Introduction

These days, home PCs are a desirable target for attackers. Most of these systems run Microsoft Windows and often are not properly patched or secured behind a firewall, leaving them vulnerable to attack. In addition to these direct attacks, indirect attacks against programs the victim uses are steadily increasing. Examples of these indirect attacks include malicious HTML-files that exploit vulnerabilities in Microsoft's Internet Explorer or attacks using malware in Peer-to-Peer networks. Especially machines with broadband connection that are always on are a valuable target for attackers. As broadband connections increase, so to do the number of potential victims of attacks. Crackers benefit from this situation and use it for their own advantage. With automated techniques they scan specific network ranges of the Internet searching for vulnerable systems with known weaknesses. Attackers often target Class B networks (/16 in CIDR notation) or smaller net-ranges. Once these attackers have compromised a machine, they install a so called IRC bot - also called zombie or drone - on it.

Internet Relay Chat (IRC) is a form of real-time communication over the Internet. It is mainly designed for group (one-to-many) communication in discussion forums called channels, but also allows one-to-one communication.
More information about IRC can be found on Wikipedia.

We have identified many different versions of IRC-based bots (in the following we use the term bot) with varying degrees of sophistication and implemented commands, but all have something in common. The bot joins a specific IRC channel on an IRC server and waits there for further commands. This allows an attacker to remotely control this bot and use it for fun and also for profit. Attackers even go a step further and bring different bots together. Such a structure, consisting of many compromised machines which can be managed from an IRC channel, is called a botnet. IRC is not the best solution since the communication between bots and their controllers is rather bloated, a simpler communication protocol would suffice. But IRC offers several advantages: IRC Servers are freely available and are easy to set up, and many attackers have years of IRC communication experience.

Due to their immense size - botnets can consist of several ten thousand compromised machines - botnets pose serious threats. Distributed denial-of-service (DDoS) attacks are one such threat. Even a relatively small botnet with only 1000 bots can cause a great deal of damage. These 1000 bots have a combined bandwidth (1000 home PCs with an average upstream of 128KBit/s can offer more than 100MBit/s) that is probably higher than the Internet connection of most corporate systems. In addition, the IP distribution of the bots makes ingress filter construction, maintenance, and deployment difficult. In addition, incident response is hampered by the large number of separate organizations involved. Another use for botnets is stealing sensitive information or identity theft: Searching some thousands home PCs for password.txt, or sniffing their traffic, can be effective.

The spreading mechanisms used by bots is a leading cause for "background noise" on the Internet, especially on TCP ports 445 and 135. In this context, the term spreading describes the propagation methods used by the bots. These malware scan large network ranges for new vulnerable computers and infect them, thus acting similar to a worm or virus. An analysis of the traffic captured by the German Honeynet Project shows that most traffic targets the ports used for resource sharing on machines running all versions of Microsoft's Windows operating system:

  • Port 445/TCP (Microsoft-DS Service) is used for resource sharing on machines running Windows 2000, XP, or 2003, and other CIFS based connections. This port is for example used to connect to file shares.
  • Port 139/TCP (NetBIOS Session Service) is used for resource sharing on machines running Windows 9x, ME and NT. Again, this port is used to connect to file shares.
  • Port 137/UDP (NetBIOS Name Service) is used by computers running Windows to find out information concerning the networking features offered by another computer. The information that can be retrieved this way include system name, name of file shares, and more.
  • And finally, port 135/TCP is used by Microsoft to implement Remote Procedure Call (RPC) services. An RPC service is a protocol that allows a computer program running on one host to cause code to be executed on another host without the programmer needing to explicitly code for this.

The traffic on these four ports cause more then 80 percent of the whole traffic captured. Further research with tools such as
Nmap, Xprobe2 and p0f reveal that machines running Windows XP and 2000 represent the most affected software versions. Clearly most of the activity on the ports listed above is caused by systems with Windows XP (often running Service Pack 1), followed by systems with Windows 2000. Far behind, systems running Windows 2003 or Windows 95/98 follow.

But what are the real causes of these malicious packets? Who and what is responsible for them? And can we do something to prevent them? In this paper we want to show the background of this traffic and further elaborate the causes. We show how attackers use IRC bots to control and build networks of compromised machines (botnet) to further enhance the effectiveness of their work. We use classical GenII-Honeynets with some minor modifications to learn some key information, for example the IP address of a botnet server or IRC channel name and password. This information allows us to connect to the botnet and observe all the commands issued by the attacker.
At times we are even able to monitor their communication and thus learn more about their motives and social behavior. In addition, we give some statistics on the quantitative information we have learned through monitoring of more than one hundred botnets during the last few months. Several examples of captured activities by attackers substantiate our presentation.

For this research, a Honeynet of only three machines was used. One dial-in host within the network of the German ISP T-Online, one dial-in within the network of the German ISP NetCologne and one machine deployed at RWTH Aachen University. The hosts in the network of the university runs an unpatched version of Windows 2000 and is located behind a Honeywall. The dial-in hosts run a newly developed software called mwcollectd2, designed to capture malware. We monitor the botnet activity with our own IRC client called drone. Both are discussed in greater detail later in this paper.

Almost all Bots use a tiny collection of exploits to spread further. Since the Bots are constantly attempting to compromise more machines, they generate noticeable traffic within a network. Normally bots try to exploit well-known vulnerabilities. Beside from the ports used for resource sharing as listed above, bots often use vulnerability-specific ports. Examples of these ports include:

The vulnerabilities behind some of these exploits can be found with the help of a search on Microsoft's Security bulletins (sample):