Conference minotr::security_advisory

144.0. "Information Only: CERT CA-97.14, metamail" by MINOTR::NOBLE (Your Data, Your Job...Protect Both) Thu May 22 1997 07:43

    The Digital Corporate Information Security Group has received the
    following external security notice.  It should be noted that DIGITAL
    UNIX Software Engineering has determined that the problem reported
    below is not present in DIGITAL ULTRIX or DIGITAL UNIX.
    
    This third party notice is being provided as information only.  No
    security action is required.  Nor is any recommendation being made with
    regards the solution(s) identified.
    
    Should you happen to be using any third party software impacted by
    this notice, please use due discretion before implementing any or all
    of the solution(s) identified.
    
    If you are an internal user of the software impacted in this notice,
    and have any questions regarding this notice, then please contact your
    IT Cluster Security Manager.  An up-to-date contact list can be found
    at URL http://www-is-security.mso.dec.com/
      
                                  -=<>=-
    
    
============================================================================
CERT* Advisory CA-97.14
Original issue date: May 21, 1997
Last revised: --
              
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.

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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. 

            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.


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 30, 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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* 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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