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CERT Advisory CA-96.04 - Corrupt Information from Network Servers

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CERT Advisory

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Feb 22, 1996, 3:00:00 AM2/22/96

=============================================================================
CERT(sm) Advisory CA-96.04
February 22, 1996

Topic: Corrupt Information from Network Servers
-----------------------------------------------------------------------------

The CERT Coordination Center has received reports of intruders exploiting
systems by corrupting data provided by a Domain Name Service (DNS) server.
Although these reports have focused only on DNS, this vulnerability could
apply to any network service from which data is received and subsequently
used.

Section III.A contains a pointer to two subroutines that address the DNS
problem. These subroutines, written in the C programming language, can be used
to validate host names and IP addresses according to RFCs 952 and 1123, as
well as names containing characters drawn from common practice, namely "_" and
"/".

In the specific case of sendmail, the problem has already been addressed by
patches (see Section III.B).

As we receive additional information relating to this advisory, we will place
it in
ftp://info.cert.org/pub/cert_advisories/CA-96.04.README

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

-----------------------------------------------------------------------------

I. Description

Information provided by an information server may be of a form that
could cause programs to operate in unexpected ways. The subroutines and
programs transferring data from that information server could check the
data for correctness of form; however, programs that *use* that data are
ultimately responsible for ensuring adherence to the documents that
define the correct form.

For example, consider a program that uses the host name returned by
gethostbyname() as part of the string given to the popen() or system()
subroutines. Because gethostbyname() may use an information server
beyond your control, the data returned could be of a form that causes
the popen() or system() subroutines to execute other commands besides
the command specified by that program.

This advisory speaks to a specific instance of a problem caused by the
information returned by DNS, but information from any server should be
checked for validity. Examples of other information servers are YP, NIS,
NIS+, and netinfo.

II. Impact

Programs that do not check data provided by information servers may
operate in unpredictable ways and give unexpected results. In
particular, exploitation of this vulnerability may allow remote access
by unauthorized users. Exploitation can also lead to root access by both
local and remote users.

III. Solution

For programs that you write or have written, consider integrating the
general solution in Section A below.

In the specific case of the sendmail mail delivery program, Eric Allman,
the original author of sendmail, has produced patches that address the
problem. Section B provides details about these, along with vendor
information and additional steps you should take to protect sendmail.

A. General solution for Internet host names

Use the host name and IP address validation subroutines available
at the locations listed below. Include them in all programs that
use the result of the host name lookups in any way.

ftp://info.cert.org/pub/tools/ValidateHostname/IsValid.c
ftp://ftp.cert.dfn.de/pub/tools/net/ValidateHostname/IsValid.c

Checksum:
MD5 (IsValid.c) = 9456f2f5dcb226eaa19a72256c8d1922

The IsValid.c file contains code for the IsValidHostname and
IsValidIPAddress subroutines. This code can be used to check host
names and IP addresses for validity according to RFCs 952 and 1123,
well as names containing characters drawn from common practice,
namely "_" and "/".

B. Specific solutions in the case of sendmail

Install a patch from your vendor when it becomes available (see B.1)
or install Eric Allman's patch (B.2). In both cases, install the
sendmail restricted shell program (B.3).

1. Install a patch from your vendor.

Below is a summary of the vendors who have reported status to us as
of the date of this advisory. More complete information is provided in
the appendix of this advisory and reproduced in the CA-96.04.README
file. We will update the README file as we receive more information.

If your vendor's name is not on this list, please contact the vendor
directly.

Vendor or Source Status
---------------- ------------
Eric Allman Patches available.
Hewlett-Packard Co. Vulnerable, watch readme file for updates.
IBM Corporation Working on fixes for sendmail.
Silicon Graphics Inc. Patch planned.

2. Install a patch to sendmail.

If you are presently running sendmail 8.6.12, there is a patch that
makes version 8.6.13.

Similarly, if you are presently running sendmail 8.7.3, there is a
patch that makes version 8.7.4.

