What it is
A 51% attack is a theoretical vulnerability in proof-of-work blockchains like Bitcoin, where a malicious actor gains control over the majority of the network's computational power (hash rate). This control would allow them to manipulate the order of transactions, prevent new transactions from being confirmed, and reverse their own transactions, enabling double-spending. The economic incentive to maintain network integrity typically outweighs the cost and effort required for such an attack on large networks.
While highly improbable for Bitcoin due to its vast hash rate, a 51% attack is a critical security concern for smaller proof-of-work chains. News about a network's hash rate distribution or a significant concentration of mining power can raise fears of such an attack. A successful attack would severely undermine confidence in the blockchain's immutability and security, potentially leading to a collapse in its asset's value.
Why it matters
Understanding a 51% attack highlights the security mechanisms of proof-of-work blockchains. It explains why a distributed hash rate is crucial for network integrity.
Reviewed under editorial standardsUpdated September 26, 2026Not investment advice