51% Attack

A 51% attack occurs when a single entity or group controls over 50% of a blockchain’s computing power or stake, allowing them to manipulate the network’s transaction ledger.

What is 51% Attack?

A 51% attack, also known as a majority attack, is a potential vulnerability in blockchain networks. By controlling more than half of the network’s computational power (in Proof of Work, PoW) or staked digital assets (in Proof of Stake, PoS), an attacker can influence the blockchain’s consensus process. This enables them to double-spend digital assets, reverse confirmed transactions, or prevent new transactions from being validated, undermining the network’s integrity.

In a PoW system like Bitcoin, an attacker would need to amass over 50% of the network’s hash rate, which is extremely costly given Bitcoin’s total hash rate of approximately 1000 EH/s (exahashes per second) as of 2025. For context, renting enough mining power for a 51% attack on Bitcoin could cost millions of dollars per hour, making it economically impractical for large networks. Smaller PoW blockchains, like Ethereum Classic, have been targeted successfully, with attacks in 2019 and 2020 causing losses of millions due to double-spending.

In PoS systems, an attacker would need to own or control over 50% of the staked assets, which is also costly but feasible if stake distribution is centralized.

While a 51% attack can disrupt a blockchain, it doesn’t allow full control, such as stealing assets from other wallets. The risk is mitigated by network size, decentralization, and economic incentives, but smaller or less secure networks remain vulnerable.