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  • CERT Advisory CA-1996-20 Sendmail Vulnerabilities

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Original issue date: September 18, 1996
Last revised: December 9, 1998
Updated vendor information for Silicon Graphics, Inc.

A complete revision history is at the end of this file. See also CA-96.24.sendmail.daemon.mode.html for information about additional vulnerabilities in sendmail.

The CERT Coordination Center has received reports of two security problems in sendmail that affect all versions up to and including 8.7.5. By exploiting the first of these vulnerabilities, users who have local accounts can gain access to the default user, which is often daemon. By exploiting the second vulnerability, any local user can gain root access.

The CERT/CC team recommends installing vendor patches or upgrading to the current version of sendmail (8.7.6). Until you can do so, we urge you to apply the workaround provided in Sec. III.C. In all cases, be sure to take the extra precautions listed in Sec. III.D.

For beta testers of sendmail 8.8: The vulnerabilities described in this advisory have been fixed in the beta version.

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

There are two vulnerabilities in all versions of sendmail up to and including sendmail 8.7.5. The first vulnerability is a resource starvation problem and the second is a buffer overflow problem.

Resource Starvation

When email is forwarded to a program using a .forward file or an :include: statement within a .forward or alias file, that program is executed as the owner of the .forward file or the file referenced by the :include: statement. Similarly, if email is forwarded to a file, that file is opened as the owner of the .forward file or the file referenced by the :include: statement. The file owner is called the "controlling user."

If the message cannot be delivered immediately, the name of the controlling user is written into the queue file along with the other delivery information so that the appropriate permissions can be acquired when the mail queue is processed.

Only the name of the controlling user is written in the queue file. This name is derived by calling the system routine getpwuid(3) on the user id of the file owner. If getpwuid fails, the sendmail default user (defined by the DefaultUser option in 8.7 and by the "u" and "g" options in older releases) is assumed.

In some cases, the system can be forced into resource starvation, thus forcing getpwuid(3) to fail even though an entry exists in /etc/passwd corresponding to that uid. Since getpwuid has no way of portably returning an error meaning "resource failure" as distinct from "user id not found," sendmail has no way of distinguishing between these cases; it assumes that the uid is unknown and falls back to the default user.

By starving sendmail of specific resources, sendmail will create files owned by the default user. Once created, these files can be used to access other files owned by the default user. In addition, these files owned by the default user can be used to leverage access to other privileged users on the system.

Buffer Overflows

There are several buffer overflows present in sendmail version 8.7.5 and earlier. Some of the buffer overflows could result in local users gaining unauthorized root access.

Significant work has been done on sendmail version 8.8 (now in beta test) to eliminate the problem, and the code changes originally planned for 8.8 have been backported to 8.7.6 to address these vulnerabilities.

II. Impact

Resource Starvation

Anyone with access to an account on the system can run programs or write files as the default user. The danger of compromising the default user depends primarily on the other files in your system owned by that user.

For example, on many systems the line printer spool directory (e.g., /var/spool/lpd) is owned by daemon; because the line printer subsystem runs setuid root, it may be possible to gain additional privileges. However, some other systems have no files owned by user daemon on the default system, and the only files owned by group daemon are not writable by that group; hence, the danger is minimal.

Buffer Overflows

Anyone with access to an account on the system can gain root access.

III. Solution

Install a patch from your vendor if one is available (Sec. A) or upgrade to the current version of sendmail (Sec. B). Until you can take one of those actions, we recommend applying the workaround described in Sec. C. This workaround addresses the resource starvation problem but not buffer overflows.

In all cases, you should take the precautions listed in Sec. D.

Note to beta testers of sendmail 8.8: The vulnerabilities described in this advisory have been fixed in the beta version of 8.8.

A. Install a vendor patch.

Below is a list of the 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.

    Digital Equipment Corporation
    Hewlett-Packard Company
    IBM Corporation
    Open Software Foundation
    The Santa Cruz Operation
    Silicon Graphics Inc.
    Sun Microsystems, Inc.

B. Upgrade to the current version of sendmail.

Install sendmail 8.7.6. This version is a "drop in" replacement for 8.7.x. There is no patch for 8.6.x. If you are using version 8.6 or earlier, you need to upgrade to the current version and rebuild your files. Upgrading to version 8.7.6 addresses both vulnerabilities described in this advisory.

Sendmail 8.7.6 is available from*

MD5 (sendmail.8.7.6.tar.gz) = 4a1f2179c53c9106bc8d7738f4d55667

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>

We strongly recommend that when you change to a newXS version of sendmail 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 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.

