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TUCoPS :: Unix :: General :: cert0130.txt

CERT Advisory CA-96.25 sendmail groups


CERT(sm) Advisory CA-96.25
Original issue date: December 10, 1996
Last revised: -- December 20, 1996
                 Appendix A - Added Cray vendor information

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

Topic: Sendmail Group Permissions Vulnerability
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The CERT Coordination Center has received reports of a security problem in
sendmail affecting version 8. By exploiting this vulnerability, a local user
can run programs with group permissions of other users. For the exploitation
to be successful, group-writable files must be available on the same file
system as a file that the attacker can convince sendmail to trust.

The CERT/CC team recommends installing vendor patches or upgrading to the
current version of sendmail (8.8.4). Until you can do so, we urge you to
apply the workaround provided in Section III.C. In all cases, be sure to take
the extra precautions listed in Section III.D.

We will update this advisory as we receive additional information. Please
check advisory files regularly for updates that relate to your site. In
addition, you can check
to identify the most current version of sendmail.

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I.  Description

     When sendmail causes mail to be delivered to a program listed in a
     .forward or :include: file, that program is run with the group
     permissions possessed by the user who owns that .forward or :include:
     file. The file's owner attribute is used to initialize the list of group
     permissions that are in force when the program is run. This list is
     determined by scanning the /etc/group file, NIS or NIS+ group maps, or
     other similar vendor-specific databases (such as netinfo on OpenStep).

     It is possible for users to obtain group permissions they should not
     have by linking to a file that is owned by someone else, but on which
     they have group write permissions. By changing that file, users can
     acquire the group permissions of the owner of that file.

     Exploitation is possible if the attacked user has a file that is
     group writable by the attacker on the same file system as either (a) the
     attacker's home directory or (b) an :include: file that is referenced
     directly from the aliases file and is in a directory writable by the
     attacker. The first (.forward) attack only works against root. This
     attack does not give users root "owner" permissions, but does give them
     access to the groups that list root in /etc/group.

II.  Impact

     A local attacker can gain the group permissions of another user.

III. Solution

     Install a patch from your vendor if one is available (Section A) or
     upgrade to the current version of sendmail (Section B). Until you can
     take one of those actions, we recommend applying the workaround described
     in Section C. In all cases, you should take the precautions described in
     Section D.

     A.  Install a vendor patch.

         Below is a list of vendors who have provided information about
         sendmail. 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, please contact the vendor directly.

            Berkeley Software Design, Inc. (BSDI)
            Cray Research
            Digital Equipment Corporation
            FreeBSD, Inc.
            Hewlett-Packard Company
            IBM Corporation
            NEC Corporation
            The Santa Cruz Operation, Inc. (SCO)
            Silicon Graphics Inc
            Solbourne (Grumman Support Systems)
            Sun Microsystems, Inc.

     B.  Upgrade to the current version of sendmail.

         Install sendmail 8.8.4. This version is a "drop in" replacement for
         8.8.x. There is no patch for any version of sendmail before 8.8.0.
         If you are running such a version, strongly consider moving to
         version 8.8.4.

         Sendmail 8.8.4 is available from

         MD5 (sendmail.8.8.4.tar.gz) = 64ce6393a6968a0dc7c6652dace127b0

         Also in that directory are .Z and .sig files. The .Z file contains
         the same bits as the .gz file, but is compressed using UNIX compress
         instead of gzip. The .sig is Eric Allman's PGP signature for the
         uncompressed tar file. The key fingerprint is

  Type bits/keyID    Date       User ID
  pub  1024/BF7BA421 1995/02/23 Eric P. Allman <eric@CS.Berkeley.EDU>
           Key fingerprint =  C0 28 E6 7B 13 5B 29 02  6F 7E 43 3A 48 4F 45 29
                                Eric P. Allman <eric@Reference.COM>
                                Eric P. Allman <eric@Usenix.ORG>
                                Eric P. Allman <eric@Sendmail.ORG>
                                Eric P. Allman <eric@CS.Berkeley.EDU>

         When you change to a new version of sendmail, we strongly recommend
         also changing to the configuration files that are provided with that
         version. Significant work has been done to make this task easier.
         (In fact, it is highly likely that older configuration files will
         not work correctly with sendmail version 8.) It is now possible to
         build a sendmail configuration file ( using the
         configuration files provided with the sendmail release. Consult the
         cf/README file for a more complete explanation. Creating your
         configuration files using this method makes it easier to incorporate
         future changes to sendmail into your configuration files.

