Dec 4, 2012
Downside of data URI's is security!
It appears that the worm took advantage of Tumblr's reblogging feature, meaning that anyone who was logged into Tumblr would automatically reblog the infectious post if they visited one of the offending pages.
The Base 64 string was actually encoded JavaScript, hidden inside an iFrame that was invisible to the naked eye, that dragged content from a url. Once decoded, the intention of the code becomes more clear.