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CA-2001-20 Continuing Threats to Home Users


CERT Advisory CA-2001-20 Continuing Threats to Home Users

Original release date: July 20, 2001
Source: CERT/CC

A complete revision history can be found at the end of this file.

Need to Protect Home Systems

This year, we have seen a significant increase in activity resulting
in compromises of home user machines. In many cases, these machines
are then used by intruders to launch attacks against other
organizations. Home users have generally been the least prepared to
defend against attacks. Many home users do not keep their machines up
to date with security patches and workarounds, do not run current
anti-virus software, and do not exercise caution when handling email
attachments. Intruders know this, and we have seen a marked increase
in intruders specifically targeting home users who have cable modem
and DSL connections.

Most of the subscribers to the CERT Advisory Mailing List and many
visitors to our web site are technical staff responsible for
maintaining systems and networks. But all of us know people who have
home computers and need advice about how to secure them. We recently
released a document on our web site providing some basic security
information and references for home users. The document, "Home Network
Security," is available on our web site at

We encourage the technical readers of our mailing list to reach out to
your parents, children, and other relatives and friends who might not
be as technically oriented, point them to this document and help them
understand the basics of security, the risks, and how they can better
defend themselves. We have a long road to travel in educating home
users on the security risks of the Internet. But all of us working
together to educate home users will improve the security of the
Internet as a whole.

Worms and DDoS Tools

The CERT/CC is currently tracking the activity of several large-scale
incidents involving new worms and distributed denial-of-service (DDoS)
tools. Some of these worms include a command and control structure
that allows the intruder to dynamically modify the behavior of the
worm after it has infected a victim system. In some cases, the command
and control structure allows the intruder to issue a single command to
all the infected systems without needing to know which systems have
actually been infected. This ability to change the behavior of the
worm (including wholesale replacement), makes it substantially more
difficult to develop "one size fits all" solutions to the problem.
Additionally, many of these worms have targeted home users as victims.

With these facts in mind, and the large number of hosts involved in
these incidents, it is imperative for everyone to take precautions to
patch the vulnerabilities involved and recover compromised systems.

W32/Leaves worm

The W32/Leaves worm, described in IN-2001-07 primarily affects systems
that have been previously compromised by the SubSeven Trojan horse
program. We have received reports that over 23,000 machines have been
compromised by this worm. This worm includes functionality that allows
a remote intruder to control the network of compromised machines.

"Code Red" worm

The "Code Red" worm, described in CA-2001-19 exploits a vulnerability
in the Indexing Service on systems running Microsoft IIS. Current
reports indicate that over 225,000 hosts have already been compromised
by this worm.

"Power" worm

A worm, known by the name of "Power" is also compromising systems
vulnerable to the IIS Unicode vulnerability described in CA-1999-16.
It uses the Internet Relay Chat (IRC) as a control channel for
coordinating compromised machines in DDoS attacks. Based on reports
that we have received, over 10,000 machines have already been
compromised by this worm.

"Knight" distributed attack tool

An attack tool known as "Knight" has been found on approximately 1,500
hosts. This tool appears to be a DDoS tool and also uses IRC as a
control channel. It has been reported that the tool is commonly being
installed on machines that were previously compromised by the
BackOrifice Trojan horse program. So far, there has been no indication
that this tool is a worm; it does not contain any logic to propagate

Protective Measures

For all of these problems, the deployment and maintenance of some
these simple defenses are relatively effective:

1. Install and Maintain Anti-Virus Software

The CERT/CC strongly recommends using anti-virus software. Most
current anti-virus software products are able to detect and alert the
user that an intruder is attempting to install a Trojan horse program
or that one has already been installed.

In order to ensure the continued effectiveness of such products, it is
important to keep them up to date with current virus and attack
signatures supplied by the original vendors. Many anti-virus packages
support automatic updates of virus definitions. We recommend using
these automatic updates when available.

2. Deploy a Firewall

The CERT/CC also recommends using a firewall product, such as a
network appliance or a personal firewall software package. In some
situations, these products may be able to alert users to the fact that
their machine has been compromised. Furthermore, they have the ability
to block intruders from accessing backdoors over the network. However,
no firewall can detect or stop all attacks, so it is important to
continue to follow safe computing practices.

For additional information about securing home systems and networks,
please see the "Home Network Security" tech tip at

If these protective measures reveal that the machine has already been
compromised, more drastic steps need to be taken to recover. When a
computer is compromised, any installed software could have been
modified, including the operating system, applications, data files,
and memory. In general, the only way to ensure that a compromised
computer is free from backdoors and intruder modifications is to
re-install the operating system from the distribution media and
install vendor-recommended security patches before connecting back to
the network. Merely identifying and fixing the vulnerability that was
used to initially compromise the machine may not be enough.

Often, these worms rely on Trojan horses to initially compromise a
system. For more information on Trojan horses, see

Additionally, these worms often spread by exploiting vulnerabilities
in systems. For information on vulnerabilities affecting popular
products, please see

Author(s): Jeff Carpenter, Chad Dougherty, Shawn Hernan

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 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

To subscribe to the CERT mailing list for advisories and bulletins,
send email to Please include in the body of your

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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 2001 Carnegie Mellon University.

Revision History
Jul 20, 2001: Initial release

Version: PGPfreeware 5.0i for non-commercial use
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