This is an attack vector in Proof of Work (PoW) secured blockchains that can occur when a miner or mining pool has control of a majority of the available hash power.
In theory if an actor has control of more than 50% of the hash power then they will be able to solve PoW puzzles faster than the rest of the miners and thus take control over that blockchain. It should be remembered that the ability to solve puzzles is pseudo-randomly determined, likewise, the finality of any given block is always probabilistic depending on which chain has the majority of hashing power behind it.
In theory, an attacker should be disincentivized from conducting a 51% attack out of self-interest. The reasoning is that:
- To gain a majority of the hashpower an actor would have to invest significant resources
- Once the attacker gained control of a majority of the network hashing power the value of the underlying cryptotoken would fall as other actors on the network would question the validity of the network
- This devaluing of the cryptotoken would result in a low or even negative return on investment for the attacker