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TUCoPS :: Unix :: General :: ciac-g09.txt

Unix Sendmail Vulnerability


                       The U.S. Department of Energy
                    Computer Incident Advisory Capability
                           ___  __ __    _     ___
                          /       |     /_\   /
                          \___  __|__  /   \  \___

                             INFORMATION BULLETIN

                          Unix sendmail vulnerability

January 31, 1996 15:00 GMT                                           Number G-09
PROBLEM:       Sendmail (prior to Sendmail 8.6.10) contains a vulnerability
               which allows local and remote users to execute privileged
PLATFORM:      All versions of "sendmail" prior to Version 8.6.10, including
               Sendmail 5.67+IDA-1.5 and most vendor versions
DAMAGE:        Local and remote users can gain privileged access
SOLUTION:      Upgrade to a latest version of Sendmail (current version is
VULNERABILITY  This vulnerability is being actively exploited on the Internet

The following information has been reprinted from the Automated
Systems Security Incident Support Team (ASSIST) 96-603 Bulletin.


SUMMARY: All versions of "sendmail" prior to Version 8.6.10,
including Sendmail 5.67+IDA-1.5 and most vendor versions, contain
a vulnerability that could allow unauthorized root access.  This
vulnerability is being actively exploited on the Internet.

BACKGROUND: The vulnerability is exploited through the use of the SMTP
"EXPN" and "VRFY" commands offered by all versions of "sendmail."  A
buffer-overrun problem is present in the implementation of these
commands that allows the executable code of the "sendmail" process to
be overwritten.  This executable code can do anything the author
wants, and is run with super-user permissions.

This vulnerability was fixed in Sendmail Version 8.6.10, which was
released on 21 February 1995.  The current version of Sendmail is
8.7.3, which was released on 3 December 1995.  The versions of
"sendmail" provided by most UNIX workstation vendors are based on
Versions 5.64, 5.65, or 5.67.  Unless the vendor has taken
considerable care to secure their version of "sendmail" (most vendors
have not), these versions are vulnerable to this problem.  The only
vendor version of "sendmail" that has been confirmed as not vulnerable
to this problem is the one shipped with Solaris 2.5 (but not earlier
versions of Solaris) from Sun Microsystems; this version is based on
"sendmail" 8.6.12.  The "IDA" version of "sendmail," a popular public
domain version that offers enhanced mail delivery and user database
features, is based on Version 5.67, and is thus vulnerable to this

IMPACT: Successful exploitation of this vulnerability allows an
attacker to execute arbitrary commands on the local system with
super-user ("root") permissions and gain unrestricted access to
system resources.

There are four possible solutions to this problem:

A. Install Sendmail Version 8.7.3.  This version of "sendmail" is
under active development and offers major improvements, especially
in the area of security.  Sendmail 8.7.3 is available via the
Internet by anonymous FTP from (
in /ucb/sendmail/sendmail.8.7.3.tar.Z.  Other information
resources for sendmail are the "comp.mail.sendmail" USENET
newsgroup and the book "Sendmail," by Bryan Costales, Eric
Allman, and Neil Rickert, published by O'Reilly & Associates
(  Note: While compiling and installing a new
version of sendmail is not difficult, , modifying the
configuration file (the file that specifies how mail is to be
delivered) to work with the new version of the program is often
non-trivial.  A complete rewrite of the existing configuration
file may be required, and this is a process that may require
several days at a minimum to complete.  Option A is not
recommended unless you have extensive UNIX system administration

B. Install the "smap" and "smapd" programs on all UNIX systems
connected to the Internet.  The "smap" program provides a minimal
SMTP implementation that accepts mail messages from the network
and stores them for later delivery.  The "smapd" program
periodically runs "sendmail" on these stored messages, and thus
"sendmail" is no longer reachable directly from the network and
its security vulnerabilities can no longer be exploited.

The "smap" and "smapd" programs are part of the Trusted
Information Systems (TIS) Firewall Toolkit, available from TIS,
Inc.  The TIS is available on the Internet via anonymous FTP,
you will need the files:
/pub/firewalls/toolkit/fwtk.tar.Z and fwtk-doc-only.tar.Z
NOTE: Installation of "smap" and "smapd" is relatively simple, but
it does represent a significant change in the way electronic mail
will be handled at your site.  Careful consideration should
be given to requirements and installation options, and entensive
testing performed prior to site-wide deployment.

C. Disable the "EXPN" and "VRFY" commands in your current version
of the "sendmail" command.  This will prohibit an attacker from
exploiting the vulnerability.

