CERT-SEI

Trojan Horse Sendmail Distribution

Original release date: October 08, 2002
Last revised: March 25, 2003
Source: CERT/CC

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

Overview

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:

sendmail.8.12.6.tar.Z
sendmail.8.12.6.tar.gz

These files began to appear in downloads from the FTP server ftp.sendmail.org 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 of Sendmail

The primary distribution site for Sendmail is

http://www.sendmail.org/

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 <sendmail@Sendmail.ORG>
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 sendmail.org 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

http://www.cert.org/incident_notes/IN-2001-06.html

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

Reporting

The CERT/CC is interested in receiving reports of this activity. If machines under your administrative control are compromised, please send mail to cert@cert.org 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 comments.

Apple Computer, Inc.

Mac OS X and Mac OS X Server do not contain the vulnerability described in this report.

Debian

We can confirm that Debian does *not* ship the version with the trojan horse. Our version predates it.

Red Hat Inc.

"Red Hat Linux has not distributed version 8.12.6 of sendmail and is therefore not affected by this vulnerability"

Xerox

A response to this advisory is available from our web site:

http://www.xerox.com/security.



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 Lindner.

Copyright 2002 Carnegie Mellon University.

Revision History

October 08, 2002: Initial release
October 09, 2002: Fix simple error in sendmail.org URL
October 09, 2002: Added Red Hat vendor statement
October 09, 2002: Added Debian vendor statement
October 14, 2002: Added Apple Vendor statement
March 25, 2003:   Added vendor statement from Xerox