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TUCoPS :: Unix :: General :: cert0129.txt

CERT Advisory CA-96.24 sendmail daemon mode


CERT(sm) Advisory CA-96.24
Original issue date: November 21, 1996
Last revised: May 8, 1997
              Appendix A - updated vendor information for Hewlett-Packard.
              A complete revision history is at the end of this file.
Topic: Sendmail Daemon Mode Vulnerability
- -----------------------------------------------------------------------------

The CERT Coordination Center has received reports of a serious security
problem in sendmail that affects versions 8.7 through 8.8.2. By exploiting
this vulnerability, any local user can gain root access. Exploitation details
involving this vulnerability have been widely distributed.

Independent of this new vulnerability, there are other security problems
with older sendmail versions. Even if you are not running a version between
8.7 and 8.8.2, we strongly encourage you to upgrade to the current version
of sendmail (8.8.3). See Section IV for details.

The CERT/CC team recommends installing vendor patches or upgrading to the
current version of sendmail (8.8.3). 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.

- -----------------------------------------------------------------------------

I.   Description

     Sendmail is often run in daemon mode so that it can "listen" for
     incoming mail connections on the standard SMTP networking port, usually
     port 25. The root user is the only user allowed to start sendmail this
     way, and sendmail contains code intended to enforce this restriction.

     Unfortunately, due to a coding error, sendmail can be invoked in daemon
     mode in a way that bypasses the built-in check. When the check is
     bypassed, any local user is able to start sendmail in daemon mode. In
     addition, as of version 8.7, sendmail will restart itself when it
     receives a SIGHUP signal. It does this restarting operation by
     re-executing itself using the exec(2) system call. Re-executing is done
     as the root user. By manipulating the sendmail environment, the user can
     then have sendmail execute an arbitrary program with root privileges.

II.  Impact

     Local users can gain root privileges on the local machine.

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)
            Data General Corporation
            Digital Equipment Corporation
            Hewlett-Packard Company
            IBM Corporation
            NeXT Software, Inc.
            Open Software Foundation (OSF)
            The Santa Cruz Operation, Inc. (SCO)
            Silicon Graphics, Inc.
            Sun Microsystems, Inc.

     B.  Upgrade to the current version of sendmail.

         Install sendmail 8.8.3. 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 8.8.3.

         Sendmail 8.8.3 is available from*

         MD5 (sendmail.8.8.3.tar.gz) = 0cb58caae93a19ac69ddd40660e01646

         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

         This vulnerability relies on a coding error that has existed in
         sendmail since November 1982, allowing non-root users to start up an
         SMTP daemon by invoking sendmail as smtpd. However, that error did
         not have the current negative implications until sendmail added the
         ability to re-execute when a SIGHUP signal was received; this was
         added in 8.7.

         The anti-smtpd program given in Appendix B refuses to permit sendmail
         to be invoked as smtpd by a non-root user. It should be installed
         setuid root in place of sendmail (e.g., as /usr/sbin/sendmail or
         /usr/lib/sendmail, depending on your system); the real sendmail
         should be moved to another place. That location should be set in the
         REAL_SENDMAIL definition, and it should not be accessible by ordinary
         users. This permits the anti-smtpd program to moderate access to

     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 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

     Two other sendmail vulnerabilities are described in CERT advisory
     CA-96.20; see that advisory for details.

     Since the release of CA-96.20, two additional sendmail vulnerabilities
     have been discovered and fixed. By upgrading to sendmail version 8.8.3,
     the two problems, noted below, are also fixed. Note that the wrapper
     described in Section III.C does not address these vulnerabilities. The
     best advice is to upgrade to sendmail version 8.8.3.

     A. A vulnerability in sendmail Versions 8.8.0 and 8.8.1 has been
        discovered that allows remote users to execute arbitrary commands
        with root privileges. This vulnerability exploits exploiting a
        problem related to a buffer overflow when converting between 7-bit
        and 8-bit MIME messages. Versions prior to Version 8.8.0 do not
        contain this vulnerability. Versions before 8.7.6 contain other
        unrelated vulnerabilities. This vulnerability is fixed in version
        8.8.2 and beyond. The Australian Emergency Response Team (AUSCERT)
        issued an advisory on this vulnerability, AA-96.06a, available from


     B. A problem in sendmail has been reported that permits users on a
        system to redirect any email in the queue addressed to an unqualified
        domain name to a host of their choosing; that is, they can steal queued
        email. In some versions of the resolver, they may also be able to
        steal queued email addressed to fully qualified addresses. This bug
        is believed to exist in all versions of sendmail up to and including
        8.8.0. It is fixed in version 8.8.1 and beyond.


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. (BSDI)
BSD/OS is vulnerable to the sendmail daemon problem and we have issued an
official patch (U210-029) which may be obtained from our mail-back patches
server at patches@BSDI.COM or via anonymous ftp from:

Data General Corporation
The sendmail included with Data General's DG/UX is not subject to this

Digital Equipment Corporation
DIGITAL Engineering is aware of these reported problems and testing is
currently underway to determine the impact against all currently supported
releases of DIGITAL UNIX and ULTRIX.  Patches will be developed (as
necessary) and made available via your normal DIGITAL Support
channel. Notice will be through normal AES services and DIGITAL'S Web site

All currently shipping releases of FreeBSD are affected, including the just
released 2.1.6. An update for 2.1.6 will be available shortly. This problem
has been corrected in the -current sources. In the mean time, FreeBSD users
should follow the instructions in the CERT advisory. Sendmail will compile
and operate "out of the box" on FreeBSD systems.

