"Over the weekend, Twitter became the victim of a cross
site scripting attack based worm that spread spam tweets. According
to Twitter, nearly 200 accounts were compromised and some 10,000
messages in total were pegged as being worm spam generated.
"... In total, there have been four different variants of the
worm that hit Twitter over the weekend and now includes Monday.
Early Monday Twitter claimed it was successfully fighting the
fourth variant."