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Up until this worm appeared, most malware was the work of vandals. Mydoom was, according to Microsoft’s security analysts, “one of the earliest examples of a botnet and for-profit malware.” You can read technical details in these writeups from Avira and SecureList.
Upon execution, the malware opened a message window in Notepad, displaying nonsense text. In the background, it installed its payload, which then proceeded to send out email messages using its own SMTP engine and the victim’s address book. If the recipient clicked the attachment, they became part of the botnet and began spreading it to their friends as well.
The worm also used file-sharing programs like Kazaa to spread its payload.
The authors of Mydoom included several taunting references to Netsky in their code.