Bruce Schneier of Counterpane Security explains why the Witty Worm is so awful.
Witty was very well written. It was less than 700 bytes long. It used a random-number generator to spread itself, avoiding many of the problems that plagued previous worms. It spread by sending itself to random IP addresses with random destination ports, a trick that made it easier to sneak through firewalls. It was — and this is a very big deal — bug-free. This strongly implies that the worm was tested before release.
Witty was exceptionally nasty. It was the first widespread worm that destroyed the hosts it infected. And it did so cleverly. Its malicious payload, erasing data on random accessible drives in random 64KB chunks, caused immediate damage without significantly slowing the worm's spread.
