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Home : Advisories : Sendmail Vulnerabilities
| Title: |
Sendmail Vulnerabilities |
| Released by: |
CERT |
| Date: |
18th September 1996 |
| Printable version: |
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CERT(*) Advisory CA-96.20
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.
Topic: Sendmail Vulnerabilities
- -----------------------------------------------------------------------------
** See also CA-96.24.sendmail.daemon.mode 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 http://info.cert.org/pub/latest_sw_versions/sendmail
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
FreeBSD
Hewlett-Packard Company
IBM Corporation
Linux
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
sendmail.cf files. Upgrading to version 8.7.6 addresses both
vulnerabilities described in this advisory.
Sendmail 8.7.6 is available from
http://ftp.sendmail.org/ucb/src/sendmail/sendmail.8.7.6.tar.gz
http://info.cert.org/pub/tools/sendmail/sendmail.8.7.6.tar.gz
http://ftp.cert.dfn.de/pub/tools/net/sendmail/sendmail.8.7.6.tar.gz
http://ftp.auscert.org.au/pub/mirrors/ftp.cs.berkeley.edu/ucb/sendmail/*
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
Key fingerprint = C0 28 E6 7B 13 5B 29 02 6F 7E 43 3A 48 4F 45 29
Eric P. Allman
Eric P. Allman
Eric P. Allman
Eric P. Allman
We strongly recommend that when you change to a new 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 (sendmail.cf) 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:
mailnull:*:32765:
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/sendmail.cf reading
O DefaultUser=1:1
to read
O DefaultUser=mailnull
If you are running 8.6.*, you will have to change the lines reading
Ou1
Og1
to read
Ou32765
Og32765
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 sendmail.mc:
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 sendmail.mx 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:
http://ftp.BSDI.COM/bsdi/patches/patches-2.1/U210-024
Digital Equipment Corporation
=============================
[About the resource starvation problem]
Source:
Software Security Response Team
Copyright (c) Digital Equipment Corporation 1996. All rights reserved.
08.SEP.1996
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.
http://ftp.service.digital.com/public/{OS}/{vn.n}
| |
| |--version
|--osf or ultrix
9/96 - DIGITAL EQUIPMENT CORPORATION
FreeBSD
=======
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
http://freebsd.org/pub/CERT/patches/SA-96:18/
when available.
Hewlett-Packard Company
=======================
HPSBUX9704-059
HEWLETT-PACKARD SECURITY BULLETIN: #00059, 30 April 1997
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:
http://us-support.external.hp.com
(for US, Canada, Asia-Pacific, & Latin-America)
http://europe-support.external.hp.com
(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,
http://service.software.ibm.com/aixsupport/
or send e-mail to aixserv@austin.ibm.com with a subject of "FixDist".
IBM and AIX are registered trademarks of International Business Machines
Corporation.
Linux
=====
[For the resource starvation problem:]
Debian Linux: not vulnerable (uses smail)
Red Hat and derivatives:
http://ftp.redhat.com/pub/redhat-3.0.3/i386/updates/RPMS/sendmail*
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)
-------------
http://ftp.sco.COM/SSE/oss443a (SLS image)
http://ftp.sco.COM/SSE/oss443a.ltr.sse (cover letter/install notes)
Compuserve
----------
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)
Checksums
---------
sum -r
------
13804 630 oss443a
35304 14 oss443a.ltr.sse
MD5
---
MD5 (oss443a) = 549260a71ca76f4e98dd38bccb72748c
MD5 (oss443a.ltr.sse) = 7475d83f0db64a1af69eb66cd392a9d3
Be sure to keep track of the README file at http://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)
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
sgigate.sgi.com (204.94.209.1). 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
http://www.sgi.com/Support/security/security.html.
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.
- -----------------------------------------------------------------------------
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 http://info.cert.org/pub/FIRST/first-contacts).
CERT/CC Contact Information
- ----------------------------
Email cert@cert.org
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
USA
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.
You can get the CERT PGP key from http://info.cert.org/pub/CERT_PGP.key
Location of CERT PGP key
http://info.cert.org/pub/CERT_PGP.key
Getting security information
CERT publications and other security information are available from
http://www.cert.org/
http://info.cert.org/pub/
CERT advisories and bulletins are also posted on the USENET newsgroup
comp.security.announce
To be added to our mailing list for advisories and bulletins, send your
email address to
cert-advisory-request@cert.org
- ------------------------------------------------------------------------------
Copyright 1996, 1997 Carnegie Mellon University. Conditions for use,
disclaimers, and sponsorship information can be found in
http://www.cert.org/legal_stuff.html and http://ftp.cert.org/pub/legal_stuff .
If you do not have FTP or web access, send mail to cert@cert.org with
"copyright" in the subject line.
CERT is registered in the U.S. Patent and Trademark Office.
- ---------------------------------------------------------------------------
This file:
http://info.cert.org/pub/cert_advisories/CA-96.20.sendmail_vul
http://www.cert.org
click on "CERT Advisories"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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 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
earlier.
Appendix, FreeBSD - added an entry.
Sep. 18, 1996 Appendix, BSDI - added an entry containing patch information.
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