51% Attack
An attack where one entity or coalition controls enough consensus power to reorganize blocks, censor transactions, or attempt double spending.Definition
An attack where one entity or coalition controls enough consensus power to reorganize blocks, censor transactions, or attempt double spending.Why it matters
Security concepts help users and developers recognize common blockchain attack surfaces and defenses.How it works
The attacker acquires enough computational power or staked tokens to control the majority of block production. They then ignore the honest network nodes, building a secret fork that follows their own rules. Eventually, they publish this chain to the network, which, due to the protocol’s longest-chain rule, is accepted by nodes as the legitimate version.Real-world example
The Ethereum Classic (ETC) network has suffered several 51% attacks where attackers reorganized blocks to perform double-spend operations on exchanges.Advantages
- Identifies weak consensus mechanisms
- Encourages wider distribution of mining power
- Validates robustness of network security
Limitations
- Extremely expensive to execute on large networks
- Damages the reputation of the attacked blockchain
- Difficult to maintain over a long duration
Common misconceptions
- People think 51% attacks can allow hackers to steal everyone’s private keys. In reality, they cannot access individual wallets but can only reorder or block transactions.
- Some believe these attacks are common on major chains like Bitcoin, but the cost required makes such an attack virtually impossible.
Related knowledge
- Phishing — term
- Smart Contract Audit — term
- Sybil Attack — term
Canonical knowledge ID:
glossary:51-attack