During the last years, we have seen a shift in how systems are being attacked. After a successful compromise, a bot (also referred to as zombie or drone) is often installed on the system. This small program provides a remote
This technique is used by attackers to form networks of compromised machines (so-called botnets) under a common Command and Control (C&C) infrastructure. With the help of a botnet, attackers can control several hundred or even thousands of bots at the same time, thus enhancing the effectiveness of their attack. In this section we discuss concepts behind bots and botnets. We show how bots can be used to attack other systems or how they can be used as spyware and provide several real-world examples of this threat.
Historically, the first bots were programs used in Internet Relay Chat (IRC, defined in RFC2810) networks. IRC was developed in the late 1980s and allows users to talk to each other in so-called IRC channels in real time. Bots offered services to other users -- for example, simple games or message services. But malicious behavior evolved soon and resulted in the so-called IRC wars, one of the first documented DDoS attacks. A DDoS attack is a distributed attack on a computer system or network that causes a loss of service to users.
Nowadays, the term bot describes a remote control program loaded on a computer, usually after a successful invasion, that is often used for nefarious purposes. During the last few years, bots like Agobot , SDBot, RBot, and many others, were often used in attacks against computer systems. Moreover, several bots can be combined into a botnet, a network of compromised machines that can be remotely controlled by the attacker. Botnets in particular pose a severe threat to the Internet community, since they enable an attacker to control a large number of machines. Attackers primarily use them for attacks against other systems, mass identity theft, or sending spam.
Three attributes characterize a bot: a remote control facility, the implementation of several commands, and a spreading mechanism to propagate it further. Let's look at each one in more detail:
- 1. A remote control lets an attacker manipulate infected machines. Bots
currently implement several different approaches for this mechanism.
• Typically, the bots controller uses a central IRC server for Command and Control (C&C). All bots join a specific channel on this server and interpret all the messages they receive here as commands. This structure is usually secured with the help of passwords to connect to the server, join a specific channel, or issue commands. Several bots also use SSL-encrypted communication.
• In other situations, such as when some bots avoid IRC and use covert communication channels, the controller uses, for example, communication channels via an HTTP or DNS tunnel instead of an inappropriate IRC protocol. They can, for example, encode commands to the bots inside HTTP requests or within DNS TXT records. Another possibility is to hide commands in images (steganography).
• Some bots use peer-to-peer (P2P) communication mechanisms to avoid a central C&C server because it's a single point of failure. Expect to see more and more bots implement P2P communication in the near future, since researchers have come up with several ways to track todays's IRC-based botnets.
- 2. Typically, two types of commands are implemented over the remote control
network: DDoS attacks and updates. DDoS attacks include SYN and UDP
flooding or more clever ones such as spidering attacks — those that start from
a given URL and follows all links in a recursive way — against websites.
Update commands instruct the bot to download a file from the Internet and
execute it. This lets the attacker issue arbitrary commands on the victim's
machine and dynamically enhance the bot's features. Other commands
include functions for sending spam, stealing sensitive information from the
victim (such as passwords or cookies), or using the victim's computer for other
The remote control facility and the commands that can be executed from it differentiate a bot from a worm, a program that propagates itself by attacking other systems and copying itself to them.
- 3. But like a worm, most bots also include a mechanism to spread further,
usually by automatically scanning whole network ranges and propagating
themselves via vulnerabilities. These vulnerabilities usually appear in the
Windows operating system, the most common being DCOM (MS03-026,
buffer overrun in RPC interface could allow code execution), LSASS
(MS04-011, security update for Microsoft Windows), or one of the newer
Microsoft security bulletins.
Attackers also integrate recently published exploits into their bots to react quickly to new trends. Propagation via network shares and weak passwords on other machines is another common technique: The bot uses a list of passwords and usernames to log on to remoteshares and then drops its copy. Propagation as an e-mail attachement, similar to e-mail worms, can also be used as a propagation vector. Some bots propagate by using P2P filesharing protocols, such as Kazaa and Limewire. Using interesting filenames, the bot drops copies of itself into these program's shared folders. It generates the filename by randomly choosing from sets of strings and hopes that an innocent user downloads and executes this file.
