In theory, such an attack is possible, in practice -- very unlikely at this point given the number of miners out there. Here is what it was in 2015: http://www.

In theory, such an attack is possible, in practice -- very unlikely at this point given the number of miners out there. Here is what it was in 2015: http://www.zerohedge.com/news/2015-11-19/bitcoins-computing-network-more-powerful-525-googles-and-more-10000-banks http://www.zerohedge.com/news/2015-11-19/bitcoins-computing-... Today it's even higher than ever: https://news.bitcoin.com/why-bitcoin-is-close-to-a-record-breaking-3-exahashes-processing-power/ https://news.bitcoin.com/why-bitcoin-is-close-to-a-record-br...