A 51% attack occurs when an entity gains control of more than half of a blockchain's consensus power, allowing them to manipulate transactions and double-spend tokens. This is a significant risk for proof-of-work (PoW) systems like Bitcoin and Ethereum Classic. Smaller blockchain networks are particularly vulnerable due to the lower cost of renting mining power. Ethereum Classic has experienced multiple 51% attacks, prompting exchanges such as Coinbase to slow down transaction confirmations to mitigate risks. While proof-of-stake (PoS) networks are technically susceptible to such attacks, they are less likely due to slashing penalties that deter malicious behavior. According to Crypto51, a 24-hour attack on Ethereum Classic would cost over $144,000, whereas a similar attack on Bitcoin would require nearly $53 million.