USENIX Association SRUTI ’05: Steps to Reducing Unwanted Traffic on the Internet Workshop 39
The Zombie Roundup: Understanding, Detecting, and Disrupting Botnets
Evan Cooke,* Farnam Jahanian,*† Danny McPherson†
Abstract
Global Internet threats are undergoing a profound transformation from attacks designed solely to disable
infrastructure to those that also target people and organizations. Behind these new attacks is a large pool of
compromised hosts sitting in homes, schools, businesses, and governments around the world. These systems are
infected with a bot that communicates with a bot controller and other bots to form what is commonly referred to as
a zombie army or botnet. Botnets are a very real and quickly evolving problem that is still not well understood or
studied. In this paper we outline the origins and structure of bots and botnets and use data from the operator
community, the Internet Motion Sensor project, and a honeypot experiment to illustrate the botnet problem today.
We then study the effectiveness of detecting botnets by directly monitoring IRC communication or other command
and control activity and show a more comprehensive approach is required. We conclude by describing a system to
detect botnets that utilize advanced command and control systems by correlating secondary detection data from
multiple sources.
1 Introduction
Global Internet threats are undergoing a profound transformation from attacks designed solely to disable
infrastructure to those that also target people and organizations. This frightening new class of attacks directly
impacts the day-to-day lives of millions of people and endangers businesses around the world. For example, new
attacks steal personal information that can be used to damage reputations or lead to significant financial losses.
Current mitigation techniques focus on the symptoms of the problem, filtering the spam, hardening web browsers,
or building applications that warn against phishing tricks. While tools such as these are important, it is also critical
to disrupt and dismantle the infrastructure used to perpetrate the attacks.
At the center of these threats is a large pool of compromised hosts sitting in homes, schools, businesses, and
governments around the world. These systems are infected with a bot that communicates with a bot controller and
other bots to form what is commonly referred to as a zombie army or botnet. A bot can be differentiated from other
threats by a communication channel to a controller. Many bots found in the the wild today are a hybrid of previous
threats combined with a communication system They can propagate like worms, hide from detection like many
viruses, and include attack methods from toolkits.
The magnitude of the botnet problem is just beginning to be carefully documented. According to a recent
report, the number of new bots observed each day rose from less than 2,000 to more than 30,000 over the first six
months of 2004 [7]. The total number of bot infected systems has been measured to be between 800,000 to
900,000 and CERT has described botnets with more than 100,000 members [12, 6].
Botnets are a very real and quickly evolving problem that is still not well understood. In this paper, we
outline the problem and investigate methods of stopping bots. We identify three approaches for handling botnets:
(1) prevent systems from being infected, (2) directly detect command and control communication among bots and
between bots and controllers, and, (3) detect the secondary features of a bot infection such as propagation or
attacks.
The first approach is to prevent systems from being infected. There are a range of existing techniques,
including anti-virus software, firewalls, and automatic patching.
The second approach is to directly detect botnet command and control traffic. Botnets today are often
controlled using Internet Relay Chat (IRC) and one possible method of detecting IRC-based botnets is to monitor
TCP port 6667 which is the standard port used for IRC traffic [9]. One could also look for non-human behavioral