The patches are available for anonymous FTP from

ftp://info.cert.org/pub/tools/sendmail/
ftp://ftp.cs.berkeley.edu/ucb/src/sendmail/
ftp://ftp.auscert.org.au/pub/mirrors/ftp.cs.berkeley.edu/ucb/sendmail/
ftp://ftp.cert.dfn.de/pub/tools/net/sendmail/

Checksums for the 8.6.13 release:
MD5 (sendmail.8.6.13.base.tar.Z) = e8cf3ea19876d9b9def5c0bcb793d241
MD5 (sendmail.8.6.13.cf.tar.Z) = 4492026fa9e750cd33974322cb5a6fb9
MD5 (sendmail.8.6.13.misc.tar.Z) = 7ec5d31656e93e08a3892f0ae542b674
MD5 (sendmail.8.6.13.xdoc.tar.Z) = e4d3caebcdc4912ed2ecce1a77e45712

Checksum for the 8.6.13 patch:
MD5 (sendmail.8.6.13.patch) = 6390b792cb5513ff622da8791d6d2073

Checksum for the 8.7.4 release:
MD5 (sendmail.8.7.4.tar.Z) = 4bf774a12752497527aae11e2bdbab36

Checksum for the 8.7.4 patch:
MD5 (sendmail.8.7.4.patch) = ef828ad91fe56e4eb6b0cacced864cd5

3. Run smrsh as additional protection for sendmail.

With all versions of sendmail, we recommend that you install and use
the sendmail restricted shell program (smrsh). We urge you to do
this whether you use the vendor's supplied sendmail, install sendmail
yourself, or patch an earlier version of sendmail.

Beginning with version 8.7.1, 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.

---------------------------------------------------------------------------
The CERT Coordination Center thanks Eric Allman of Pangaea Reference Systems,
Andrew Gross of San Diego Supercomputer Center, Eric Halil of AUSCERT,
Wolfgang Ley of DFN-CERT, and Paul Vixie for their support in the development
of this advisory.
---------------------------------------------------------------------------

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).

We strongly urge you to encrypt any sensitive information you send by email.
The CERT Coordination Center can support a shared DES key and PGP. Contact the
CERT staff for more information.

Location of CERT PGP key
ftp://info.cert.org/pub/CERT_PGP.key

CERT Contact Information
------------------------
Email ce...@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

To be added to our mailing list for CERT advisories and bulletins, send your
email address to
cert-advis...@cert.org

CERT publications, information about FIRST representatives, and other
security-related information are available for anonymous FTP from
ftp://info.cert.org/pub/

CERT advisories and bulletins are also posted on the USENET newsgroup
comp.security.announce

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.

.........................................................................
Appendix A: Vendor Information

Current as of February 22, 1996
See CA-96.04.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.

-----------------------
Eric Allman (original author of sendmail)

Install a patch to sendmail.

If you are presently running sendmail 8.6.12, there is a patch that
makes version 8.6.13.

Similarly, if you are presently running sendmail 8.7.3, there is a
patch that makes version 8.7.4.

The patches are available for anonymous FTP from

Checksum for the 8.6.13 patch:
MD5 (sendmail.8.6.13.patch) = 6390b792cb5513ff622da8791d6d2073

Checksum for the 8.7.4 release:
MD5 (sendmail.8.7.4.tar.Z) = 4bf774a12752497527aae11e2bdbab36

Checksum for the 8.7.4 patch:
MD5 (sendmail.8.7.4.patch) = ef828ad91fe56e4eb6b0cacced864cd5

----------------------
Hewlett-Packard Company

Vulnerable, watch readme file for updates.

----------------------
IBM Corporation

IBM is working on fixes for sendmail.

----------------------
Silicon Graphics Inc.

At the time of writing of this document, a patch is planned for
IRIX versions 5.2, 5.3, 6.0, 6.0.1 and 6.1 for this issue.

This patch along with previous SGI Advisories and security patches
can be obtained via anonymous FTP from sgigate.sgi.com or its
mirror, ftp.sgi.com. SGI security patches and advisories are
provided freely to all interested parties. The advisories
and patches can be found in either the ~ftp/patches or ~ftp/security
directories along with more current pertinent information.

For assistance obtaining or working with security patches, please
contact your SGI support provider.

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