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CERT(sm) Advisory CA-96.25
Original issue date: December 10, 1996
Last revised: -- December 20, 1996
Appendix A - Added Cray vendor information
A complete revision history is at the end of this file.
Topic: Sendmail Group Permissions Vulnerability
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The CERT Coordination Center has received reports of a security problem in
sendmail affecting version 8. By exploiting this vulnerability, a local user
can run programs with group permissions of other users. For the exploitation
to be successful, group-writable files must be available on the same file
system as a file that the attacker can convince sendmail to trust.
The CERT/CC team recommends installing vendor patches or upgrading to the
current version of sendmail (8.8.4). 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 ftp://info.cert.org/pub/latest_sw_versions/sendmail
to identify the most current version of sendmail.
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I. Description
When sendmail causes mail to be delivered to a program listed in a
.forward or :include: file, that program is run with the group
permissions possessed by the user who owns that .forward or :include:
file. The file's owner attribute is used to initialize the list of group
permissions that are in force when the program is run. This list is
determined by scanning the /etc/group file, NIS or NIS+ group maps, or
other similar vendor-specific databases (such as netinfo on OpenStep).
It is possible for users to obtain group permissions they should not
have by linking to a file that is owned by someone else, but on which
they have group write permissions. By changing that file, users can
acquire the group permissions of the owner of that file.
Exploitation is possible if the attacked user has a file that is
group writable by the attacker on the same file system as either (a) the
attacker's home directory or (b) an :include: file that is referenced
directly from the aliases file and is in a directory writable by the
attacker. The first (.forward) attack only works against root. This
attack does not give users root "owner" permissions, but does give them
access to the groups that list root in /etc/group.
II. Impact
A local attacker can gain the group permissions of another user.
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)
Cray Research
Digital Equipment Corporation
FreeBSD, Inc.
Hewlett-Packard Company
IBM Corporation
NEC Corporation
The Santa Cruz Operation, Inc. (SCO)
Silicon Graphics Inc
Solbourne (Grumman Support Systems)
Sun Microsystems, Inc.
B. Upgrade to the current version of sendmail.
Install sendmail 8.8.4. 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 8.8.4.
Sendmail 8.8.4 is available from
ftp://ftp.sendmail.org/ucb/src/sendmail/sendmail.8.8.4.tar.gz
ftp://info.cert.org/pub/tools/sendmail/sendmail.8.8.4.tar.gz
ftp://ftp.cert.dfn.de/pub/tools/net/sendmail/sendmail.8.8.4.tar.gz
ftp://ftp.auscert.org.au/pub/mirrors/ftp.cs.berkeley.edu/ucb/sendmail/
MD5 (sendmail.8.8.4.tar.gz) = 64ce6393a6968a0dc7c6652dace127b0
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 (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.
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
workaround. Note that this workaround is for sendmail 8.8.3. If you
are running a version less than 8.8.3 we strongly recommend to
upgrade at least to that version (or install the appropriate vendor
patches). See CERT advisories CA-95:08 and CA-96.24 for more
information on vulnerabilities in older sendmail versions.
Set the UnsafeGroupWrites option in the sendmail.cf file. This
option tells sendmail that group-writable files should not be
considered safe for mailing to programs or files, causing sendmail
to refuse to run any programs referenced from group-writable files.
Setting this option is a good idea in any case, but may require
your users to tighten permissions on their .forward files and
:include: files.
The command "find <filesystem> -user root -type f -perm -020 -print"
will print the names of all files owned by root that are group
writable on a given file system. While this is only a partial
solution we encourage you to carefully check all entries in your
alias and .forward files (incl. aliases obtained via NIS, NIS+,
or similar information systems) to check for group writable files.
In addition, group memberships should be audited regularly. Users
should not be in groups without a specific need. In particular,
root generally does not need to be listed in most groups.
As a policy matter, root should have a umask of 022 so that
group-writable files are made consciously. Also, the aliases
file should not reference :include: files in writable directories.
While checking for writable directories, it's not enough to check the
permissions of the directory the file itself lives in. You also have
to check all other directories "on top" of that dir. If you, for
example, want to check the permissions of the file
/where/ever/here/file you have to check for group-write permissions
not only in the directory /where/ever/here but also check the
directories /where/ever and /where.
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 Version 8 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 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.
IV. Additional Notes
Three other sendmail vulnerabilities are described in CERT advisory
CA-96.20 and CA-96.24; see those advisories for details.
Sendmail 8.8.4 also fixes a denial-of-service attack. If your system
relies on the TryNullMXList option to forward mail to third-party MX
hosts, an attacker can force that option off, thereby causing mail to
bounce. As a workaround, you can use the mailertable feature to deliver
to third party MX hosts regardless of the setting of the TryNullMXList
option.