         Sun sendmail users: A paper is available to help you convert your
         sendmail configuration files from the Sun version of sendmail to one
         that works with sendmail version 8.8.x. The paper is entitled
         "Converting Standard Sun Config Files to Sendmail Version 8" and was
         written by Rick McCarty of Texas Instruments Inc. It is included in
         the distribution and is located in contrib/converting.sun.configs.

     C.  Apply a workaround.

         Eric Allman, the author of sendmail, has provided the following
         workaround. Note that this workaround is for sendmail 8.8.3. If you
         are running a version less than 8.8.3 we strongly recommend to
         upgrade at least to that version (or install the appropriate vendor
         patches). See CERT advisories CA-95:08 and CA-96.24 for more
         information on vulnerabilities in older sendmail versions.

         Set the UnsafeGroupWrites option in the file. This
         option tells sendmail that group-writable files should not be
         considered safe for mailing to programs or files, causing sendmail
         to refuse to run any programs referenced from group-writable files.
         Setting this option is a good idea in any case, but may require
         your users to tighten permissions on their .forward files and
         :include: files.

         The command "find <filesystem> -user root -type f -perm -020 -print"
         will print the names of all files owned by root that are group
         writable on a given file system. While this is only a partial
         solution we encourage you to carefully check all entries in your
         alias and .forward files (incl. aliases obtained via NIS, NIS+,
         or similar information systems) to check for group writable files.

         In addition, group memberships should be audited regularly. Users
         should not be in groups without a specific need. In particular,
         root generally does not need to be listed in most groups.

         As a policy matter, root should have a umask of 022 so that
         group-writable files are made consciously. Also, the aliases
         file should not reference :include: files in writable directories.

         While checking for writable directories, it's not enough to check the
         permissions of the directory the file itself lives in. You also have
         to check all other directories "on top" of that dir. If you, for
         example, want to check the permissions of the file
         /where/ever/here/file you have to check for group-write permissions
         not only in the directory /where/ever/here but also check the
         directories /where/ever and /where.

     D.  Take additional precautions

         Regardless of which solution you apply, you should take these extra
         precautions to protect your systems. These precautions do not address
         the vulnerabilities described herein, but are recommended as good
         practices to follow for the safer operation of sendmail.

         * Use the sendmail restricted shell program (smrsh)

           With *all* versions of sendmail, use the sendmail restricted shell
           program (smrsh). You should do this whether you use vendor-supplied
           sendmail or install sendmail yourself. Using smrsh gives you
           improved administrative control over the programs sendmail executes
           on behalf of users.

           A number of sites have reported some confusion about the need to
           continue using the sendmail restricted shell program (smrsh) when
           they install a vendor patch or upgrade to a new version of
           sendmail. You should always use the smrsh program.

           smrsh is included in the sendmail Version 8 distribution in the
           subdirectory smrsh. See the RELEASE_NOTES file for a description
           of how to integrate smrsh into your sendmail configuration file.

           smrsh is also distributed with some operating systems.

         * Use mail.local

           If you run /bin/mail based on BSD 4.3 UNIX, replace /bin/mail with
           mail.local, which is included in the sendmail distribution. As of
           Solaris 2.5 and beyond, mail.local is included with the standard
           distribution. It is also included with some other operating systems
           distributions, such as FreeBSD.

           Although the current version of mail.local is not a perfect
           solution, it is important to use it because it addresses
           vulnerabilities that are being exploited. For more details, see
           CERT advisory CA-95:02.

           To use mail.local, replace all references to /bin/mail with
           /usr/lib/mail.local. If you are using the M4(1)-based configuration
           scheme provided with sendmail 8.X, add the following to your
           configuration file:

              define(`LOCAL_MAILER_PATH', /usr/lib/mail.local)

         * WARNING: Check for setuid executable copies of old versions of
                    mail programs

           If you leave setuid executable copies of older versions of
           sendmail installed in /usr/lib (on some systems, it may be
           installed elsewhere), the vulnerabilities in those versions could
           be exploited if an intruder gains access to your system. This
           applies to as well as other sendmail programs. Either
           delete these versions or change the protections on them to be

           Similarly, if you replace /bin/mail with mail.local, remember to
           remove old copies of /bin/mail or make them non-executable.

IV.  Additional Notes

     Three other sendmail vulnerabilities are described in CERT advisory
     CA-96.20 and CA-96.24; see those advisories for details.

     Sendmail 8.8.4 also fixes a denial-of-service attack. If your system
     relies on the TryNullMXList option to forward mail to third-party MX
     hosts, an attacker can force that option off, thereby causing mail to
     bounce. As a workaround, you can use the mailertable feature to deliver
     to third party MX hosts regardless of the setting of the TryNullMXList


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.