An additional characteristic applies to most bots we have captured in the wild. Most of them have at least one executable packer, a small program that compresses/encrypts the actual binary. Typically, the attacker uses tools such as UPX or Morphine to pack the executable. The packing hampers analysis and makes reverse engineering of the malware binary harder.
11.1.1 Examples of Bots
We now want to take a closer look at some specific bot variants to give you an overview of what type of bots can be found in the wild.
Agobot and Variants
Presumably the best-known family of bots includes Agobot/Gaobot; its variants Phatbot, Forbot, and XtrmBot; and several others. Currently, the antivirus (AV) vendor Sophos lists more than 1500 known different versions of Agobot, and this number is steadily increasing. The source code for Agobot was published at various websites in April 2004, resulting in many new variants being created each week.
Agobot was written by a young German man who was arrested and charged under the computer sabotage law for creating malicious computer code in May 2004. The bot is written in C++ with cross-platform capabilities and shows a very high abstract design. It is structured in a very modular way, and it is very easy to add commands or scanners for other vulnerabilities.For remote control, this family of bots typically uses a central C&C IRC server. Some variants also use P2P communication via the decentralizedWASTE network, thus avoiding a central server. In the variant we have analyzed, eight DoS-related functions were implemented and six different update mechanisms. Moreover, at least ten mechanisms to spread further exist. This malware is also capable of terminating processes that belong to antivirus and monitoring applications. In addition, some variants modify the hosts file, which contains the host name to IP address mappings. The malware appends a list of website addresses—for example, of AV vendors—and redirects them to the loopback address. This prevents the infected user from accessing the specified location.
Agobot and its variants use a packet sniffing library (libpcap) and Perl Compatible Regular Expressions (PCRE) to sniff and sort network traffic passing by the victim's computer. This can be used to retrieve sensitive information from the victim. In addition, Agobot can use NTFS Alternate Data Stream (ADS) to hide itself and offers rootkit capabilities like file and process hiding to hide its own presence on a compromised host. Furthermore, reverse-engineering this malware is harder, since it includes functions to detect debuggers and virtual machines, and it encrypts the configuration in the binary.
Upon startup, the program attempts to run a speed test for Internet connectivity. By accessing several servers and sending data to them, this bot tries to estimate the available bandwidth of the victim. This activity of the bot allows us to estimate the actual number of hosts compromised by this particular bot. This works by taking a look at log files — for example, Agobot uses www.belwue.de as one of the domains for this speed test. So the administrators of this domain can make an educated guess about the actual deployment of the bot by looking at how often this speed test was performed. In May 2004, about 300,000 unique IP addresses could be identified in this way per day . This shows that bots are a real threat nowadays.
A detailed analysis of this bot is available by LURHQ .
SDBot and Variants
SDBot and its variants RBot, UrBot, UrXBot, Spybot, are at the moment the most active bots in the wild. The whole family of SDBots is written in C and literally thousands of different versions exist, since the source code is public. The source code of this bot is not as well designed or written as the source code of Agobot. It offers similar features as Agobot, although the command set is not as large nor the implementation as sophisticated. Nevertherless, many attackers use this family of bots.
For remote control, this bot typically only offers the usage of a central IRC server. But there are also variants that used HTTP to command the bots. Again, the typical commands for remote control are implemented. More than ten DDoS attacks and four update functions were implemented in the bots we have analyzed. Moreover, this bot incorporates many different techniques to propagate further. Similar to Agobot and its variants, the whole family of SDBots includes more than ten different possibilities to spread further, including exploit to compromise remote systems and propagation with other mechanisms.
The evolution of bots through time can be observed by means of this family of bots. Each new version integrates some new features, and each new variant results in some major enhancements. New vulnerabilities are integrated in a couple of days after public announcement, and once one version has new spreading capabilities, all others integrate it very fast. In addition, small modifications exist that implement specific features (e.g., encryption of passwords within the malware) that can be integrated in other variants.
We will introduce some advanced functionalities of SDbot and its variants in Section 11.1.2, where we point out some use cases for bots as spyware.
