Computerworm
A computerworm is a standalone malware computer program that replicates itself inorder to spread to other computers. Often, it uses a computer network to spreaditself, relying on security failures on the target computer to access it.Unlike a computer virus, it does not need to attach itself to an existingprogram. Worms almost always cause at least some harm to the network, even ifonly by consuming bandwidth, whereas viruses almost always corrupt or codifyfiles on a targeted computer.
Many worms that have been created aredesigned only to spread, and do not attempt to change the systems they passthrough. However, as the Morris worm and Mydoom showed, even these “payloadfree” worms can cause major disruption by increasing network traffic and other unintendedeffects. A “payload” is code in the worm designed to do more than spread theworm – it might delete files on a host system (e.g., the ExploreZip worm), encryptfiles in a ransomware attack, or send documents via email. A very commonpayload for worms is to install a backdoor in the infected computer to allowthe creation of a “zombie” computer under control of the worm author. Networksof such machines are often referred to as botnets and are very commonly used byspam 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 ofsuch worms, and the worm writhers have been caught selling lists of IPaddresses of infected machines. Others try to backmail companies or schoolswith threatening DoS attacks.
Users can minimize the threat posed byworms by keeping their computers’ operating system and other software up to date,avoiding opening unrecognized or unexpected email and running firewall andantivirus software.
Backdoors can be exploited by other malware, including worms. Examplesinclude Doomjuice, which can spread using the backdoor opened by Mydoom, and atleast one instance of malware taking advantage of the rootkit and backdoorinstalled by the Sony/BMG DRM software utilized by millions of music CDs priorto late 2005.