All blockchains have a loophole that gives any entity with over 50% of the network’s hash rate or computational power control over a given network. “Control” refers to the ability to change the rules of the network, transaction values, transaction recipients, and more.
Almost every blockchain in existence has been the recipient of an attack of this nature, including Bitcoin. 51% attacks are extremely unlikely to succeed, to the point that they are virutally impossible. A given group of miners would need to maintain 51% ownership of the hash power – which in and of itself extremely difficult – while simultaneously winning consecutive blocks through proof of work mining in order to maintain the ability to manipulate transaction values.