51% Attack

A 51% attack is an adversarial attempt to gain a majority of a blockchain's validation power to disrupt the consensus state, such as by tampering with transactions or reordering blocks. Such attacks can occur through individual entities, such as a large stakeholder in a Proof of Stake network, or coordinated groups of entities, such as mining pools in a Proof of Work network. When an entity has over half of the hash power or stake, it can reverse transactions, double spend, prevent some transactions from being included in blocks, produce all blocks, and prevent other entities from creating blocks. However, the attacker cannot enforce changes to the protocol codebase, change block rewards, or spend others' tokens.