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CERT Advisory CA-95:13 - Syslog Vulnerability (with sendmail workaround)

  • From: CERT Advisory
  • Date: Thu Oct 19 14:26:06 1995

CA-95:13                         CERT Advisory
                                 October 19, 1995
                   Syslog Vulnerability - A Workaround for Sendmail

The CERT Coordination Center has received reports of problems with the
syslog(3) subroutine. To the best of our current knowledge, the problem is
present in virtually all versions of the UNIX Operating System except the
          Sony's NEWS-OS 6.X
          SunOS 5.5 (Solaris 2.5)
          Linux with libc version 4.7.2, released May 1995 

We have received reports indicating that the vulnerability is being exploited
with a script that has been written to be used with sendmail.

This advisory includes a workaround that you can use with sendmail. It
*does not* include workarounds for any other programs that use the syslog(3)
subroutine--telnetd, ftpd, httpd, etc.

The CERT Coordination Center recommends installing all appropriate
syslog-related patches as soon as they are available from vendors. But, in
the meantime, we suggest addressing at least the syslog problem in sendmail by
installing sendmail version 8.7.1. We are aware that several workarounds
concerning the syslog vulnerability have been published on the Internet, but
the CERT staff has not formally evaluated them.

As we receive additional information relating to this advisory, we will
place it in

We encourage you to check our README files regularly for updates on advisories
that relate to your site.


I.   Description

     The syslog(3) subroutine uses an internal buffer for building messages
     that are sent to the syslogd(8) daemon. This subroutine does no range
     checking on data stored in this buffer. It is possible to overflow the
     internal buffer and rewrite the subroutine call stack. It is then
     possible to execute arbitrary programs. 

     This problem is present in virtually all versions of the UNIX
     Operating System except the following:

        Sony's NEWS-OS 6.X
        SunOS 5.5 (Solaris 2.5)
        Linux with libc version 4.7.2 released in May, 1995

     The sendmail(8) program uses the syslog(3) subroutine, and a script has
     been written and is being used to exploit the vulnerability.

II.  Impact

     Local and remote users can execute commands. Prior access to the system
     is not needed. Exploitation can lead to root access.

III. Solution

     We recommend that you do all of A, B, and C.

     A. Install syslog patches from your vendor when they become available.

        Information we received from vendors as of the date of this advisory
        is attached as Appendix A and reproduced in CA-95:13.README.
        We will update the README file as vendors send updated information.

        When you install patches, you will need to recompile/relink any
        programs built on your system that have been compiled without shared
        libraries, that is, compiled statically. Be especially careful of
        programs that contain their own versions of the syslog(3) subroutine.
        You may need to do significant extra work to compile those programs to
        use the vendor-supplied patches.

     B. Install sendmail version 8.7.1. 

        NOTE: This workaround addresses the syslog(3) vulnerability in 
              sendmail only. The vulnerability still exists in all other
              programs that use syslog(3). 

        When your vendor(s) provides a patch, we recommend that you rebuild
        sendmail version 8.7.1 with the patched syslog(3) and place that
        newly compiled version into service. 
        Sendmail is available by anonymous FTP from


           MD5 (sendmail.8.7.1.tar.Z) = 4a66d07a059d1d5af5e9ea53ff1b396a

        Depending upon your currently installed sendmail program, switching to
        a different sendmail may require significant effort (such as rewriting
        the file). See Section VI for additional notes on

        In addition, Sections IV and V below contain scripts for building
        sendmail 8.7.1 for SunOS 4.1.X and Solaris 2.X, respectively. 

     C. Install smrsh.
        To restrict the sendmail program mailer facility, install and
        use the sendmail restricted shell program (smrsh). We recommend
        that you do this regardless of whether you use the vendor's supplied
        sendmail or you install sendmail version 8.7.1.  
        Smrsh is now included in the sendmail 8.7.1 distribution in the 
        subdirectory smrsh. See the RELEASE_NOTES file for a description
        of how to integrate smrsh into your sendmail configuration file.

IV. Building this package for SunOS 4.1.X

    Here is a script that is given as an illustration of how to build
    sendmail 8.7.1 for SunOS 4.1.X. Please refer to READ_ME in the src
    subdirectory for a more complete explanation of other options available
    during the compilation process. 

    % uname -sr
    SunOS 4.1.2
    % ls
    % zcat sendmail.8.7.1.tar.Z | tar xf -
    % cd sendmail-8.7.1/src
    % ./makesendmail LIBS='-lresolv' DBMDEF='-DNDBM -DNIS' \
        INCDIRS= LIBDIRS= sendmail
      Configuration: os=SunOS, rel=4.1.2, rbase=4, arch=sun4, sfx=
      Creating obj.SunOS.4.1.2.sun4 using Makefile.SunOS
      Making dependencies in obj.SunOS.4.1.2.sun4
      Making in obj.SunOS.4.1.2.sun4

    See Section VI for final installation steps.

