A 51% attack refers to a scenario where an entity controls more than half of the blockchain consensus, which can pose significant risks such as transaction reversals and double spending, particularly in smaller proof-of-work (PoW) networks. While such attacks could theoretically occur on proof-of-stake (PoS) chains as well, the costs are higher and the risks greater due to token slashing. Data shows that the cost of a 24-hour attack on Ethereum Classic exceeds $144,000, while the cost of attacking Bitcoin approaches $53 million. Past incidents, including multiple attacks on Ethereum Classic from 2019 to 2020 and the reorganization of Monero in 2025, highlight the vulnerabilities that exist, although larger networks are becoming more resilient as security measures improve.

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