Hewlett-Packard Company

  Description: Sendmail patches for HP-UX releases 9.X thru 10.20

  Security Bulletins are available from the HP Electronic 
  Support Center via electronic mail.
  User your browser to get to the HP Electronic Support 
  Center page at:
           (for US, Canada, Asia-Pacific, & Latin-America)
           (for Europe)
IBM Corporation
See the appropriate release below to determine your action.

  AIX 3.2
    No fix required. AIX 3.2 sendmail is not vulnerable.

  AIX 4.1
    No fix required. AIX 4.1 sendmail is not vulnerable.

  AIX 4.2
    AIX 4.2 sendmail is vulnerable.
    APAR IX63068 will be available shortly.

    To determine if you have this APAR on your system, run the following

       instfix -ik IX63068

  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:

    or send e-mail to with a subject of "FixDist".

  IBM and AIX are registered trademarks of International Business Machines


Linux has provided these URLs for S.u.S.E. Linux:

Checksums for the files in these directories:

   6279df0597c972bff65623da5898d5dc  sendmail.tgz
   0c0d20eecb1019ab4e629b103cac485c  sendmail-8.8.3.dif
   0cb58caae93a19ac69ddd40660e01646  sendmail-8.8.3.tar.gz

- -----
Caldera OpenLinux has released a security advisory, available from

- -----
Red Hat has patched sendmail 8.7.6. The fixes are available from

Red Hat Linux/Intel:
   rpm -Uvh

Red Hat Linux/Alpha:
   rpm -Uvh

NeXT Software, Inc.
NeXT is not vulnerable to the problem described in Section IV.A.
NeXT is vulnerable to the problem described in Section IV.B, and it
will be fixed in release 4.2 of OpenStep/Mach.

Open Software Foundation (OSF)
OSF/1 R1.3 is not vulnerable to this problem.

The Santa Cruz Operation, Inc. (SCO)
SCO is investigating the problem and will have more information in the
near future.

If we find that patches are needed, please check the following URLs
and this advisory appendix.


Silicon Graphics, Inc.
Silicon Graphics has historically provided a version 8.6.x sendmail
program.   The most recent SGI sendmail patch (1502) provides a version
8.6.12 sendmail program also.

The versions of sendmail provided in the distributed Silicon Graphics IRIX
operating system versions 5.2, 5.3, 6.0, 6.0.1, 6.1, 6.2 and 6.3 (and in
SGI patch 1502, which is the latest released patch for sendmail) are not
vulnerable to the exploitation as described in the CERT Advisory CA-96:24.

No further action is required.

Silicon Graphics also published an advisory for their customers on November
21, 1996--SGI advisory number 19961103-01-I. SGI advisories are
available from

Sun Microsystems, Inc.
No Sun versions of sendmail are affected by this vulnerability.


Appendix B - anti-smtpd.c

Below is the code for the anti-smtpd.c sendmail wrapper. Here is an example
of how to compile and install this wrapper. You may have to change these
commands for your system. Further, you may have to change the code for
anti-smtpd.c to get it to compile on your system. Finally, you may also have
to turn off sendmail before running these commands and then turn sendmail back
on after running them. Run these commands as root.

        # mkdir /usr/hidden
        # chmod 700 /usr/hidden
        # mv /usr/lib/sendmail /usr/hidden/sendmail
        # cc anti-smtpd.c -o anti-smtpd
        # mv anti-smtpd /usr/lib/sendmail
        # chmod u+s /usr/lib/sendmail

Here is the code for anti-smtpd.c:

#include <stdio.h>
#include <string.h>
#include <syslog.h>
#include <sysexits.h>

static char *Version = "Version 1.0 November 21, 1996";

**  Sendmail wrapper for CA-96:24 HUP to smtpd problem
**      This is trivial -- it just ensures that sendmail cannot be
**      invoked as smtpd.
**      To install this, move the real sendmail into /usr/hidden,
**      which should be a mode 700 directory owned by root. Install
**      this program setuid root in place of sendmail.

# define REAL_SENDMAIL  "/usr/hidden/sendmail"

main(argc, argv)
        int argc;
        char **argv;
        char *p;
        extern int errno;

        if (argc < 1)
                fprintf(stderr, "sendmail: need a program name\n");

        p = strrchr(argv[0], '/');
        if (p == NULL)
                p = argv[0];
        if (strcmp(p, "smtpd") == 0 && getuid() != 0)
                fprintf(stderr, "sendmail: cannot be invoked as smtpd\n");
                syslog(LOG_ALERT, "sendmail: invoked as smtpd by %d", getuid());
        execv(REAL_SENDMAIL, argv);
        fprintf(stderr, "sendmail: cannot exec %s: errno = %d\n",
                REAL_SENDMAIL, errno);

- -----------------------------------------------------------------------------
The CERT Coordination Center thanks Eric Allman and AUSCERT 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

May 8, 1997    Appendix A - updated vendor information for Hewlett-Packard.
Nov. 22, 1996  Updates - added vendor information for Silicon Graphics.
               Modified Hewlett Packard's patch information.

Version: 2.6.2


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