We subsume all mIRC-based bots as GT-bots, since there are so many different versions of them that it is hard to get an overview of all forks. mIRC itself is a popular IRC client for Windows. GT is an abbreviation for "Global Threat," and this is the common name used for all mIRC-scripted bots. These bots launch an instance of the mIRC chat-client with a set of scripts and other binaries, so the remote control mechanism is IRC. One binary that we found in almost all cases is a HideWindow executable used to make the mIRC instance unseen by the user. The other binaries are mainly Dynamic Link Libraries (DLLs) linked to mIRC that add some new features the mIRC scripts can use. The mIRC scripts are used to control the bot and to implement several commands. They can access the spreading functions in the DLLs and thus enable further propagation. GT-bots spread by exploiting weaknesses on remote computers and uploading themselves to compromised hosts. One handicap is their large file size; they are sometimes larger than 1 MB. But besides this handicap, they can be classified with the scheme we have just presented.
Strictly speaking, Zotob is just a variant of Rbot. It gained much media attention, since it affected machines from companies such as CNN, ABC, and the New York Times. Zotob was one of the first bots to include the exploit for the Microsoft security bulletin MS05-039 (vulnerability in the Plug-and- Play component of Windows 2000), released on August 9, 2005. Only four days after the security bulletin, Zotob began spreading and compromised unpatched machines. It spread quickly and during a live show, reporters from CNN reported that their computers were affected by a new worm. But the fame for the botherders was short-lived: 13 days after the first release, on August 26, the Moroccan police arrested, at the request of the FBI, the suspected bot-herder. At the same time, another young man from Turkey was arrested as the suspected coder of Zotob.
Storm WormStarting with Nugache , we have seen more and more bots that use p2p based protocols for botnet command and control. One prominent example is Storm Worm, for which a detailed and very nice analysis is available by Joe Stewart . This particular piece of malware uses a variation of the eDonkey protocol to exchange command and update messages between the bots. Storm Worm was mainly used to attack a number of antispam websites via DDoS attacks. Since this botnet does not have a central server used for C&C, it is rather hard to track it, and shutting it down is even harder.
Other Interesting Bots
Besides these five types of bots, which are well known, five are also other bots are not that widespread. Some bots offer interesting features and are worth mentioning here.
Xot and its successor XT Bot are two bots that implement a feature called Dynamic Remote Settings Stub (DRSS).With the help of DRSS, the communication flow between the attacker and bots is hidden. This works by embedding the commands in a file -- for example, within an image. This file is then uploaded by the attacker to a server. The bot at the victim's computer downloads the file, extracts the information, and interprets the commands. Thus, the command flow is hidden with the help of steganography.
The Dataspy Network X (DSNX) bot is written in C++ and has a convenient plug-in interface. An attacker can easily write scanners and spreaders as plug-ins and extend the bot's features. This bot has one major disadvantage: The default version does not come with any spreaders. But plug-ins are available to overcome this "gap." Furthermore, plug-ins that offer services like DDoS attacks, portscan interface, or hidden web server are available and can be used by an attacker.
An interesting approach in the area of bots is Bobax. It uses HTTP requests as communication channel and thus implements a stealthier remote control than IRC-based C&C. In addition, it implements mechanisms to spread further and to download and execute arbitrary files. In contrast to other bots, the primary purpose of Bobax is sending spam.With the help of Bobax, an automated spamming network can be setup very easily. A detailed analysis of Bobax can be found at .
Presumably one of the most widespread bots is Toxbot. It is a variant of Codbot, a widespread family of bots. Common estimations of the botnet size achieved by Toxbot reach from a couple of hundred thousand compromised machines to more than one million. Giving an exact number is presumably not possible, but it seems like Toxbot was able to compromise a large number of hosts.
Another special bot is called aIRCBot. It is very small -- only 2560 bytes. It is not a typical bot because it only implements a rudimentary remote control mechanism. The bot only understands raw IRC commands. In addition, functions to spread further are completely missing. But due to its small size, it can nevertheless be used by attackers.
Q8Bot and kaiten are very small bots, consisting of only a few hundred lines of source code. Both have one additional noteworthiness: They are written for Unix/Linux systems. These programs implement all common features of a bot: dynamic updating via HTTP-downloads, various DDoS-attacks (e.g., SYN-flooding and UDP-flooding), a remote control mechanism, and many more. In the version we have captured, spreaders are missing, but presumably versions of these bots exist that also include mechanisms to propagate further.
