One of the most well known blockchain attacks is the 51% attack. In a 51% attack, a malicious actor is able to control more than 50% of the hashing power and temporarily control the network for their benefit. Due to the way that Proof of Work systems operate, an individual (or coalition) with 51% hashing power could disrupt the block creation process in a variety of ways including: excluding or reordering transactions, preventing transaction from being confirmed, or preventing other miners from mining. In this post, I'll explain how a 51% attack works, what it can do, what it can't do, and try to offer some simple ways of mitigating this risk.