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CERT Advisory CA-99-17 Denial-of-Service Tools
From: aleph1 () UNDERGROUND ORG (Aleph One)
Date: Wed, 29 Dec 1999 00:01:53 -0800
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Note -- On Tuesday, December 28, 1999, beginning at 6:00 PM Eastern
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while routine maintenance is done.
CERT(R) Advisory CA-99-17 Denial-of-Service Tools
Original release date: December 28, 1999
A complete revision history is at the end of this file.
* All systems connected to the Internet can be affected by
denial-of-service attacks. Tools that run on a variety of UNIX and
UNIX-like systems and Windows NT systems have recently been
released to facilitate denial-of-service attacks. Additionally,
some MacOS systems can be used as traffic amplifiers to conduct a
New Distributed Denial-of-Service Tools
Recently, new techniques for executing denial-of-service attacks have
been made public. A tool similar to Tribe FloodNet (TFN), called Tribe
FloodNet 2K (TFN2K) was released. Tribe FloodNet is described in
Like TFN, TFN2K is designed to launch coordinated denial-of-service
attacks from many sources against one or more targets simultaneously.
It includes features designed specifically to make TFN2K traffic
difficult to recognize and filter, to remotely execute commands, to
obfuscate the true source of the traffic, to transport TFN2K traffic
over multiple transport protocols including UDP, TCP, and ICMP, and
features to confuse attempts to locate other nodes in a TFN2K network
by sending "decoy" packets.
TFN2K is designed to work on various UNIX and UNIX-like systems and
TFN2K obfuscates the true source of attacks by spoofing IP addresses.
In networks that employ ingress filtering as described in , TFN2K
can forge packets that appear to come from neighboring machines.
Like TFN, TFN2K can flood networks by sending large amounts of data to
the victim machine. Unlike TFN, TFN2K includes attacks designed to
crash or introduce instabilities in systems by sending malformed or
invalid packets. Some attacks like this are described in
Also like TFN, TFN2K uses a client-server architecture in which a
single client, under the control of an attacker, issues commands
simultaneously to a set of TFN2K servers. The servers then conduct the
denial-of-service attacks against the victim(s). Installing the server
requires that an intruder first compromise a machine by different
Asymmetric traffic from MacOS 9
MacOS 9 can be abused by an intruder to generate a large volume of
traffic directed at a victim in response to a small amount of traffic
produced by an intruder. This allows an intruder to use MacOS 9 as a
"traffic amplifier," and flood victims with traffic. According to ,
an intruder can use this asymmetry to "amplify" traffic by a factor of
approximately 37.5, thus enabling an intruder with limited bandwidth
to flood a much larger connection. This is similar in effect and
structure to a "smurf" attack, described in
Unlike a smurf attack, however, it is not necessary to use a directed
broadcast to achieve traffic amplification.
Intruders can flood networks with overwhelming amounts of traffic or
cause machines to crash or otherwise become unstable.
The problem of distributed denial-of-service attacks is discussed at
length in , available at
Managers, system administrators, Internet Service Providers (ISPs) and
Computer Security Incident Response Teams (CSIRTs) are encouraged to
read this document to gain a broader understanding of the problem.
For the ultimate victim of distributed denial-of-service attacks
Preparation is crucial. The victim of a distributed denial-of-service
attack has little recourse using currently available technology to
respond to an attack in progress. According to :
The impact upon your site and operations is dictated by the
(in)security of other sites and the ability of of a remote
attackers to implant the tools and subsequently to control
and direct multiple systems worldwide to launch an attack.
Sites are strongly encouraged to develop the relationships and
capabilities described in  before you are a victim of a distributed
For all Internet Sites
System and network administrators are strongly encouraged to follow
the guidelines listed in . In addition, sites are encouraged to
implement ingress filtering as described in . CERT/CC recommends
implementing such filtering on as many routers as practical. This
method is not foolproof, as mentioned in :
While the filtering method discussed in this document does
absolutely nothing to protect against flooding attacks which
originate from valid prefixes (IP addresses), it will
prohibit an attacker within the originating network from
launching an attack of this nature using forged source
addresses that do not conform to ingress filtering rules.
Because TFN2K implements features designed specifically to take
advantage of the granularity of ingress filtering rules, the method
described in  means that sites may only be able to determine the
network or subnet from which an attack originated.
Sites using manageable hubs or switches that can track which IP
addresses have been seen at a particular port or which can restrict
which MAC addresses can be used on a particular port may be able to
further identify which machine(s) is responsible for TFN2K traffic.
For further information, consult the documentation for your particular
hub or switch.
The widespread use of this type of filtering can significantly reduce
the ability of intruders to use spoofed packets to compromise or
Preventing your site from being used by intruders
TFN2K and similar tools rely on the ability of intruders to install
the client. Preventing your system from being used to install the
client will help prevent intruders from using your systems to launch
denial-of-service attacks (in addition to whatever damage they may
cause to your systems).
Popular recent attacks can be found at
Sites are encouraged to regularly visit this page and address any
issues found there.
For the "Mac Attack"
Apple is developing a patch, as described in Appendix A. This advisory
will be updated when the patch is available.
Appendix A contains information provided by vendors for this advisory.
We will update the appendix as we receive or develop more information.
If you do not see your vendor's name in Appendix A, the CERT/CC did
not hear from that vendor. Please contact your vendor directly.
Appendix A. Vendor Information
We've reproduced the problem in our lab and we are working now to
create a fix that can be easily distributed to our customers. The
problem only affects customers running our most recent release of
networking software on machines that are continuously attached to the
While most Macintosh customers are not affected by this problem, we
are moving quickly to put a solution in place.
 RFC2267, Network Ingress Filtering: Defeating Denial of Service
Attacks which employ IP Source Address Spoofing , P. Ferguson, D.
Senie, The Internet Society, January, 1998, available at
 Results of the Distributed-Systems Intruder Tools Workshop, The
CERT Coordination Center, December, 1999, available at
 The "Mac Attack," a Scheme for Blocking Internet Connections, John
A. Copeland, December, 1999, available at
http://www.csc.gatech.edu/~copeland. Temporary alternate URL:
The CERT Coordination Center thanks Jeff Schiller of the Massachusetts
Institute of Technology, Professor John Copeland and Jim Hendricks of
the Georgia Institute of Technology, Jim Ellis of Sun Microsystems,
Wietse Venema of IBM, Rick Forno of Network Solutions, Inc., Dave
Dittrich of the University of Washington, Steve Bellovin of AT&T, and
Jim Duncan and John Bashinski of Cisco Systems for input and technical
assistance used in the construction of this advisory.
This document is available from:
CERT/CC Contact Information
Email: cert () cert org
Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890
CERT personnel answer the hotline 08:00-20:00 EST(GMT-5) / EDT(GMT-4)
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December 28, 1999: Initial release
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CERT Advisory CA-99-17 Denial-of-Service Tools Aleph One (Dec 29)
Re: majordomo local exploit Christopher X. Candreva (Dec 29)
The "Mac DoS Attack," a Scheme for Blocking Internet Connections John Copeland (Dec 29)
Re: majordomo local exploit Olaf Kirch (Dec 29)
Re: majordomo local exploit Spidey (Dec 29)
Fwd: Sun Security Bulletin #00192 Bryan Blackburn (Dec 29)