provided
it is used for noncommercial purposes and the copyright statement is
included.
The CERT Coordination Center is part of the Software Engineering Institute
(SEI). The SEI is sponsored by the U.S. Department of Defense.
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This file: ftp://info.cert.org/pub/cert_advisories/CA-97.14.metamail
http://www.cert.org
click on "CERT Advisories"
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Revision history
May 23, 1997 Appendix A, BSDI - added information.
May 21, 1997 Appendix A, FreeBSD - changed release date of the patch
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