The Blaster worm appeared in early August of 2003 and promptly infected millions of PCs because of a security flaw in Microsoft’s Windows OS. The hole allowed the worm to download itself to vulnerable systems, which meant it didn’t require any action by the user to infect the machine. It was also designed to launch a denial of service attack against Microsoft’s Windows Update site, where users could download a patch to protect themselves from the vulnerability. Two variants of the worm later appeared; Blaster B and Welchia.
View the original CERT advisory.