Өзін-өзі көшіретін зиянды бағдарлама: Компьютерлік құрттар

Self replicating malware program
coding of a worm

A computer worm is a standalone malware computer program that replicates itself in order to spread to other computers. It often uses a computer network to spread itself, relying on security failures on the target computer to access it. It will use this machine as a host to scan and infect other computers. When these new worm invaded computers are controlled, the worm will continue to scan and infect other computers using these computers as hosts, and this behaviour will continue. Computer worms use recursive methods to copy themselves without host programs and distribute themselves based on exploiting the advantages of exponential growth, thus controlling and infecting more and more computers in a short time. 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.

The term "worm" was first used in John Brunner's 1975 novel, The Shockwave Rider. In the 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!" "Then the answer dawned on him, and he almost laughed. Fluckner had resorted to one of the oldest tricks in the store and turned loose in the continental net a self perpetuating tapeworm, probably headed by a denunciation group "borrowed" from a major corporation, which would shunt itself from one nexus to another every time his credit code was punched into a keyboard. It could take days to kill a worm like that, and sometimes weeks." During the Morris appeal process, the U. S. Court of Appeals estimated the cost of removing the worm 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. Conficker, a computer worm discovered in 2008 that primarily targeted Microsoft Windows operating systems, is a worm that employs 3 different spreading strategies: local probing, neighborhood probing, and global probing. This worm was considered a hybrid epidemic and affected millions of computers. The term "hybrid epidemic" is used because of the three separate methods it employed to spread, which was discovered through code analysis.

Any code designed to do more than spread the worm is typically referred to as the "payload". Typical malicious payloads might delete files on a host system (e. g., the ExploreZip worm), encrypt files in a ransomware attack, or exfiltrate data such as confidential documents or passwords. Some worms may install a backdoor. This allows the computer to be remotely controlled by the worm author as a "zombie". Networks of such machines are often referred to as botnets and are very commonly used for a range of malicious purposes, including sending spam or performing DoS attacks. Some special worms attack industrial systems in a targeted manner. Stuxnet was primarily transmitted through LANs and infected thumb drives, as its targets were never connected to untrusted networks, like the internet. This virus can destroy the core production control computer software used by chemical, power generation and power transmission companies in various countries around the world in Stuxnet's case, Iran, Indonesia and India were hardest hit it was used to "issue orders" to other equipment in the factory, and to hide those commands from being detected. Stuxnet used multiple vulnerabilities and four different zero day exploits (eg: ) in Windows systems and Siemens SIMATICWinCC systems to attack the embedded programmable logic controllers of industrial machines. Although these systems operate independently from the network, if the operator inserts a virus infected drive into the system's USB interface, the virus will be able to gain control of the system without any other operational requirements or prompts.

A helpful worm or anti worm is a worm designed to do something that its author feels is helpful, though not necessarily with the permission of the executing computer's owner. Beginning with the first research into worms at Xerox PARC, there have been attempts to create useful worms. Those worms allowed John Shoch and Jon Hupp to test the Ethernet principles on their network of Xerox Alto computers. Similarly, the Nachi family of worms 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. Regardless of their payload or their writers' intentions, security experts regard all worms as malware. Another example of this approach is Roku OS patching a bug allowing for Roku OS to be rooted via an update to their screensaver channels, which the screensaver would attempt to connect to the telnet and patch the device. One study proposed the first computer worm that operates on the second layer of the OSI model (Data link Layer), utilizing 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. Anti worms have been used to combat the effects of the Code Red, Blaster, and Santy worms. Welchia is an example of a helpful worm. Utilizing the same deficiencies exploited by the Blaster worm, Welchia infected computers and automatically began downloading Microsoft security updates for Windows without the users' consent. Welchia automatically reboots the computers it infects after installing the updates. One of these updates was the patch that fixed the exploit.