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Mydoom is a computer worm that affects Windows computers and was first identified in 2004. Mydoom is spread by email, using an attachment that uses a user's address book to send copies of itself to other users. It has been blamed for several major slowdowns in internet traffic as the worm spread through infected computers.
Mydoom works by targeting a backdoor on port 3127 on unpatched versions of Microsoft Windows. The worm consists of an email attachment that searches for address book files and sends multiple copies of itself as email attachments in turn. This is one of the reasons users are urged to avoid opening unsolicited email attachments. Mydoom also spread via the Kazaa file sharing application.
Mydoom consists of a payload targeting the website of SCO, which at the time was involved in a lawsuit with IBM claiming that Linux source code was copied from the original Unix, which SCO held the copyrights to at the time the worm was identified. Only about 25 percent of infected computers actually targeted SCO. There are two versions of the worm: Mydoom.A and Mydoom.B. The latter contains a payload that target's Microsoft's website as well as SCO.
Mydoom was notable for being the fastest-spreading email worm at the time. The author of Mydoom is unknown, but many security researchers believe the worm originated in Russia.