There are many different version of very simple bots based on the programming language Perl. These bots contain, in most cases, only a few hundred lines of source code and offer only a rudimentary set of commands (most often only DDoS attack capabilities). This type of bots is also typically used on Unix-based systems.
Spyware in the Form of Bots
Currently, identity theft and stealing of sensitive information are some of the most severe threats in the Internet. Attackers often use spyware to steal this kind of information from compromised machines. A program is classified as spyware if it covertly collects sensitive information about the system it is running on, often without the knowledge of the owner of the system.
Spyware has become a major threat in today's Internet. For example, in May 2005 an incident in Israel showed how dangerous spyware can be. Several large companies in Israel are suspected to have used a malicious program to steal sensitive information from their rivals. In this espionage case, the malicious program was a kind of spyware that is able to retrieve sensitive data (e.g., spreadsheets or screen captures) from the victim's computer. This information is then sent to an FTP server controlled by the attacker. It allows him to collect all kinds of information for his nefarious purposes. The incident in Israel is just one of many examples of spyware used today.
In the following, we introduce several bots and show how they can be employed to spy on the users of the compromised machines. Our treatment of different bot types is, of course, incomplete, but we discuss the most prevalent uses. In addition to spying, an attacker can also issue arbitrary commands, since the vast majority of bots allow an attacker to install arbitrary programs on the victim's computer.
One of the most dangerous bot features is a keylogger. With the help of this functionality, an attacker can observe everything the victim is doing. A keylogger can reveal very sensitive information on the victim because she does not suspect that everything she types or clicks is observable by the attacker. Figure 11.1 shows an example output of a keylogger. The attacker can observe that the victim currently uses MSN Messenger, an instant messaging tool. In addition, he observes that the victim is using a search engine.
Figure 11.1 Example of keylogging feature
Another way to spy on the victim is to grab e-mail addresses or other contact information from the compromised machine. For example, Agobot supports searching for e-mail addresses or AOL contact information on the infected host. Via this spying mechanism, it is possible for an attacker to send customized spam or phishing e-mails to further victims (so-called spear phishing). More detailed information about the mechanics behind phishing attacks can be found in a whitepaper published by the Honeynet Project  or in the cases studies presented in Chapter 10.
Bots often include functions to steal CD-keys from the victim's hard disk. A CD-key is a credential to prove that a specific software has been legally purchased. For example, we found a version of Agobot that is capable of grabbing 26 different CD-keys from a compromised machine, ranging from popular games like Half-Life or Fifa to applications like Windows product IDs. Bots retrieve this information from theWindows registry. They search for characteristic keys and send this data to their controller, as shown in Figure 11.2. Furthermore, there are several other bots that allow the attacker to read arbitrary registry entries from the victim's computer. Another basic spy functionality is retrieving information about the victim's host. This information includes the speed of the CPU, the uptime, and IP address. For example, SDBot provides the attacker with several facts about the compromised host. Figure 11.3 shows the output of the two commands sysinfo and netinfo. We see that an attacker gets an overview of the hardware configuration and the network connectivity. Similarly, 4x10m, a rather uncommon bot, implements several functions to retrieve the registered owner and company of the compromised machine. This kind of information is especially interesting if the attacker plans to sell or rent his bots to others.
Figure 11.2 Example of an attack that steals CD keys from compromised machines.
Figure 11.3 Example of attack that retrieves information about the victim.
Many bots also include functions to search the hard drive of all victims for sensitive files, based on a regular expression. Moreover, these bots implement functions to download these files from the victim's computer. As an example, we take a look at a bot called reverb. This bot implements a function called weedfind that can be used to retrieve information. An example is the command ".weedfind c:.xls or c:finance*." This command lists all Excel spreadsheets and all files that contain the string finance on compromised machines.
