I believe that it is possible for an attack to happen, but extremely unlikely. Mempool floods are an attack that can be pulled off within the foreseeable future as said in a post before mine. a 51% attack is close to impossible on bitcoin, the hashrate needed to pull off an attack such as a 51% is tremendous, they're arent enough ASICs in production to be able to buy enough to do a 51% attack. Supercomputers are out of the picture as well, they are not very good at mining BTC. An attack is possible, but so unlikely, that I dont worry about it.