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CA-2002-30 Trojan Horse tcpdump and libpcap Distributions


CERT Advisory CA-2002-30 Trojan Horse tcpdump and libpcap Distributions

Original issue date: November 13, 2002
Last revised: --
Source: CERT/CC

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


The CERT/CC has received reports that several of the released source
code distributions of the libpcap and tcpdump packages were modified
by an intruder and contain a Trojan horse.

We strongly encourage sites that use, redistribute, or mirror the
libpcap or tcpdump packages to immediately verify the integrity of
their distribution.

I. Description

The CERT/CC has received reports that some copies of the source code
for libpcap, a packet acquisition library, and tcpdump, a network
sniffer, have been modified by an intruder and contain a Trojan horse.

The following distributions were modified to include the malicious


md5sum 3a1c2dd3471486f9c7df87029bf2f1e9 tcpdump-3.6.2.tar.gz
md5sum 3c410d8434e63fb3931fe77328e4dd88 tcpdump-3.7.1.tar.gz


md5sum 73ba7af963aff7c9e23fa1308a793dca libpcap-0.7.1.tar.gz

These modified distributions began to appear in downloads from the
HTTP server on or around Nov 11 2002 10:14:00 GMT. The
tcpdump development team disabled download of the distributions
containing the Trojan horse on Nov 13 2002 15:05:19 GMT. However, the
availability of these distributions from mirror sites is unknown. At
this time, it does not appear that related projects such as WinPcap
and WinDump contain this Trojan horse.

The Trojan horse version of the tcpdump source code distribution
contains malicious code that is run when the software is compiled.
This code, executed from the tcpdump configure script, will attempt to
connect (via wget, lynx, or fetch) to port 80/tcp on a fixed hostname
in order to download a shell script named services. In turn, this
downloaded shell script is executed to generate a C file (conftes.c),
which is subsequently compiled and run.

When executed, conftes.c makes an outbound connection to a fixed IP
address (corresponding to the fixed hostname used in the configure
script) on port 1963/tcp and reads a single byte. Three possible
values for this downloaded byte are checked, each causing conftes.c to
respond in different ways:

* 'A' will cause the Trojan horse to exit

* 'D' will cause the Trojan to fork itself, spawn a shell, and
redirect this shell to the connected IP address (Note that
communication to and from this shell is obfuscated by XORing all
bytes with the constant 0x89.)

* 'M' will cause the Trojan horse to close the connection and sleep
for 3600 seconds

To mask the activity of this Trojan horse in tcpdump, libpcap, the
underlying packet-capture library of tcpdump, has been modified
(gencode.c) to explicitly ignore all traffic on port 1963 (i.e., a BPF
expression of "not port 1963").

II. Impact

An intruder operating from (or able to impersonate) the remote address
specified in the malicious code could gain unauthorized remote access
to any host that compiled a version of tcpdump with this Trojan horse.
The privilege level under which this malicious code would be executed
would be that of the user who compiled the source code.

III. Solution

We encourage sites using libpcap and tcpdump to verify the
authenticity of their distribution, regardless of where it was

Where to get libpcap and tcpdump

While the compromise of these distributions is being investigated, the
tcpdump and libpcap maintainers recommend using the following
distribution sites:

Sites that mirror the source code are encouraged to verify the
integrity of their sources. We also encourage users to inspect any and
all other 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.

Verifying checksums

The MD5 hashes of the vendor suggested updates for libpcap and tcpdump
are as follows:


md5sum 03e5eac68c65b7e6ce8da03b0b0b225e tcpdump-3.7.1.tar.gz


md5sum 0597c23e3496a5c108097b2a0f1bd0c7 libpcap-0.7.1.tar.gz

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

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


We have checked all our released libpcap and tcpdump packages and
confirmed that they do not contain the trojan code.


Problematic packages are only distributed in Debian/unstable. I
have examined both source packages and they did not contain the
trojan code the HLUG reported on their web page. Hence, I guess
that Debian distributes safe source.

MontaVista Software, Inc.

We have examined our sources, and our software does not contain
this trojan. We are not vulnerable to this advisory.


SuSE Linux products are not vulnerable.

Feedback can be directed to the author: Roman Danyliw, Chad Dougherty.

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

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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
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Copyright 2002 Carnegie Mellon University.

Revision History
November 13, 2002: Initial release

Version: PGP 6.5.8


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