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CA-2002-28 Trojan Horse Sendmail Distribution


CERT Advisory CA-2002-28 Trojan Horse Sendmail Distribution

Original release date: October 08, 2002
Last revised: --
Source: CERT/CC

A complete revision history is at the end of this file.


The CERT/CC has received confirmation that some copies of the source
code for the Sendmail package were modified by an intruder to contain
a Trojan horse.

Sites that employ, redistribute, or mirror the Sendmail package should
immediately verify the integrity of their distribution.

I. Description

The CERT/CC has received confirmation that some copies of the source
code for the Sendmail package have been modified by an intruder to
contain a Trojan horse.

The following files were modified to include the malicious code:


These files began to appear in downloads from the FTP server on or around September 28, 2002. The Sendmail
development team disabled the compromised FTP server on October 6,
2002 at approximately 22:15 PDT. It does not appear that copies
downloaded via HTTP contained the Trojan horse; however, the CERT/CC
encourages users who may have downloaded the source code via HTTP
during this time period to take the steps outlined in the Solution
section as a precautionary measure.

The Trojan horse versions of Sendmail contain malicious code that is
run during the process of building the software. This code forks a
process that connects to a fixed remote server on 6667/tcp. This
forked process allows the intruder to open a shell running in the
context of the user who built the Sendmail software. There is no
evidence that the process is persistent after a reboot of the
compromised system. However, a subsequent build of the Trojan horse
Sendmail package will re-establish the backdoor process.

II. Impact

An intruder operating from the remote address specified in the
malicious code can gain unauthorized remote access to any host that
compiled a version of Sendmail from this Trojan horse version of the
source code. The level of access would be that of the user who
compiled the source code.

It is important to understand that the compromise is to the system
that is used to build the Sendmail software and not to the systems
that run the Sendmail daemon. Because the compromised system creates a
tunnel to the intruder-controlled system, the intruder may have a path
through network access controls.

III. Solution

Obtain an authentic version Sendmail

The primary distribution site for Sendmail is

Sites that mirror the Sendmail source code are encouraged to verify
the integrity of their sources.

Verify software authenticity

We strongly encourage sites that recently downloaded a copy of the
Sendmail distribution to verify the authenticity of their
distribution, regardless of where it was obtained. Furthermore, we
encourage users to inspect any and all software that may have been
downloaded from the compromised site. Note that it is not sufficient
to rely on the timestamps or sizes of the file when trying to
determine whether or not you have a copy of the Trojan horse version.

Verify PGP signatures

The Sendmail source distribution is cryptographically signed with the
following PGP key:

pub 1024R/678C0A03 2001-12-18 Sendmail Signing Key/2002
Key fingerprint = 7B 02 F4 AA FC C0 22 DA 47 3E 2A 9A 9B 35 22 45

The Trojan horse copy did not include an updated PGP signature, so
attempts to verify its integrity would have failed. The
staff has verified that the Trojan horse copies did indeed fail PGP
signature checks.

Verify MD5 checksums

In the absence of PGP, you can use the following MD5 checksums to
verify the integrity of your Sendmail source code distribution:
Correct versions:

73e18ea78b2386b774963c8472cbd309 sendmail.8.12.6.tar.gz
cebe3fa43731b315908f44889d9d2137 sendmail.8.12.6.tar.Z
8b9c78122044f4e4744fc447eeafef34 sendmail.8.12.6.tar.sig

As a matter of good security practice, the CERT/CC encourages users to
verify, whenever possible, the integrity of downloaded software. For
more information, see

Employ egress filtering

Egress filtering manages the flow of traffic as it leaves a network
under your administrative control.

In the case of the Trojan horse Sendmail distribution, employing
egress filtering can help prevent systems on your network from
connecting to the remote intruder-controlled system. Blocking outbound
TCP connections to port 6667 from your network reduces the risk of
internal compromised machines communicating with the remote system.

Build software as an unprivileged user

Sites are encouraged to build software from source code as an
unprivileged, non-root user on the system. This can lessen the
immediate impact of Trojan horse software. Compiling software that
contains Trojan horses as the root user results in a compromise that
is much more difficult to reliably recover from than if the Trojan
horse is executed as a normal, unprivileged user on the system.

Recovering from a system compromise

If you believe a system under your administrative control has been
compromised, please follow the steps outlined in

Steps for Recovering from a UNIX or NT System Compromise


The CERT/CC is interested in receiving reports of this activity. If
machines under your administrative control are compromised, please
send mail to with the following text included in the
subject line: "[CERT#33376]".

Appendix A. - Vendor Information

This appendix contains information provided by vendors for this
advisory. As vendors report new information to the CERT/CC, we will
update this section and note the changes in our revision history. If a
particular vendor is not listed below, we have not received their

The CERT Coordination Center thanks the staff at the Sendmail
Consortium for bringing this issue to our attention.

Feedback can be directed to the authors: Chad Dougherty, Marty

This document is available from:

CERT/CC Contact Information

Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
Postal address:
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890

CERT/CC personnel answer the hotline 08:00-17:00 EST(GMT-5) /
EDT(GMT-4) Monday through Friday; they are on call for emergencies
during other hours, on U.S. holidays, and on weekends.

Using encryption

We strongly urge you to encrypt sensitive information sent by email.
Our public PGP key is available from

If you prefer to use DES, please call the CERT hotline for more

Getting security information

CERT publications and other security information are available from
our web site

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* "CERT" and "CERT Coordination Center" are registered in the U.S.
Patent and Trademark Office.

Any material furnished by Carnegie Mellon University and the Software
Engineering Institute is furnished on an "as is" basis. Carnegie
Mellon University makes no warranties of any kind, either expressed or
implied as to any matter including, but not limited to, warranty of
fitness for a particular purpose or merchantability, exclusivity or
results obtained from use of the material. Carnegie Mellon University
does not make any warranty of any kind with respect to freedom from
patent, trademark, or copyright infringement.

Conditions for use, disclaimers, and sponsorship information

Copyright 2002 Carnegie Mellon University.

Revision History
October 08, 2002: Initial release

Version: PGP 6.5.8


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