Computer worm
A computer worm is a self-replicating malware
Malware, short for malicious software, consists of programming that is designed to disrupt or deny operation, gather information that leads to loss of privacy or exploitation, or gain unauthorized access to system resources, or that otherwise exhibits abusive behavior...

 computer program
Computer program
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...

, which uses a computer network
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....

 to send copies of itself to other nodes (computers on the network) and it may do so without any user intervention. This is due to security shortcomings on the target computer. Unlike a computer virus
Computer virus
A computer virus is a computer program that can replicate itself and spread from one computer to another. The term "virus" is also commonly but erroneously used to refer to other types of malware, including but not limited to adware and spyware programs that do not have the reproductive ability...

, it does not need to attach itself to an existing program. Worms almost always cause at least some harm to the network, even if only by consuming bandwidth
Bandwidth (computing)
In computer networking and computer science, bandwidth, network bandwidth, data bandwidth, or digital bandwidth is a measure of available or consumed data communication resources expressed in bits/second or multiples of it .Note that in textbooks on wireless communications, modem data transmission,...

, whereas viruses almost always corrupt or modify files on a targeted computer.


Many worms that have been created are only designed to spread, and don't attempt to alter 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. A "payload
Payload (software)
Payload in computing is the cargo of a data transmission. It is the part of the transmitted data which is the fundamental purpose of the transmission, to the exclusion of information sent with it solely to facilitate delivery.In computer security, payload refers to the...

" is code in the worm designed to do more than spread the worm–it might delete files on a host system (e.g., the ExploreZip
ExploreZip, also known as I-Worm.ZippedFiles, is a destructive computer worm which attacks machines running Microsoft Windows. It was first discovered in Israel on June 6, 1999.Worm.ExploreZip is a worm that contains a malicious payload. The worm utilizes Microsoft Outlook, Outlook Express, or...

 worm), encrypt files in a cryptoviral extortion
Cryptovirology is a field that studies how to use cryptography to design powerful malicious software. The field was born with the observation that public-key cryptography can be used to break the symmetry between what an antivirus analyst sees regarding a virus and what the virus writer sees...

 attack, or send documents via e-mail
Electronic mail, commonly known as email or e-mail, is a method of exchanging digital messages from an author to one or more recipients. Modern email operates across the Internet or other computer networks. Some early email systems required that the author and the recipient both be online at the...

. A very common payload for worms is to install a backdoor in the infected computer to allow the creation of a "zombie" computer under control of the worm author. Networks of such machines are often referred to as botnets and are very commonly used by spam
E-mail spam
Email spam, also known as junk email or unsolicited bulk email , is a subset of spam that involves nearly identical messages sent to numerous recipients by email. Definitions of spam usually include the aspects that email is unsolicited and sent in bulk. One subset of UBE is UCE...

 senders for sending junk email or to cloak their website's address. Spammers are therefore thought to be a source of funding for the creation of such worms, and the worm writers have been caught selling lists of IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...

es of infected machines. Others try to blackmail companies with threatened DoS
Denial-of-service attack
A denial-of-service attack or distributed denial-of-service attack is an attempt to make a computer resource unavailable to its intended users...


Backdoors can be exploited by other malware, including worms. Examples include Doomjuice, which spreads better using the backdoor opened by Mydoom, and at least one instance of malware taking advantage of the rootkit
A rootkit is software that enables continued privileged access to a computer while actively hiding its presence from administrators by subverting standard operating system functionality or other applications...

 and backdoor installed by the Sony/BMG DRM
Digital rights management
Digital rights management is a class of access control technologies that are used by hardware manufacturers, publishers, copyright holders and individuals with the intent to limit the use of digital content and devices after sale. DRM is any technology that inhibits uses of digital content that...

 software utilized by millions of music CDs prior to late 2005.

Worms with good intent

Beginning with the very first research into worms at Xerox PARC
Xerox PARC
PARC , formerly Xerox PARC, is a research and co-development company in Palo Alto, California, with a distinguished reputation for its contributions to information technology and hardware systems....