Spybot, a quite popular bot nowadays, implements several methods to retrieve sensitive information from a victim. An analysis revealed that this specific spyware implements at least ten functions that can be used for spying purposes. Besides functions to retrieve a file listing and retrieve files, this bot also implements a function to delete files. In addition, Spybot offers a method to log keystrokes on the victim's machine. To achieve this, two functions are implemented: startkeylogger is used to start the logging of keystrokes and stopkeylogger to stop this function. The logged keystrokes are sent directly to the attacker. Moreover, keystrokes can also be sent to the victim's computer and thus arbitrary key-sequences can be simulated with the help of the sendkeys [keys] command. Spybot also implements functions that return information about the running processes. With the function listprocesses, a listing of all running processes can be retrieved, and killprocess [processname] can then be used to stop processes on the victim's machine—for example, an antivirus scanner or some kind of personal firewall. Our analysis revealed two additional functions to retrieve sensitive information from the victim's machines. First, the command passwords lists the Remote Access Service (RAS) password from computers running Windows. Second, the command cashedpasswords lists all passwords that are returned by the Windows API function WNetEnumCachedPasswords(). Table 11.1 gives a short summary of all functions from Spybot that are spyware-related, including examples of how an attacker could use these commands to retrieve sensitive information.Table 11.1 Summary of Spyware-Related Options in Spybot
Botnet Control Structure
After having introduced various different bots and a closer look at the spyware functionality of current bots, we now describe how attackers use the individual bots to form botnets.
Usually, the controller of the botnet compromises a series of systems using various tools and then installs a bot to enable remote control of the victim computer. As communication protocol for this remote command channel, most attackers use IRC or HTTP, but also other -- sometimes even proprietary -- communication protocols can be used.
A typical setup of a botnet is shown in Figure 11.4. A central IRC server is used for C&C. Normally, attackers use dynamic DNS names for their servers because it allows a botnet to be distributed across multiple servers. In addition, it allows an attacker to relocate the bots to another server in case one of the C&C servers goes down.
The bots connect to the server at a predefined port and join a specific channel. The attacker can issue commands in this channel, and these commands are sent via the C&C server to the individual bots, which then execute these commands. In this example, an attacker instructs all bots to attack a certain server via a distributed denial of service attack. All bots send as many packets as possible to the victim, effectively prohibiting the normal service.
Figure 11.4 Communication flow in a botnet.
Another protocol that is often used for the communication channel is HTTP. Within a request, the bot running on an infected machine encodes status information. This can, for example, look like the following request:
GET /cgi-bin/get.cgi?port=4260&ID=866592496&OS=WindowsME&CONN=LAN& TIME=11:28:55&new=true&kent_new=true
The infected machines tries to reach a web server running at a certain IP address. A CGI script is used as communication endpoint. Within the parameters of the script, the bot encodes several information:
As a reply, the server sends to the infected machine the command it should execute. In contrast to the push-based IRC C&C, HTTP-based C&C is more like a poll-based mechanism. The infected machine periodically queries the central server to retrieve new commands.
Another option for a botnet structure are P2P based protocols. We have seen some of these bots with Sinit , Nugache , and StormWorm . Common among all these bots is that they use one of the common P2P protocols — for example, eDonkey — and adopt it to their needs. The changes are usually small, but enough to command a bot via this protocol. In the following, we focus mainly on IRC-based botnets, since these are still the vast majority of bots spreading in the Internet.
Most bots can automatically scan whole network ranges and propagate themselves using vulnerabilities and weak passwords on other machines. After successful invasion, a bot uses TFTP, FTP, HTTP, CSend (a custom protocol used by some bots to send files to other users), or another custom protocol to transfer itself to the compromised host. The binary is started and tries to connect to the hard-coded master IRC server on a predefined port, often using a server password to protect the botnet infrastructure. This server acts as the C&C server to manage the botnet. Often a dynamic DNS name is provided rather than a hard-coded IP address, so the bot can be easily relocated. Using a specially crafted nickname, the bot tries to join the master's channel, often using a channel password, too. In this channel, the bot can be remotely controlled by the attacker.