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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.
==============================
BSD/OS is vulnerable to this problem and a patch (U210-030) is
available from our mail-back patches server at <patches@BSDI.COM>
or via ftp at
ftp://ftp.BSDI.COM/bsdi/patches/patches-2.1/U210-030
Cray Research
=============
Sendmail version 8 has not been included in any released Unicos
system, so this is not a problem for current Unicos systems.
Digital Equipment Corporation
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This problem is currently under review by engineering to determine if
it impacts DIGITAL UNIX and DIGITAL ULTRIX sendmail implementations.
FreeBSD, Inc.
=============
FreeBSD versions 2.1.5, 2.1.6, and 2.1.6.1 are affected by the group
vulnerability. Versions 2.1.6 and 2.1.6.1 are affected by the denial of
service vulnerability. All known sendmail security problems will have been
addressed prior to the upcoming 2.2 release. Given the complex nature of
the patches produced by the sendmail author, user's are encouraged to follow
the workarounds described in this advisory or apply and install patches
available directly from the author to upgrade to Sendmail 8.8.4 available
from the URLs listed in this advisory.
We believe FreeBSD version 2.1.0 and prior to be unaffected by these
particular vulnerabilities, however there are significant other security
vulnerabilities in the sendmail supplied in prior releases. All FreeBSD
users should consider upgrading to sendmail 8.8.4 or removing sendmail from
their systems if they are concerned about unauthorized root access from an
unprivileged user account.
Hewlett-Packard Company
=======================
Vulnerabilities
---------------
1. Sendmail Group Permissions Vulnerability
2. Denial of Service Attack using the sendmail configuration variable
TryNullM\XList.
Vulnerable releases
--------------------
9.x
pre-10.2 10.x
10.2
The 9.x, pre-10.2 10.x sendmail is vulnerable with respect to the "Sendmail
Group Permissions Vulnerability".
The 10.2 sendmail is vulnerable with respect to both the reported security
holes.
Patches for these vulnerabilities are in progress.
IBM Corporation
===============
The version of sendmail that ships with AIX is vulnerable to the
conditions listed in this advisory. A fix is in progress and the
APAR numbers will be available soon.
IBM and AIX are registered trademarks of International Business Machines
Corporation.
NEC Corporation
===============
Checking out the vulnerability. Contacts for further information
by e-mail:UX48-security-support@nec.co.jp.
The Santa Cruz Operation, Inc. (SCO)
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Any SCO operating system running a version of sendmail provided by SCO
is vulnerable to this problem. SCO will soon be providing a Support Level
Supplement, (SLS), to address this issue for the following releases of SCO
software:
SCO Internet FastStart release 1.0.0, 1.1.0
SCO OpenServer releases 5.0.0 and 5.0.2
The SLS will provide a version of sendmail release 8.8.4 for these
platforms.
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.
Please watch the following URLs for availability information:
ftp://ftp.sco.COM/SLS/README
ftp://ftp.sco.COM/SSE/README
Silicon Graphics Inc.
=====================
Currently Silicon Graphics Inc does not provide a 8.8.x sendmail
version but instead provides a 8.6.12 version. Silicon Graphics
has evaluated this issue as possibly applicable to the 8.6.12 version
provided by Silicon Graphics and has not found this version to be
vulnerable. No further action is required.
Solbourne (Grumman Support Systems)
==================================
Solbourne customers running the supported sendmail version
SendMail version 1.1 of 92/11/12
are not vulnerable to this 'denial-of-service' attack.
Those Solbourne customers running later versions of sendmail
are probably vulnerable and should consider applying the
workaround or installing the latest version of sendmail.
No patches are available.
Sun Microsystems, Inc.
======================
All Sun sendmails are susceptible to both vulnerabilities. We will
produce and announce patches for all supported versions of SunOS.
We expect the patches to be available later this month.
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The CERT Coordination Center thanks Eric Allman, AUSCERT, Terry Kyriacopoulos
of Interlog Internet Services, and Dan Bernstein of the University of
Illinois, Chicago for their contributions to the development of this advisory.
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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 ftp://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.
Location of CERT PGP key
ftp://info.cert.org/pub/CERT_PGP.key
Getting security information
CERT publications and other security information are available from
http://www.cert.org/
ftp://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
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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
included.
CERT is a service mark of Carnegie Mellon University.
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This file:
ftp://info.cert.org/pub/cert_advisories/CA-96.25.sendmail_groups
http://www.cert.org
click on "CERT Advisories"
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Revision history
Dec. 20, 1996 Appendix A, Cray - added vendor information.
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