V.  Building this package for Solaris 2.X

    Here is a typescript that is given as an illustration for how to build
    sendmail 8.7.1 for Solaris 2.X. Note that this procedure assumes that
    you have the GNU gcc system. The examples below used gcc version 2.6.3.
    Again, please refer to READ_ME in the src sub-directory for a more
    complete explanation of other options available during the compilation

    % uname -sr
    SunOS 5.4
    % ls
    % zcat sendmail.8.7.1.tar.Z | tar xf -
    % cd sendmail-8.7.1/src
    % ./makesendmail LIBS='-lresolv -lsocket -lnsl -lelf' \
        INCDIRS= LIBDIRS= sendmail
      Configuration: os=SunOS, rel=5.4, rbase=5, arch=sun4, sfx=
      Creating obj.SunOS.5.4.sun4 using Makefile.SunOS.5.4
      Making dependencies in obj.SunOS.5.4.sun4

    Note: If you wish sendmail version 8.7.1 to use the aliases and
    configuration file directory conventions from SunOS 5.4, use the
    following command:

          ./makesendmail LIBS='-lresolv -lsocket -lnsl -lelf' \
            LIBDIRS= sendmail

VI. Final Installation Notes

    Sendmail can then be installed and configured with new configuration
    files as needed. We strongly recommend that if you change to sendmail
    8.7.1, you also change to the configuration files that are provided with
    that version. 

    Significant work has been done to make this task easier. It is now
    possible to build a sendmail configuration file ( using
    the configuration files provided with this release. Consult the
    cf/READ_ME file for a more complete explanation. We recommended that you
    create your configuration files using this method because it provides a
    technique for incorporating any future changes to sendmail into your
    configuration files. 

    In addition, we recommend that you recreate your configuration file
    ( using the configuration files provided with 8.7.1.

    Finally, for Sun users, a paper is available to help you convert your
    sendmail configuration files from the Sun version of sendmail to one that
    works with version 8.7.1. 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. 

The CERT Coordination Center staff thanks Eric Allman and Wolfgang Ley for
their involvement in the development of this advisory, and thanks Karl
Strickland and Neil Woods for reporting the vulnerability.
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 (FIRST).

If you wish to send sensitive incident or vulnerability information to
CERT staff by electronic mail, we strongly advise that the email be
encrypted.  The CERT Coordination Center can support a shared DES key, PGP
(public key available via anonymous FTP on, or PEM (contact
CERT staff for details).

Internet email: [email protected]
Telephone: +1 412-268-7090 (24-hour hotline)
           CERT personnel answer 8:30 a.m.-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

CERT advisories and bulletins are posted on the USENET newsgroup If you would like to have future advisories and
bulletins mailed to you or to a mail exploder at your site, please send mail
to [email protected]

Past CERT publications, information about FIRST representatives, and other
information related to computer security are available for anonymous  
FTP from 

Copyright 1995 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.

CERT is a service mark of Carnegie Mellon University.


Appendix A: Vendor Information

Current as of October 19, 1995
See CA-95.13.README for updated information.

Below is information we have received from vendors concerning the
vulnerability described in this advisory. If you do not see your vendor's
name, please contact the vendor directly for information.

In addition to vendor information, note that the freely available Linux with
libc version 4.7.2, released May 1995, is not vulnerable.

Eric Allman

Sendmail version 8.7.1 is not vulnerable.  
This version is available by anonymous FTP from


      MD5 (sendmail.8.7.1.tar.Z) = 4a66d07a059d1d5af5e9ea53ff1b396a

Berkeley Software Design, Inc.

Users of BSD/OS V2.0 and V2.0.1 by Berkeley Software Design, Inc. should
install patch U201-001 which works for both versions. The patch is available
to all BSDI customers in: 

md5 checksum: 88b3fd8c83a5926589d7b87b55bc4e14 

Cray Research

Information about fixes for the syslog problem can be found in FN #2011,
dated October 10, 1995. Customers should receive this information from 
their Cray Research service representative.

For all source installations, your Cray Research service representative can
obtain the fix via the getfix tool.

Due to the number of executables which use this library routine, it is not
possible to provide getfix packages for all binary installations. UNICOS
binary update packages and include this mod.

                         Release Level            Fix Package
 Affected Product        Containing Fix           Availability
 ================        ==============           ===========
 UNICOS 8.0             UNICOS *         source only
 UNICOS 8.3             **                       source only
 UNICOS 9.0             UNICOS *         source only

 * This update is not yet available.
 ** No more updates planned

Digital Equipment Corporation

At the time of writing this document, patches(binary kits) for Digital's
ULTRIX platforms are in final testing and packaging. V4.3 (both VAX and RISC)
thru V4.5.  

Similar patches(binary kits) for OSF/1 versions are in progress and testing
is expected to begin the week of October 23, 1995 and then packaged for
Customer distribution estimated to available in November. Digital will provide
notice of the completion of the kits through AES services (DIA, DSNlink
FLASH) and be available from your normal Digital Support channel.
                        Digital's Software Security Response Team    10/18/95

Open Software Foundation

OSF cannot reproduce the security hole in OSF/1. However we have reproduced
the problem with syslog(3).  We have a fix for the syslog(3) problem. Support
customers should contact OSF for the fix. The fix will be included in the
OSF/1 R1.3.2 update release. 

Silicon Graphics Inc.

SGI has been in coordination with CERT regarding this issue.

Specific SGI information was not complete before CERTs submission deadline for
this advisory. 

SGI does have pending information and this information will be available via
anonymous ftp ( and/or your SGI service provider and potential
future CERT advisory addendums.

Solbourne (Grumman)

Solbourne 2.5 is not vulnerable.

Sony Corporation

NEWS-OS 6.0.3 and 6.1 are not vulnerable.

Sun Microsystems, Inc.

SunOS 5.5 is not vulnerable.