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6. Santy (2004)
A specialised worm that demonstrated quite how subtle attack vectors could be, Santy used search engines — Google at first, then Yahoo and AOL — to find vulnerable sites running phpBB bulletin board software, which it then attacked. It spread worldwide in less than three hours.
The most unusual aspect of the worm was that someone then produced an anti-Santy worm that used the same techniques to find and infect phpBB installations, but then patched the problem and inoculated the sites against further attack.
Photo credit: CNET News