A computer worm is a standalone malware computer program that replicates itself in order to spread to other computers. Often, it uses a computer network to spread itself, relying on security failures on the target computer to access it. Worms almost always cause at least some harm to the network, even if only by consuming bandwidth, whereas viruses almost always corrupt or modify files on a targeted computer.
Many worms are designed only to spread, and do not attempt to change the systems they pass through. However, as the Morris worm and Mydoom showed, even these "payload-free" worms can cause major disruption by increasing network traffic and other unintended effects.
On November 2, 1988, Robert Tappan Morris, a Cornell University computer science graduate student, unleashed what became known as the Morris worm, disrupting a large number of computers then on the Internet, guessed at the time to be one tenth of all those connected. During the Morris appeal process, the U.S. Court of Appeals estimated the cost of removing the virus from each installation at between $200 and $53,000; this work prompted the formation of the CERT Coordination Center and Phage mailing list. Morris himself became the first person tried and convicted under the 1986 Computer Fraud and Abuse Act.
Probably the most common payload for worms is to install a backdoor. This allows the computer to be remotely controlled by the worm author as a "zombie computers". Networks of such machines are often referred to as botnets and are very commonly used for a range of malicious purposes, including sending e-mail spam or performing DoS attacks.
Users need to be wary of opening unexpected email, Threat Description Email-Worm: VBS/LoveLetter and should not run attached files or programs, or visit web sites that are linked to such emails. However, as with the ILOVEYOU worm, and with the increased growth and efficiency of phishing attacks, it remains possible to trick the end-user into running malicious code.
Anti-virus and Antispyware software are helpful, but must be kept up-to-date with new pattern files at least every few days. The use of a firewall is also recommended.
In the April–June 2008 issue of IEEE Transactions on Dependable and Secure Computing, computer scientists described a new and potentially effective way to combat internet worms. The researchers discovered how to contain worms that scanned the Internet randomly, looking for vulnerable hosts to infect. They found that the key was to use software to monitor the number of scans that machines on a network send out. When a machine started to send out too many scans, it was a sign that it has been infected, which allowed administrators to take it off line and check it for malware. In addition, machine learning techniques can be used to detect new worms, by analyzing the behavior of the suspected computer.Moskovitch R., Elovici Y., Rokach L. (2008), " Detection of unknown computer worms based on behavioral classification of the host", Computational Statistics and Data Analysis, 52(9):4544–4566,
Users can minimize the threat posed by worms by keeping their computers' operating system and other software up to date, avoiding opening unrecognized or unexpected emails and running firewall and antivirus software.
Mitigation techniques include:
Several worms, like XSS Worm, have been written to research how worms spread. For example, the effects of changes in social activity or user behavior. One study proposed what seems to be the first computer worm that operates on the second layer of the OSI model (Data link Layer), it utilizes topology information such as Content-addressable memory (CAM) tables and Spanning Tree information stored in switches to propagate and probe for vulnerable nodes until the enterprise network is covered.