Commands can be sent to the bot in two different ways: via sending an ordinary command directly to the bot or via setting a special topic in the command channel that all bots interpret. For example, the topic.asc dcom135 50 5 999 -c –s
tells the bots to spread further with the help of a known vulnerability (theWindows DCOM vulnerability) on TCP port 135. The bots start 50 concurrent threads that scan with a delay of 5 seconds for 999 seconds. The scans target machines within the same Class C network of the bot (parameter -c) and the bots are silent (parameter -s), — that is, they do not send any report about their activity to the master. As another example, the topic.update http://
instructs the bots to download a binary from the Internet via HTTP to the local filesystem and execute it (parameter 1). Finally, as a third example, the command.ddos.ack 85.131.xxx.xxx 22 500
orders the bots to attack a specific IP address with a DDoS attack. All bots send packets to the specified IP address on TCP port 22 for 500 seconds.
If the topic does not contain any instructions for the bot, then it does nothing but idle in the channel, awaiting commands. That is fundamental for most current bots. They do not spread if they are not told to spread in their master's channel.
To remotely control the bots, the controller of a botnet has to authenticate himself before issuing commands. This authentication is done with the help of a classical authentication scheme. At first, the controller has to log in with his username. Afterward, he has to authenticate with the correct password to approve his authenticity. The whole authentication process is usually only allowed from a predefined domain, so only certain people can start this process. Once an attacker is authenticated, he has complete control over the bots and can execute arbitrary commands as just shown.
DDoS Attacks Caused by Botnets
Today, botnets are very often used to mount DDoS attacks in the Internet. A DDoS attack is an attack on a computer system or network that causes a loss of service to users, typically the loss of network connectivity and services by consuming the bandwidth of the victim network or overloading the computational resources of the victim system. Using available tools , it is relatively easy to mount DDoS attacks against remote networks. For the (connection-oriented) Internet protocol TCP, the most common technique is called TCP SYN flooding [9,77] and consists of creating a large number of "half open" TCP connections on the target machine, thereby exhausting kernel data structures and making it impossible for the machine to accept new connections. For the (connectionless) protocol UDP, the technique of UDP flooding consists of overrunning the target machine with a large number of UDP packets, thereby exhausting its network bandwidth and other computational resources.
DDoS attacks are one of the most dangerous threats in the Internet today, since they are not limited to web servers.Virtually any service available on the Internet can be the target of such an attack. Higher-level protocols can be used to increase the load even more effectively by using very specific attacks, such as running exhausting search queries on bulletin boards or mounting web spidering attacks — that is, starting from a given website and then recursively requesting all links on that site.
In the past, there have been several examples of severe DDoS attacks. In February 2000, an attacker targeted major e-commerce companies and news sites . The network traffic flooded the available Internet connection so that no users could access these websites for several hours. In recent years, the threat posed by DDoS attacks grew and began to turn into real cybercrime. An example of this professionalism are the blackmail attempts against a betting company during the European soccer championship in 2004 . The attacker threatened to take the website of this company offline unless the company payed money. Similar documented cybercrime cases happened during other major sport events. Furthermore, paid DDoS attacks to take competitor's websites down were reported in 2004 . These types of attacks often involve botnets, since such a remote control network lets an attacker control a large number of compromised machines at the same time. Botnets often consist of several thousand machines and enable an attacker to cause serious damage. Botnets are regularly used for DDoS attacks, since their combined bandwidth overwhelms the available bandwidth of most target systems. In addition, several thousand compromised machines can generate so many packets per second that the target is unable to respond to that many requests.
All common bots include several different possibilities to participate in these attacks. Most commonly implemented, and also very often used, are TCP SYN and UDP flooding attacks. For example, the command .ddos.syn XXX.XXX.XXX. XXX 80 600 instructs the bots within the botnet to start a TCP SYN flooding attack against the specified IP address against TCP port 80 for 600 seconds. Now imagine that 1000 infected machines participate in this attack. If each of those users is connected to the Internet with an upstream connection of 1024 KBit/s, the resulting flow of 1000 * 1024 KBit/s will surely cause large problems at many sites. And this is no uncommon situation, as the following log shows:
Several bots send a report back to their controller, telling him with how many KB/s they flooded the victim. As you can see, botnets are a quite severe threat in the area of DDoS attacks.
Virtual Honeypots: From Botnet Tracking to Intrusion Detection
Home: Virtual honeypots: Tracking botnets
1: Bot and botnet 101
2: Tracking botnets
3: Case studies
4: Defending against bots
This was first published in October 2007