   1. Sendmail versions 8.6.x or 8.7.x:
   Sendmail Version 8 provides the "p" configuration file option
   that allows you to configure the privacy and security features
   of your "sendmail" process.  The format of the option is:


   where "what" is one of several keywords that enable the
   various features.  By listing the "noexpn" and "novrfy"
   keywords in this list, you can disable access to the "EXPN" and
   "VRFY" commands.  After adding (or changing) this line in your
   configuration file, you must kill and restart the "sendmail"

   2. Sendmail versions other than 8.6.x or 8.7.x:
   Prior to Version 8, there is no way to disable these commands
   through the configuration file or the command line.  However,
   the commands can be disabled by patching the "sendmail" binary
   as follows.

      A. Make a backup copy of the sendmail binary, Patching binary
      files is not always successful and you will need to be able
      to recover.
      # cp -p /usr/lib/sendmail /usr/lib/
      # chmod 700 /usr/lib/

      B. Use the "strings" command to find the addresses of the
      "vrfy" and "expn" command name strings:
      # strings -o /usr/lib/sendmail | grep vrfy
       240248 novrfy
       240264 needvrfyhelo
       276648 vrfy
      # strings -o /usr/lib/sendmail | grep expn
       240256 noexpn
       240280 needexpnhelo
       276640 expn
       280328 expn
      NOTE: The output you see may be different, the important
      information to note are the numbers in front of the "expn"
      and "vrfy" strings.

      C. Use "adb" to write a null byte on each of these commands.
      Note: Use the addresses output by your run of the "strings"
      command, not the numbers from the example above.
      # adb -w - /usr/lib/sendmail
       not core file = /usr/lib/sendmail
       a$d                     <-- converts to decimal addresses
       276648/w 0              <-- writes zero byte on "vrfy"
       0x438a8:        30322   =       0
       276640/w 0              <-- writes zero byte on first "expn"
       0x438a0:        25976   =       0
       280328/w 0              <-- writes zero byte on second "expn"
       0x44708:        25976   =       0
       ^D                      <-- CTRL-D exits adb

      D. Verify the strings were removed:
      # strings -o /usr/lib/sendmail | grep vrfy
       240248 novrfy
       240264 needvrfyhelo
      # strings -o /usr/lib/sendmail | grep expn
       240256 noexpn
       240280 needexpnhelo

      E. Kill and restart the "sendmail" daemon.  NOTE: Some mail
      user agents (the programs that users use to read and send mail)
      rely on the presence of the "VRFY" command which you just
      disabled.  After performing the above steps, test all of the
      mail programs in use at your site to insure that they still
      operate correctly.

D. Install the appropriate "sendmail" patch from your vendor.  At the
time this bulletin was issued, most major vendors were working this
matter.  Some will ship fixes to customers, others will issue patches
that will be available for download.  ASSIST will be sending out
updates to this bulletin as additional information becomes available.


ASSIST would like to thank the IBM-ERS for information contained in
this bulletin.



CIAC wishes to acknowledge ASSIST and IBM-ERS for providing the
information contained in this bulletin.

CIAC, the Computer Incident Advisory Capability, is the computer
security incident response team for the U.S. Department of Energy (DOE) and the
National Institute of Health (NIH). CIAC is located at the Lawrence Livermore
National Laboratory in Livermore, California. CIAC is also a founding member of
FIRST, the Forum of Incident Response and Security Teams, a global organization
established to foster cooperation and coordination among computer security teams


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This document was prepared as an account of work sponsored by an agency of the
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LAST 10 CIAC BULLETINS ISSUED (Previous bulletins available from CIAC)

(F-27)  Incorrect Permissions on /tmp
(F-28)  Vulnerability in SunOS 4.1.* Sendmail (-oR option)
(G-1)   Telnetd Vulnerability
(G-2)   SunOS 4.1.X Loadmodule Vulnerability
(G-3)   AOLGOLD Trojan Program
(G-4)   X Authentication Vulnerability
(G-5)   HP-UX FTP Vulnerability Bulletin
(G-06A) Win95 Vulnerabilities
(G-07)  SGI Object Server Vulnerability
(G-08)  splitvt(1) vulnerability

RECENT CIAC NOTES ISSUED (Previous Notes available from CIAC)

Notes 07 - 3/29/95   A comprehensive review of SATAN

Notes 08 - 4/4/95    A Courtney update

Notes 09 - 4/24/95   More on the "Good Times" virus urban legend

Notes 10 - 6/16/95   PKZ300B Trojan, Logdaemon/FreeBSD, vulnerability
                     in S/Key, EBOLA Virus Hoax, and Caibua Virus

Notes 11 - 7/31/95   Virus Update, Hats Off to Administrators,
                     America On-Line Virus Scare, SPI 3.2.2 Released,
                     The Die_Hard Virus

Notes 12 - 9/12/95   Securely configuring Public Telnet Services, X Windows,
                     beta release of Merlin, Microsoft Word Macro Viruses,
                     Allegations of Inappropriate Data Collection in Win95

Version: 2.6.2


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