51% Attack Definition

A 51% attack occurs when a single participant or consortium gains more than half of the consensus power in a blockchain that uses Proof of Work (PoW) or Proof of Stake (PoS). With majority control, the attackers can:

• Spend the same coins twice by reorganizing earlier blocks
• Reorder, exclude, or censor transactions
• Halt new block production by blocking validation

This majority erodes confidence in the ledger because decentralization no longer protects transaction history.

Cost of mounting a 51% attack

The required resources depend on the network’s size and consensus model.

Proof of Work

An attacker must exceed the combined hash rate of honest miners, which demands extensive specialized hardware and continuous electricity. Achieving this on Bitcoin costs billions of dollars in equipment alone, with ongoing power expenses added. Smaller PoW chains such as Bitcoin Gold or Ethereum Classic require far less combined hash rate, so an attack might cost only hundreds of thousands or a few million dollars.

Proof of Stake

In PoS systems an attacker purchases dominance instead of computing power. Ethereum’s move to PoS in 2022 means an attacker would need to stake more than 10 million ETH—tens of billions of dollars—to control the network, and that stake remains at risk throughout the attempt.

Practicality on Proof of Stake networks

PoS chains remain theoretically vulnerable, yet several factors deter attacks:

• Acquiring a majority stake demands extraordinary capital.
• Slashing rules allow protocols to confiscate misused stakes.
• Validator nodes operate worldwide, complicating coordination.
• Buying such a large position drives the token price higher, raising acquisition costs, while a successful attack would likely collapse the token’s value.

Smaller PoS networks with limited staking participation face greater relative risk, but the largest chains make a 51 % attack economically unattractive and operationally daunting.