What Is a 51% Attack and How Does It Compromise a Blockchain?

A 51% attack occurs when a single entity or group controls more than half (51%) of a blockchain network's total mining hash power. This majority control allows the attacker to manipulate the order of transactions and prevent new transactions from being confirmed.

The primary compromise is the ability to execute a "double-spend" attack, where the attacker spends the same coins twice. They achieve this by confirming one transaction on their private chain and a conflicting one on the public chain, then overriding the public chain with their longer, private chain.