For sites that use the ampersand character ('&') in the gecos field of /etc/passwd, they should be aware that this may cause the full name returned to be corrupted or empty. (See man (4) passwd for further details on the purpose of the ampersand character in the gecos field.) Therefore when configuring sendmail, sites may also wish to ensure that information in the gecos field is explicitly complete, rather than rely on name expansion using the ampersand character.

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 sendmail version 8.7.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.

Resource Starvation

Eric Allman, the author of sendmail, has provided the following workaround to the resource starvation vulnerability.

Using smrsh as "prog" mailer limits the programs that can be run as the default user. Smrsh does not limit the files that can be written, but less damage can be done by writing files directly.

The damage can be almost entirely constrained by ensuring that the default user is an innocuous one. Sendmail defaults to 1:1 (daemon) only because that is reasonably portable. A special "mailnull" account that is used only for this purpose would be better. This user should own no files and should have neither a real home directory nor a real shell. A sample password entry might be:

          mailnull:*:32765:32765:Sendmail Default User:/no/such/dir:/dev/null

A corresponding entry should be made in /etc/group:


These assume that there are no other users or groups with id = 32765 on your system; if there are, pick some other unique value.

NOTE: When allocating a numeric uid to the "mailnull" user you should be careful to ensure that this value is less than the value of the UID_MAX kernel variable, if your system implements this variable. To check whether your system implements this variable (and the value that it uses), check for a reference to it in the "limits.h" header file, which should be located in the directory /usr/include, or one of its subdirectories. For further information on the use and content on the "limits.h" header file, see the man (4) limits.

After creating this user, change the line in /etc/ reading

          O DefaultUser=1:1
to read
          O DefaultUser=mailnull
If you are running 8.6.*, you will have to change the lines reading
to read

Finally, if you are using the m4(1)-based sendmail configuration scheme provided with sendmail 8.7.*, you should add the following line to the m4 input file, usually named

          define(`confDEF_USER_ID', 32765:32765)

The actual values should, of course, match those in the passwd file.

Buffer Overflows

There is no workaround for the buffer overflow problem. To address this problem, you must apply your vendor's patches or upgrade to the current version of sendmail (version 8.7.6).

D. Take additional precautions.

Regardless of which solution you apply, you should take these extra precautions to protect your systems.
  • 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. 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.

    Note that as of Solaris 2.5 and beyond, mail.local is included with the standard distribution. 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 executable copies of old versions of mail programs

  • If you leave 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 non-executable.

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

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 has released a patch for BSD/OS V2.1 to update sendmail to the 8.7.6 version. The patch is available from the patches@BSDI.COM mailback server
or via ftp at: ftp://ftp.BSDI.COM/bsdi/patches/patches-2.1/U210-024

Digital Equipment Corporation

[About the resource starvation problem]


Software Security Response Team
Copyright (c) Digital Equipment Corporation 1996. All rights reserved.

At the time of writing this document, patches (binary kits) for Digital's UNIX related operating systems are being developed. Digital will provide notice of availability for remedial kits through AES services (DIA, DSNlink FLASH), placed in the public FTP patch service domain and also be available from your normal Digital Support channel.{OS}/{vn.n}
                                                |     |
                                                |     |--version
                                                |--osf or ultrix



All currently released FreeBSD distributions have this vulnerability, as we distribute sendmail 8.7.x as part of our operating system. However, our -current and -stable source distributions were updated on 18 Sep 1996 to sendmail 8.7.6. Users tracking -current or -stable are advised to upgrade and recompile sendmail at their earliest convinience.

An official FreeBSD security advisory, including patches to close this vulnerability in FreeBSD 2.1.5 will be available shortly. The security advisory will appear at when available.

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

The following APARs are being developed and will be available shortly. See the appropriate release below to determine your action.

AIX 3.2

Apply the following fixes to your system:

APAR - IX61303 IX61307

AIX 4.1

Apply the following fixes to your system: APAR - IX61162 IX61306

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

instfix -ik IX61162 IX61306

AIX 4.2

Apply the following fixes to your system: APAR - IX61304 IX61305

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

instfix -ik IX61304 IX61305

To Order

APARs may be ordered using Electronic Fix Distribution (via FixDist) or from the IBM Support Center. For more information on FixDist,

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

IBM and AIX are registered trademarks of International Business Machines Corporation.


[For the resource starvation problem:]

Debian Linux: not vulnerable (uses smail)

Red Hat and derivatives:*

Open Software Foundation

OSF's OSF/1 R1.3.2 is not vulnerable to these types of attacks described in the resource starvation sections of the advisory.

OSF's OSF/1 R1.3.2 is vulnerable to the buffer overflow problems. We will address the problem in our next maintenance release.