, there have been attempts to create useful worms. The Nachi family of worms, for example, tried to download and install patches from Microsoft's website to fix vulnerabilities in the host system–by exploiting those same vulnerabilities. In practice, although this may have made these systems more secure, it generated considerable network traffic, rebooted the machine in the course of patching it, and did its work without the consent of the computer's owner or user.

Some worms, such as XSS worms
XSS Worm
An XSS worm, sometimes referred to as a cross site scripting virus, is a malicious payload, usually written in JavaScript, that propagates among visitors of a website in the attempt to progressively infect other visitors...

, have been written for research to determine the factors of how worms spread, such as social activity and change in user behavior, while other worms are little more than a prank
Practical joke
A practical joke is a mischievous trick played on someone, typically causing the victim to experience embarrassment, indignity, or discomfort. Practical jokes differ from confidence tricks in that the victim finds out, or is let in on the joke, rather than being fooled into handing over money or...

, such as one that sends the popular image macro
Image macro
In Internet culture, an image macro is a picture captioned with superimposed text for humorous effect.-Internet forums:On Internet forums and imageboards, image macros are used to emphasize a certain phrase by superimposing it over a related picture.-Formats:Although they come in many forms, the...

 of an owl with the phrase "O RLY?
O RLY? is an Internet phenomenon, typically presented as an image macro featuring a Snowy Owl. The phrase "O RLY?", an abbreviated form of "Oh, really?", is popularly used in Internet forums in a sarcastic manner, often in response to an obvious, predictable, or blatantly false statement...

" to a print queue in the infected computer. Another research 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.

Most security experts regard all worms as malware
Malware, short for malicious software, consists of programming that is designed to disrupt or deny operation, gather information that leads to loss of privacy or exploitation, or gain unauthorized access to system resources, or that otherwise exhibits abusive behavior...

, whatever their payload or their writers' intentions.

Protecting against dangerous computer worms

Worms spread by exploiting vulnerabilities in operating systems.
Vendors with security problems supply regular security updates (see "Patch Tuesday
Patch Tuesday
Patch Tuesday is usually the second Tuesday of each month, on which Microsoft releases security patches.Starting with Windows 98, Microsoft included a "Windows Update" system that would check for patches to Windows and its components, which Microsoft would release intermittently...

"), and if these are installed to a machine then the majority of worms are unable to spread to it. If a vulnerability is disclosed before the security patch released by the vendor, a zero-day attack is possible.

Users need to be wary of opening unexpected email, and should not run attached files or programs, or visit web sites that are linked to such emails. However, as with the ILOVEYOU
ILOVEYOU, also known as Love Letter, is a computer worm that successfully attacked tens of millions of computers in 2000 when it was sent as an attachment to a user with the text "ILOVEYOU" in the subject line. The worm arrived e-mail on and after May 4, 2000 with the simple subject of "ILOVEYOU"...

 worm, and with the increased growth and efficiency of phishing
Phishing is a way of attempting to acquire information such as usernames, passwords, and credit card details by masquerading as a trustworthy entity in an electronic communication. Communications purporting to be from popular social web sites, auction sites, online payment processors or IT...

 attacks, it remains possible to trick the end-user into running a malicious code.

Anti-virus and anti-spyware software are helpful, but must be kept up-to-date with new pattern files at least every few days. The use of a firewall
Firewall (computing)
A firewall is a device or set of devices designed to permit or deny network transmissions based upon a set of rules and is frequently used to protect networks from unauthorized access while permitting legitimate communications to pass....

 is also recommended.

In the April–June, 2008, issue of IEEE Transactions on Dependable and Secure Computing, computer scientists describe a potential new way to combat internet worms. The researchers discovered how to contain the kind of worm that scans the Internet randomly, looking for vulnerable hosts to infect. They found that the key is for software to monitor the number of scans that machines on a network sends out. When a machine starts sending out too many scans, it is a sign that it has been infected, allowing administrators to take it off line and check it for viruses.. In addition, machine learning techniques can be used to detect new worms, by analyzing the behavior of the suspected computer.

Mitigation techniques

  • ACLs in routers and switches
    Network switch
    A network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...

