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Understanding and Defending against Botnets and Stealthy Malware


Jaideep Chandrashekar is a Research Scientist at Intel. His e-mail is jaideep.chandrashekar@intel.com. Carl Livadas is a Research Scientist at Intel Labs. His e-mail is clivadas@alum.mit.edu. Steve Orrin is Director of Security Solutions for Software Pathfinding and Innovation, a part of the Software and Services Group at Intel. His e-mail is steve.orrin@intel.com. Eve Schooler is a Principal Engineer at Intel Labs. Her e-mail is eve.m.schooler@intel.com. Copyright (c) 2009 Intel Corporation. All rights reserved.


With estimates of botnet infections continuing to gain in momentum, botnets are the latest scourge to hit the Internet and are the latest challenge for IT personnel. Each new botnet discovered reveals the use of more advanced technology and the use of quality software processes that are challenging the defense strategies of current intrusion detection systems (IDS). Thus, we begin this article with an overview of the state of the art of botnets and stealthy malware. We first describe the botnet lifecycle and highlight the advanced capabilities and stealth techniques in use today by botnets; we also examine and strategize about future advances in this area. We then go on to present several promising anti-botnet defense strategies, notably a collection of real traces to calibrate normalcy, the development of techniques that analyze communication with remote nodes with the goal of identifying botnet command-and-control (C&C) channels, and the application of various forms of correlation to amplify accuracy of detection and to root out stealthiness.

Botnets Defined

A botnet is a collection of distributed computers or systems that has been compromised, that is, taken over by rogue software. As a result, these machines are often called zombies or bots. Bots are controlled or directed by a bot-herder by means of one or more C&C servers. Most commonly, the bot-herder controls the botnet with C&C servers, delivered via protocols such as internet relay chat (IRC) or peer-to-peer (P2P) networking communications. Bots typically become installed on our devices via malware, worms, trojan horses, or other back-door channels. Further information on botnets can be found in [1].

The statistics for the size and growth of botnets differ widely, based on the reporting organization. According to Symantec's "Threat Horizon Report" [2], 55,000 new botnet nodes are detected every day, while a 2008 Report from USA Today states that "…on an average day, 40 per cent of the 800 million computers connected to the Internet are bots used to send out spam, viruses and to mine for sensitive personal data" [3]. USA Today also reports a tenfold increase in 2008 in the code threats reported over the same period in 2007, signifying the increase in threat surface area for botnet-style infections [3]. Various sources estimate that the best-known botnets -- Storm, Kraken, and Conficker -- have infected staggering numbers of machines. These numbers range from 85,000 machines infected by Storm, to 495,000 infected by Kraken [4], to a staggering 9 million nodes infected by Conficker [5].

The Underground Economy and Advances in Botnet Development

Like any money-driven market, botnet developers operate like a legitimate business: they take advantage of the economic benefits of cooperation, trade, and development processes, and quality. Recently, botnets have begun to use common software quality practices such as lifecycle management tools, peer reviews, object orientation, and modularity. Botnet developers are selling their software and infection vectors, providing documentation and support, as well as collecting feedback and requirements from customers.

Common economic goals are driving innovation, collaboration, and risk reduction in the Botnet communities. On-line barter and marketplace sites have sprung up to service this underground community with barter and trade forums, on-line support, and rent and lease options for bot-herders. This cooperation has led to a fairly mature economy where botnet nodes or groups are bought and sold, or where several bot-herders can cooperate when targeting an entity for attack. Botnets can be rented for the distribution of spam. Stolen identities and accounts are traded and sold among the participants.

The Botnet Lifecycle

The lifecycle of a botnet typically includes four phases: spread, infection, command and control (C&C), and attack, as shown in Figure 1. We describe each phase.

Figure 1: The Botnet Lifecycle (Source: Intel Corporation, 2009)

Spread Phase. In the spread phase in many botnets, the bots propagate and infect systems. Bots can spread through a variety of means, including SPAM e-mails, web worms, and through web downloads of malware that occur unbeknownst to users. Since the goal of the spread phase is to infect a system for the first time, bot-herders attempt to either trick the user into installing the malware payload or exploit vulnerabilities on the user system via applications or browsers, thereby delivering the malware payload.

Infection Phase. The malware payload, once on the system, uses a variety of techniques to infect the machine and obfuscate its presence. Advances in bot infection capabilities include techniques for hiding the infection and for extending the life of the infection by targeting the anti-malware tools and services that would normally detect and remove the infection. Botnets employ many of the standard malware techniques in use by viruses today. Polymorphism and rootkitting are two of the most common techniques in use.