The Santa Cruz Operation

Any SCO operating system running a version of sendmail provided by SCO is vulnerable to this problem. SCO is providing Support Level Supplement (SLS) oss443a for the following releases to address this issue:

SCO Internet FastStart release 1.0.0
SCO OpenServer releases 5.0.0 and 5.0.2

This SLS provides a pre-release version of sendmail release 8.7.6 for these platforms. SCO hopes to have a final version of sendmail 8.7.6 available to address both issues mentioned in this advisory in the near future.

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.

SCO intends to provide a similar patch for SCO UnixWare release 2.1.0 in the near future.

When configured to use a version of sendmail provided by SCO, releases prior to the ones mentioned here are also vulnerable, but no plans have yet been made concerning patches for these earlier releases.

You can download SLS oss443a as shown below.

Anonymous ftp (World Wide Web URL) ftp://ftp.sco.COM/SSE/oss443a (SLS image)
ftp://ftp.sco.COM/SSE/oss443a.ltr.sse (cover letter/install notes)


SLS oss443a is also available in the SCO Forum on Compuserve.

SCO Online Support (SOS) BBS

SLS oss443a can also be downloaded interactively via X, Y, or Z MODEM or Kermit, using the SCO Online Support System (SOS). Follow the menu selections under "Toolchest" from the main SOS menu.

The phone numbers available for interactive transfer from SOS are:

1-408-426-9495 (USA)
+44 (0)1923 210 888 (United Kingdom)


 sum -r

   13804   630 oss443a
   35304    14 oss443a.ltr.sse


   MD5 (oss443a) = 549260a71ca76f4e98dd38bccb72748c
   MD5 (oss443a.ltr.sse) = 7475d83f0db64a1af69eb66cd392a9d3

Be sure to keep track of the README file at ftp://ftp.sco.COM/SSE/README for updates to this supplement.
If you have further questions, contact your support provider. If you need to contact SCO, please send electronic mail to support@sco.COM , or contact SCO as follows:

USA/Canada: 6am-5pm Pacific Daylight Time (PDT)

1-800-347-4381 (voice)
1-408-427-5443 (fax)

Pacific Rim, Asia, and Latin American customers: 6am-5pm Pacific Daylight Time (PDT)

1-408-425-4726 (voice)
1-408-427-5443 (fax)

Europe, Middle East, Africa: 9am-5:30pm Greenwich Mean Time (GMT)

+44 (0)1923 816344 (voice)
+44 (0)1923 817781 (fax) XS

Silicon Graphics, Inc.

Please refer to Silicon Graphics Inc. Security Advisory, "IRIX mail(1)/rmail(1M)/sendmail(1M) Security Vulnerabilities," Number: 19980604-02-PX, distributed September 29, 1998 for additional information relating to this vulnerability.

The primary SGI anonymous FTP site for security information and patches is ( Security information and patches are located under the directories ~ftp/security and ~ftp/patches, respectively. The Silicon Graphics Security Headquarters Web page is accessible at the URL

Sun Microsystems, Inc.

Sun Microsystems has provided the following list of patches in response to this advisory:
    103594-10 5.5.1
    103595-10 5.5.1_86
    102980-13 5.5
    102981-13 5.5_x86
    102066-18 5.4
    102064-17 5.4_x86
    101739-17 5.3
    102423-07 4.1.4
    101665-10 4.1.3_U1

The CERT Coordination Center staff thanks Eric Allman, the author of sendmail, for his extensive assistance with this advisory, Wolfgang Ley of DFN-CERT and members of the AUSCERT for their contributions, and D. J. Bernstein of the University of Illinois at Chicago for reporting the resource starvation vulnerability.

Copyright 1996 Carnegie Mellon University.

Revision History
Dec.  9, 1998  Updated vendor information for Silicon Graphics, Inc.
Aug. 24, 1998  Updated vendor information for Silicon Graphics, Inc.
Oct. 20, 1997  Appendix A - updated vendor information for Sun.
Sep. 24, 1997  Updated copyright statement
May 8, 1997    Appendix A - updated vendor information for Hewlett-Packard.
Nov. 21, 1996  Introduction - Added a pointer to CA-96.24 for information
               on more sendmail vulnerabilities.
Nov. 19, 1996  Appendix A - Updated Hewlett-Packard information to address
               both problems.
Sep. 21, 1996  Sec. III.B - added instructions on configuring sendmail at
               sites that use '&' in the gecos filed of /etc/passwd.
	       Sec. III.C - added a note on uid for "mailnull" user.
Sep. 19, 1996  Sec. III.B - added URL in Australia for sendmail
               Acknowledgements - included reference that had been omitted
Appendix, FreeBSD - added an entry.
Sep. 18, 1996 Appendix, BSDI - added an entry containing patch information.
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