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September 15, 2004 (Computerworld) -- The economic impact of Internet worm attacks is staggering, with analysts reporting that the Bagle, Netsky and Mydoom worms combined caused several billion dollars in damage from lost productivity, business disruption, bandwidth consumption and manpower costs. While there were many worms in the past 18 months, there were only a few devastating ones, giving companies a somewhat false sense of comfort.
Now imagine a world where worm attacks frequently occur because hackers and rogue developers have access to "worm kits" or development tools that provide the basic building blocks for rapid worm development.
Historically, worms were basic clones of one another that didn't change after their original development. Simple mechanisms were used to propagate them, such as mass-mailing worms using a single subject line.
Today's worms are more sophisticated. They have the ability to mutate after development based on knowledge of how to thwart new security processes. For instance, an early worm, Code Red, attacked only Internet Information Server servers. The Nimda worm, which came later, expanded to include at least three additional attack methodologies: mail-based attacks, file-sharing-based attacks, and attacks against the Internet Explorer Web browser.
Worms become easier to create
The potential for this worm-a-day nightmare comes from several factors: the dozens of vulnerabilities that are ready to be exploited, the availability of worm source code, recycled exploits and the ease of editing existing worms.
Before a worm can be developed, a network vulnerability has to be identified. Recent research from Arbor Networks on the transition from vulnerability disclosure to worm release shows that there are dozens of vulnerabilities ready to be used as the propagation vector in Internet worms. However, only a handful are developed into worms every year, resulting in a large number of untapped vulnerabilities that attackers could use to spread their worms in the future.
All these vulnerabilities might not be a big deal if worms weren't getting so much easier to build. With available source code, worm authors can expand on these tools or recycle methods seen in successful worms. These code bases provide an excellent starting point for an aspiring worm author and drastically reduce development time.
Examples of techniques and resources hackers are using to expedite worm development include the following:
- Agobot source code is available on the Internet. Mydoom had its source code distributed by a follow-up worm, and the Bagle and Netsky worms shared source code.
- Opening a command shell on a network port and other widespread vulnerabilities can be used to download the executable file during propagation.
- Re-releasing worms is becoming more common as hackers capture a spreading worm and modify it using a hex editor, usually to distribute a new payload.
- By being able to recycle routines, techniques and code from previous worms, a worm author has less to develop and test and can reuse methods that have already proved successful.
All of these elements combine to lower the barrier to entry for the worm market, which means that more authors can write more worms. In recent years, spammers and distributed denial-of-service (DDoS) extortionists have learned that worms provide an excellent malware propagation platform, allowing them to efficiently spread spam relays, spyware and DDoS tool kits across the Internet. Their motive is often financial, so combining the ease of development and the reward potential only underscores the increased pace of worms in the coming months and years.
Worms change network traffic
Once a worm appears on the Internet or finds its way onto a corporate network, the security implications have only begun. A worm's release merely starts its spread; it isn't an isolated event. While the industry has spent a lot of time trying to combat the worm problem with signature-based detection methods, the increasing frequency and complexity of worms is forcing corporations to find out new ways to stop these attacks.
By their very nature, all worms will change the nature of the network's normal traffic patterns, which also provides a way to recognize and stop them through anomaly detection. The very nature of a worm outbreak changes the dynamics of a network and its component hosts. Traffic levels rise, and hosts begin behaving differently than they did before the worm's appearance. Interhost relationships also change, both in the nature of newly established roles and the attempted connections. Scans may also be observed as the worm actively targets new hosts. All of these events compound rapidly in almost every worm seen to date.
The anomaly-detection model typically uses several techniques to detect and characterize a worm by creating a profile of how the network and its traffic behave normally, including:
- Behavioral properties of a host, such as its role in the network and how many other nodes are linked to it and how many paths in and out of it.
- General traffic characteristics, often through statistical based approaches (such as rates and relative proportions) or relational models (such as client/server networks).
- Recognizing events such as scans or application-layer protocol anomalies.
By combining analysis of all these network parameters on a continuous basis, anomaly detection focuses on the traffic created by the worm. And when a worm mutates to avoid static detection methods, its traffic patterns will still be recognized.
The fact remains that worms are becoming easier to develop and propagate, and the number of known vulnerabilities provides plenty of opportunities for creating new worms. In preparing for the worm-a-day nightmare, network administrators need to shift to proactive solutions, such as anomaly detection or similar tools, that recognize worm behavior and stop it in real time.
Without such tools, the increasing frequency of worms will likely devastate networks and overwhelm administrators, increasing the economic impact of such events.
Jose Nazario is a worm researcher and software engineer at Arbor Networks Inc., a network security company in Lexington, Mass. He is the author of Defense and Detection Strategies Against Internet Worms (Artech House Publishers, 2003).
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