  • By polymorphism, we mean that the malware code changes with every new infection, thus making it harder for anti-virus products to detect the code. Further, the use of code-hardening techniques often employed by SW developers to protect from SW piracy and reverse engineering, are in turn used by botnet developers. These techniques include code obfuscation, encryption, and encoding that further hide the true nature of the malware code as well as making it harder for anti-virus vendors to analyze it. There are indications that malware and botnet developers are beginning to look into advanced rootkitting techniques to further hide the malware.
  • By rootkitting, we mean the stealthy installation of malicious software—called a rootkit—that is activated each time a system boots up. Rootkits are difficult to detect because they are activated before the system's operating system (OS) has completely booted up. Advances in rootkit techniques include hyperjacking and virtualization-based rootkits as well as identifying and using new targets for code insertion such as firmware and BIOS.

A virtual machine monitor (VMM) or hypervisor runs underneath an OS, making it a particularly useful means for botnet and malware developers to gain control of computer systems. Hyperjacking involves installing a rogue hypervisor that can take complete control of a system. Regular security measures are ineffective against this hypervisor, because the OS is unaware that the machine has been compromised, and software anti-virus and local firewalls are unable to detect them.

Another technique that is currently used by botnet developers is to actively target the anti-virus, local firewall and intrusion prevention and detection software (IPS/ IDS) and services. Some of the techniques employed by botnets have included attacking the anti-virus and firewall software by killing its process or blocking its ability to get updates. Two examples that we know of show how botnets blocked the security software from getting updates:

  • A botnet changed the local DNS settings of the infected system to disable the anti-virus software from reaching its update site.
  • botnet was actively detecting connection attempts to the update site and blocking them.

These update-blocking techniques prevent the security software from getting potential updated signatures from the vendor that identify the newer version of the botnet or from being able to communicate with a central vendor server for anomaly correlation and update.

Timing the infection to strike between malware detection services scan times is another infection technique employed by botnet developers. The bot slowly infects a system without generating alarms in the intrusion detection software services.

Other advanced bots spoof the local and remote scans performed by the IDS/IPS and anti-virus software. In this case, the botnet's malware presents a false image of memory or hard disk to the anti-virus software to scan, or the malware disrupts vulnerability scans by dropping packets, spoofing the network response, or redirecting traffic coming from vulnerability scanners.

Command and Control. Botnet C&C servers use one of several protocols to communicate, the most common of which up to this point has been IRC. Recently, however, a trend towards the use of protected or hardened protocols has begun to emerge. For example, the Storm botnet uses an encrypted P2P protocol (eDonkey/Overnet). Advances in C&C techniques are crucial for bot-herders to keep their Botnets from being detected and shut down. To this end, botnets have begun to leverage protocols such as HTTP and P2P that are common across networks, thus making the botnet harder to detect. HTTP is particularly advantageous to botnets because of the sheer volume and diversity of HTTP traffic coming from systems today. Also, botnet software can take advantage of the local browser software for much of its functionality and communications stack, leveraging HTTP's ability to transit firewalls. Other techniques on the horizon include the use of VoIP, web services, and the use of scripting within the HTTP communications stack. Another advanced technique uses a blind drop, a site on the Internet such as a forum, BBS, or a newsgroup, where users can leave anonymous messages. Botnet nodes can post messages to these sites, and bot-herders can anonymously check for messages from their nodes and post instructions. The botnet nodes can then poll the site for new instructions and other communications as part of a messaging-based C&C. Social networking sites are a prime target for this kind of C&C.

A key feature of modern botnet development is the ability to re-program or update the botnet node software after it has infected a system. The C&C directs the node either to download the update directly or to go to a specific infected site hosting the update. Botnets with this reprogrammability have a higher value in the underground economy, as they can be augmented to perform new and advanced attack and stealth missions as they are developed.

As mentioned previously, stealth is a key feature of botnet technology. Kracken and Conficker Botnets both target and disable anti-virus software resident on the system. Other botnets deliberately try to hide from threshold-detection software by customizing the timing of infections and the frequency of communications to hide activities from both local and network security products. Steganographic techniques are the next method by which botnet developers plan to evade detection. They include the use of covert channels for communications and steganography-based messaging, such as mimicry and stegged content (i.e., embedding messages in content such as images, streaming media, VoIP, and so on).

Attack Phase. The final phase of the botnet lifecycle is the attack phase. In many cases the attack is simply the distribution of the SPAM that is carrying the infection, and when the attack is successful, the size of the botnet itself increases. Botnets also often have been used to send SPAM as part of barter and rental deals, whereby phishers, hackers, spammers, and virus writers use the botnet to sell information and services. Botnets also have been used to perform massive distributed denial-of-service (DoS) attacks against a variety of targets including government, corporate systems, and even other botnets. Some of the newer botnets can be upgraded to use various hacker tools, fault injectors (fuzzers), and so on, to further attack the networks they have infiltrated. For example, the Asprox botnet included an SQL injection attack tool, and another botnet included a Brute Force SSH attack engine. In addition to performing remote attacks, botnets can engage in persistent local attacks to phish for identities and accounts from the infected system and its users.


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