Book Excerpt

Defending against bots

After presenting the wide spectrum of possible usage of bots as an attack tool, we now want to present several ways to stop this threat. This should help you to get an overview over possible methods to detect the presence of bots and also to detect the existence of communication channels used for C&C. If you want to report the presence of a botnet to other people, it is best to have at least the following information present:

  • Information about who you are
  • What malware is using the botnet -- that is, if you have collected a botnet binary with the help of a honeypot, send them a copy. In that case, a common proceeding is to send them an encrypted ZIP archive with password infected
  • Information about the IP address(es) and port(s) used by the botnet and all additional information (username, nickname, channel, . . . ) you have
  • Any proof you have. This can be packet traces, log files, documented exploitation attempts, or anything else that can back up your claims
  • Estimated size of the botnet to give them an estimation of the threat level
  • Information about which steps to mitigate the botnet you have already taken
  • Any additional information you have collected up to that point
  • This information then helps the responsible people to get a quick overview of the situation. In any case, be certain to have information that can back up your claims. Expect to spend quite a lot of time for each report, and do not overlook the need to build strong and positive relationships with the support community.

    If you observe that the botnet is used to harm other people -- that is, a botnet that is used to steal personal information or distribute denial of service attacks, the best approach is to have law enforcement track the botmaster down and haul him into court. If you decide to take this road, contact your local law enforcement office and give them all the information you have. They will then initiate all necessary steps to collect more information and try to track down the botnet controller. After that, it is of your hands.

    If the C&C server is hosted on an exploited server, the best approach is to get in touch with the server's operators to have them look into the problem. For legitimate IRC servers that are abused by botnets, this appproach is relatively easy and usually successful. To get a point of contact, you must be sure who handles such matters. If you have only the hostname, resolve it to an IP address. Based on the IP, you can then use the tool whois to retrieve information about the network owner. The who-is information also contains the network operator's e-mail address, often in the form of abuse@PROVIDER. Send them all the information you have and hope that they respond to your report.

    Virtual Honeypots: From Botnet Tracking to Intrusion Detection
    Download the entire chapter in full as a .pdf file

    Presumably, the most effective method to stop bots is to stop the initial establishment of a connection from a bot to the C&C server. As just explained, most bots use a central server for C&C, and, in most cases, a dynamical DNS name is used for this server. This allows us to stop a botnet effectively. Once you know this DNS name, you can contact the DNS provider and ask for help. Since many DNS providers do not tolerate the abuse of their service, they are also interested in stopping the attack. The DNS provider can easily "blackhole" the dynamic DNS name—that is, set it to an IP address in the private range as defined in RFC 1918. If an infected machine then tries to contact the C&C server, the DNS name will resolve to a private IP address, and thus the bot cannot contact the real C&C server. This method is mostly used by CERTs and similar organizations. It proved to be quite effective, and many communication channels have been disrupted in this way. Nevertheless, it requires the cooperation with the DNS provider and this is not always possible. But if you send them a polite mail with all the information you have, you might get lucky.

    Since we observe the communication flow within the botnet, we are also able to observe the IP addresses of the individual bots unless this information is obfuscated — for example, by modifying the C&C server. Thus, one possible way to stop the botnet is to contact the owner of the compromised system. This is, however, a tedious and cumbersome job, since many organizations are involved, and these organizations are spread all over the world. In addition, the large number of bots make this approach nearly infeasible; only an automated notification system could help.

    There are also several methods to stop a bot within a network that can be carried out by a network administrator or security engineer. As always, the best way to stop a threat is to stop its root cause. In this case, this would mean eliminating the attack vectors and check for signs of intrusions, such as, by patching all machines and keeping AV signatures up to date. But this is often not easily possible. A 0day exploit—an exploit that has no available patch—cannot be eliminated in all cases, and patching needs some testing because it could also break important systems. In addition, AV scanners often cannot identify targeted attacks. With the recent malware outbreaks, we have seen that the time between a proof-of-concept exploit for a new security vulnerability and the integration of it into a bot is only several hours or days. Thus patching cannot always help, but at least try to keep up to date as much as possible.

    One quite effective method to detect the presence of bots also exploits their rather noisy nature. Most bots try to spread further by exploiting security flaws on other systems. To find such a system, they have to extensively scan the network for other machines. In addition, the communication channel often uses specific, rather unusual ports. So by looking at the state of your network, you can also detect bots. Netflow/cflow is an easy-to-use solution for this problem. The collected data often allows us to spot an infected machine. A typical sign is a spike in the number of outgoing connections, most often on TCP ports 445, 135, or on ports with recent security vulnerabilities. This is caused by bots that try to propagate further via common vulnerabilities. Another sign is a high amount of traffic on rather unusual ports.We analyzed the information about more than 11,000 botnets and found out that the vast majority of botnets use TCP port 6667 for C&C. Other common ports include TCP ports 7000, 3267, 5555, 4367, and 80. TCP port 6667 is commonly used for IRC, but you should take a look at this port and the others mentioned. In addition, tools like ngrep or Snort can help to detect the presence of C&C channels. One example is to search for typical C&C messages with the help of these tools. This can, for example, be done with the regular expression, shown in Figure 11.7, as proposed by Fischer [27].

    (advscan|asc|xscan|xploit|adv.start|adv5c4n) (webdav|netbios| ntpass|dcom(2|135|445|1025)|mssql|lsass|optix|upnp|ndcass|imail)
    Figure 11.7 Possible regular expression to detect C&C channel.

    Of course, such a method requires some human supervision, since it is not error-free and could lead to false positives. In addition, the C&C commands can change with time, so regular updates are necessary.

    In Section 10.1 we introduced an approach of how to use honeypots to detect the presence of bots in a given network. That method exploits the fact that bots try to propagate further by exploiting vulnerabilities on other hosts or other side effects -- for example, sending out spam e-mails. We detect this, and the honeypot provides us with additional information related to the botnet -- for example, a bot binary captured by nepenthes.

     


    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
      5: Summary

This was first published in October 2007

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