  • Packet-filters
  • Nullrouting
    In computer networking, a null route is a network route that goes nowhere. Matching packets are dropped rather than forwarded, acting as a kind of very limited firewall. The act of using null routes is often called blackhole filtering...

  • TCP Wrapper
    TCP Wrapper
    TCP Wrapper is a host-based networking ACL system, used to filter network access to Internet Protocol servers on operating systems such as Linux or BSD...

    libwrap is a free software program library that implements generic TCP Wrapper functionality for network service daemons to use ....

     enabled network service daemon
    Daemon (computer software)
    In Unix and other multitasking computer operating systems, a daemon is a computer program that runs as a background process, rather than being under the direct control of an interactive user...



The actual term "worm"' was first used in John Brunner
John Brunner (novelist)
John Kilian Houston Brunner was a prolific British author of science fiction novels and stories. His 1968 novel Stand on Zanzibar, about an overpopulated world, won the 1968 Hugo Award for best science fiction novel. It also won the BSFA award the same year...

's 1975 novel, The Shockwave Rider
The Shockwave Rider
The Shockwave Rider is a science fiction novel by John Brunner, originally published in 1975. It is notable for its hero's use of computer hacking skills to escape pursuit in a dystopian future, and for the coining of the word "worm" to describe a program that propagates itself through a computer...

. In that novel, Nichlas Haflinger designs and sets off a data-gathering worm in an act of revenge against the powerful men who run a national electronic information web that induces mass conformity. "You have the biggest-ever worm loose in the net, and it automatically sabotages any attempt to monitor it... There's never been a worm with that tough a head or that long a tail!"

On November 2, 1988, Robert Tappan Morris
Robert Tappan Morris
Robert Tappan Morris, , is an American computer scientist, best known for creating the Morris Worm in 1988, considered the first computer worm on the Internet - and subsequently becoming the first person convicted under the Computer Fraud and Abuse Act.He went on to co-found the online store...

, a Cornell University
Cornell University
Cornell University is an Ivy League university located in Ithaca, New York, United States. It is a private land-grant university, receiving annual funding from the State of New York for certain educational missions...

 computer science graduate student, unleashed what became known as the Morris worm, disrupting perhaps 10% of the computers then on the Internet and prompting the formation of the CERT Coordination Center
CERT Coordination Center
The CERT Coordination Center was created by DARPA in November 1988 after the Morris worm struck. It is a major coordination center in dealing with Internet security problems....

 and Phage mailing list. Morris himself became the first person tried and convicted under the 1986 Computer Fraud and Abuse Act
Computer Fraud and Abuse Act
The Computer Fraud and Abuse Act is a law passed by the United States Congress in 1986, intended to reduce cracking of computer systems and to address federal computer-related offenses...


See also

  • Computer surveillance
    Computer surveillance
    Computer surveillance is the act of performing surveillance of computer activity, and of data stored on a hard drive or being transferred over the Internet....

  • Computer virus
    Computer virus
    A computer virus is a computer program that can replicate itself and spread from one computer to another. The term "virus" is also commonly but erroneously used to refer to other types of malware, including but not limited to adware and spyware programs that do not have the reproductive ability...

  • Helpful worm
    Helpful worm
    A helpful worm is variant on a computer worm which delivers its payload by doing "helpful" actions instead of malicious actions. Welchia was a major example of a helpful worm - utilizing the same exploit that caused the Blaster worm...

  • Spam
  • Timeline of notable computer viruses and worms
    Timeline of notable computer viruses and worms
    This is a timeline of noteworthy computer viruses, worms and Trojan horses.- 1966 :* The work of John von Neumann on the "Theory of self-reproducing automata" is published...

  • Trojan horse (computing)
    Trojan horse (computing)
    A Trojan horse, or Trojan, is software that appears to perform a desirable function for the user prior to run or install, but steals information or harms the system. The term is derived from the Trojan Horse story in Greek mythology.-Malware:A destructive program that masquerades as a benign...

  • XSS Worm
    XSS Worm
    An XSS worm, sometimes referred to as a cross site scripting virus, is a malicious payload, usually written in JavaScript, that propagates among visitors of a website in the attempt to progressively infect other visitors...

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.