As web 2.0 sites are growing up, common side effects of grown-up software are coming along. First sign was the worm that attacked MySpace.com last year. Only a week ago Yahoo! Mail was attacked by a javascript worm. Some argue that this is supposed to happen when software becomes Popular, but I think that is not true. Over the years, operating systems have become more and more popular yet some of them do not know viruses or worms. I think people believing that existance of viruses are normal are simply conditioned to using Microsoft Windows.
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