Day of the digital undead

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Like the living dead, armies of “zombie” computers are disrupting corporate networks and sucking the life out of business-critical systems around the world. Zombies strike fear into the hearts of IT personnel responsible for maintenance of corporate networks, and particularly those charged with protecting and ensuring the availability of vital corporate email systems.

A zombie is a computer that has been compromised by attackers, typically for the purpose of sending spam email and viruses to literally millions of recipients. Zombies are created by the use of trojans, malicious programs disguised as legitimate email attachments or file downloads. Once installed on the victim's computer, the trojan allows a remote hacker to take control of the machine and use it for any of a number of nefarious purposes. In fact, today's hackers (the "zombie masters") have become so sophisticated that they have begun creating coordinated networks of zombie computers that can launch a full-scale attack at a moment's notice. These attacks result in a wave of message traffic, usually directed at corporate systems. This traffic can include:

  • Spam
  • Phishing scams
  • Viruses
  • Distributed Denial of Service attacks
  • Redirects to websites containing malicious code
  • Once zombie networks have been established, they are often auctioned or traded among underground networks of attackers. Thus, a single compromised system may be under the control of multiple criminals, each with a different purpose in mind.

    Anatomy of a Zombie

    Zombies can be created in several ways, including via peer-to-peer networks and maliciously encoded websites. However, the most popular method for distributing the Trojans that create zombies is via an email attachment masquerading as an innocent file, such as a digital photo or contest entry form. When the user executes the attachment, the trojan installs itself onto the user's machine. Once installed, the trojan sends a notification to the network operator that the machine is now ready to be controlled remotely. There is typically no notification to the user that any of this has happened, so they are often completely unaware that their system has been compromised. In fact, many users voluntarily pass the trojan on by forwarding the email to others.

    The first generation of zombies was made up mostly of corporate-based machines such as web, email, or DNS servers. Because these machines were on high-speed networks, they provided an ideal platform from which damaging attacks could be launched. However, corporate systems have become increasingly secure and more tightly monitored, making them less attractive to hackers. Now, the hackers have turned to the next set of victims, vulnerable home computer users. These computers are easy targets, as home users often lack the internet savvy necessary to adequately protect their machines with firewalls and up-to-date antivirus protection; many will also willingly open email attachments from unknown senders, enticed by the promise of easy money or cheap prescriptions. In addition, the widespread availability of always-on, high-speed home connections using cable and DSL has made the home user an obvious target for zombie masters. In fact, this alone is the single largest contributing factor to recent escalations in spam, phishing attacks and Distributed Denial of Service (DDoS) attacks.

    How Zombies Attack

    Zombie networks can be configured to attack enterprises in a number of ways. Among the most popular is to create a globally dispersed "spam cannon," sending massive volumes of unwanted email, including dangerous phishing scams that prey on unsuspecting end users. When the attacks involve distribution of viruses via spam techniques, the stakes are raised even higher. The most damaging type of zombie attack, however, is the DDoS, which is a flood of traffic targeted at a specific network, eventually resulting in system overload and shutdown.

    These attacks have been known to continue for days and enterprises that are not prepared for them can be brought to their knees, as loss of the email system can cause a major disruption in the flow of business activities. A service degradation attack is a similar type of attack, except that rather than sending huge amounts of traffic all-at-once, the zombies increase traffic to an email system gradually until the system is eventually overloaded and knocked offline.

    The costs of a DDoS attack can be crippling to enterprises, resulting in lost sales during downtime and recovery, and more importantly, loss of trust from partners and customers should the attack become public information. Realizing the strength of the hand they hold, zombie masters have put a new twist on an old game, extorting money from corporations in exchange for assurance that the corporate networks will not be brought down by an attack. In addition, an evolving threat is the commissioning of attacks on rival companies. In one case, a business owner paid as little as $1000 to have DDoS attacks carried out on two competitors' websites in order to shut them down for hours at a time.

    To understand just how large a zombie network can grow, consider this: a Norwegian telecommunications provider recently discovered and shut down a network consisting of over 10,000 zombie machines. Taking into consideration that each zombie involved in a recent DDoS attack launched 64 connection attempts per second against the targeted corporation, it's easy to see how even the most robust systems can wilt under the massive load inflicted upon them.

    Keep Zombies at Bay

    Some basic tenets of security should be followed at all times, whether you want to protect your enterprise network from spam, viruses and DDoS attacks spewed forth by zombie networks, or protect your home computer from joining the ranks of the undead.

    Enterprise Protection

    The single most effective way to ensure that zombies don't find their way into your corporate networks is to trust your email security to a hardened gateway appliance. They offer both inbound and outbound protection via an objective, dynamic reputation system, connection management technology and robust antivirus capabilities.

    Reputation Systems

    The purpose of a reputation system is to determine who the "good" senders are. These can be defined simply as those who have historically sent legitimate mail on a regular basis. By evaluating senders based on their past behavior, a more accurate picture of their intentions and legitimacy can be discerned. Has the sender engaged in spamming, virus distribution or phishing attacks? If they have, an effective reputation system knows and flags the message. Has the sender ever been seen before? If not, a reputation system should pay close attention to ensure that the sender is not a zombie machine.

    Connection Management

    Connection management technology works by combining an objective reputation system with traffic shaping technology. If the reputation system determines that the sender is unwelcome, an effective technique for encouraging them to focus their efforts elsewhere is to reject connections from their IP address altogether for pre-defined intervals. And since having to scan fewer messages reduces throughput needs, robust connection management technology provides a significant boost to your return on investment.

    Outbound Antivirus Protection

    Should a situation arise in which a workstation on your corporate network is infected by a trojan, outbound antivirus scanning will ensure that it is unable to spread outside the network via email. You don't need to be told about the disastrous consequences of spreading viruses to partners and customers, and keeping your email server from becoming a "spam cannon" is of paramount importance to protecting your company's reputation as a responsible sender.

    Home Protection

    The best way to ensure that a personal computer is not taken over by a zombie inducing trojan is to protect it. To that end, some basic security steps should be followed at all times:

    Get Vaccinated

    Be sure to keep the antivirus software on your computer up-to-date. Many antivirus programs offer automatic updating to keep up with the ever-changing virus landscape. Take advantage of this feature if it's available from your antivirus software. If it's not, find a solution that offers it. Protection without automated updating is obsolete before it's ever installed.

    Firewall Your System

    A firewall is designed to stop unwanted traffic flowing into our out of a system, and "hide" your computer from would-be snoops. The Microsoft Windows XP operating system has built-in firewall functionality, and many free personal firewall programs are available for private use.

    Don't Talk to Strangers

    Never open unanticipated file attachments, especially when they come from unknown sources. If you're not sure about an attachment, save it to your hard drive first and scan it with antivirus software to check for dangerous files.

    Slaying the Zombies

    All jokes about shotguns and baseball bats aside, fighting zombies is not easy work. Zombies exist in huge numbers and are designed to be disposable so that if some are "killed," or taken offline, the zombie master hardly notices. The lifespan of a zombie is short, and most are useful for only a few days or weeks before they are repaired or otherwise compromised. To combat their short life expectancy, zombies are self-propagating, constantly infecting new computers.

    The author is VP and MD CipherTrust EMEA

    Copyright © SC Magazine, US edition
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