> <azonenberg> wordpress is an unauthenticated remote shell that, as a useful side feature, also contains a blog

That's an old joke and obviously an exaggeration, but it's not horribly far off. Wordpress instances fall to attack constantly, due to a combination of bugs in the base application itself (not terribly common) and extensions (not just common -- CONSTANT). In terms of breakability, I would rank Wordpress in the top 1% of applications without a second thought.