Conficker

Conficker, also known as Downup, Downadup and Kido, is a computer worm targeting the Microsoft Windows operating system that was first detected in November 2008. It uses flaws in Windows OS software (MS08-067 / CVE-2008-4250) and dictionary attacks on administrator passwords to propagate while forming a botnet, and has been unusually difficult to counter because of its combined use of many advanced malware techniques. The Conficker worm infected millions of computers including government, business and home computers in over 190 countries, making it the largest known computer worm infection since the 2003 SQL Slammer worm.

Despite its wide propagation, the worm did not do much damage, perhaps because its authors – believed to have been Ukrainian citizens – did not dare use it because of the attention it drew. Four men were arrested, and one pled guilty and was sentenced to four years in prison.