Berkeley Software Design, Inc.
  BSD/OS is vulnerable to this problem and a patch (U210-030) is
  available from our mail-back patches server at <patches@BSDI.COM>
  or via ftp at


Cray Research
  Sendmail version 8 has not been included in any released Unicos
  system, so this is not a problem for current Unicos systems.

Digital Equipment Corporation
  This problem is currently under review by engineering to determine if
  it impacts DIGITAL UNIX and DIGITAL ULTRIX sendmail implementations.

FreeBSD, Inc.
  FreeBSD versions 2.1.5, 2.1.6, and are affected by the group
  vulnerability.  Versions 2.1.6 and are affected by the denial of
  service vulnerability.  All known sendmail security problems will have been
  addressed prior to the upcoming 2.2 release.  Given the complex nature of
  the patches produced by the sendmail author, user's are encouraged to follow
  the workarounds described in this advisory or apply and install patches
  available directly from the author to upgrade to Sendmail 8.8.4 available
  from the URLs listed in this advisory.

  We believe FreeBSD version 2.1.0 and prior to be unaffected by these
  particular vulnerabilities, however there are significant other security
  vulnerabilities in the sendmail supplied in prior releases.  All FreeBSD
  users should consider upgrading to sendmail 8.8.4 or removing sendmail from
  their systems if they are concerned about unauthorized root access from an
  unprivileged user account.

Hewlett-Packard Company
   1. Sendmail Group Permissions Vulnerability
   2. Denial of Service Attack using the sendmail configuration variable

   Vulnerable releases
   pre-10.2 10.x

   The 9.x, pre-10.2 10.x sendmail is vulnerable with respect to the "Sendmail
   Group Permissions Vulnerability".

   The 10.2 sendmail is vulnerable with respect to both the reported security

   Patches for these vulnerabilities are in progress.

IBM Corporation
  The version of sendmail that ships with AIX is vulnerable to the
  conditions listed in this advisory. A fix is in progress and the
  APAR numbers will be available soon.

  IBM and AIX are registered trademarks of International Business Machines

NEC Corporation
  Checking out the vulnerability. Contacts for further information

The Santa Cruz Operation, Inc. (SCO)
  Any SCO operating system running a version of sendmail provided by SCO
  is vulnerable to this problem. SCO will soon be providing a Support Level
  Supplement, (SLS), to address this issue for the following releases of SCO

  SCO Internet FastStart release 1.0.0, 1.1.0
  SCO OpenServer releases 5.0.0 and 5.0.2

  The SLS will provide a version of sendmail release 8.8.4 for these

  Note that only SCO Internet FastStart uses sendmail as the default mail
  system. All other SCO operating systems use other mail systems such as the
  Multi-Channel Memorandum Distribution Facility (MMDF) or the "mailsurr" mail
  system as the default, and as such are not vulnerable to this problem unless
  otherwise configured to use sendmail.

  Please watch the following URLs for availability information:


Silicon Graphics Inc.
  Currently Silicon Graphics Inc does not provide a 8.8.x sendmail
  version but instead provides a 8.6.12 version. Silicon Graphics
  has evaluated this issue as possibly applicable to the 8.6.12 version
  provided by Silicon Graphics and has not found this version to be
  vulnerable. No further action is required.

Solbourne (Grumman Support Systems)

  Solbourne customers running the supported sendmail version

        SendMail version 1.1 of 92/11/12

  are not vulnerable to this 'denial-of-service' attack.

  Those Solbourne customers running later versions of sendmail
  are probably vulnerable and should consider applying the
  workaround or installing the latest version of sendmail.
  No patches are available.

Sun Microsystems, Inc.
  All Sun sendmails are susceptible to both vulnerabilities. We will
  produce and announce patches for all supported versions of SunOS.
  We expect the patches to be available later this month.

- -----------------------------------------------------------------------------
The CERT Coordination Center thanks Eric Allman, AUSCERT, Terry Kyriacopoulos
of Interlog Internet Services, and Dan Bernstein of the University of
Illinois, Chicago for their contributions to the development of this advisory.
- -----------------------------------------------------------------------------

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

CERT/CC Contact Information
- ----------------------------

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

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

Getting security information
   CERT publications and other security information are available from

   CERT advisories and bulletins are also posted on the USENET newsgroup

   To be added to our mailing list for advisories and bulletins, send your
   email address to

- ---------------------------------------------------------------------------
Copyright 1996 Carnegie Mellon University
This material may be reproduced and distributed without permission provided
it is used for noncommercial purposes and the copyright statement is

CERT is a service mark of Carnegie Mellon University.
- ---------------------------------------------------------------------------

This file:

               click on "CERT Advisories"

Revision history

Dec. 20, 1996    Appendix A, Cray - added vendor information.

Version: 2.6.2


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