Definition
A 51% attack describes a situation in which an actor controls enough consensus power to undermine a blockchain’s expected transaction ordering or confirmation security. The classic case is a majority of hash rate in a proof-of-work network. The label should not be applied mechanically to every proof-of-stake system, because thresholds for disrupting finality, censoring, or finalizing conflicting histories vary with the protocol.
How It Works
In the proof-of-work case, sustained majority hash power can allow an attacker to outpace honest miners and cause a valid competing history to replace recent blocks. This can reverse the attacker’s own recent payments or impede selected transactions. It does not let the attacker forge someone else’s signature, spend arbitrary users’ coins, or make full nodes accept blocks that break their validation rules.
Key Considerations
An exchange may respond to elevated reorganization risk by increasing confirmation requirements or pausing affected deposits. More confirmations raise the cost of many attacks but do not provide an unconditional guarantee against a sustained majority. Relevant factors include mining concentration, available competing hash power, economic incentives, and the value exposed to reorganization. A majority of visible nodes is not the same as a majority of mining power. The attack concerns consensus control, rather than theft of private keys, and its feasibility must be assessed for the specific network rather than inferred from the name alone.