51% Attack
Updated
DEFINITION
A 51% attack is an attack on a proof-of-work network in which an entity controls a majority of its mining hashpower. That advantage can let an attacker build a competing valid chain, reorganize recent transactions or censor transactions. It does not reveal other users’ private keys, permit arbitrary spending of their coins, or make rule-breaking blocks acceptable to validating nodes.
The relevant resource is mining power, not the number of user accounts or nodes. Reversing a payment the attacker made can enable a double-spend. Majority power improves the attacker’s ability to outpace honest miners, but attacks can also have a chance of success below 50%. See proof of work for the underlying mechanism.
IN A SENTENCE
A majority-hashpower attack threatens transaction history, not the secrecy of everyone’s private keys.
Key facts
- Resource
- Mining hashpower
- Main risks
- Reorganizations, double-spending and censorship
- Still enforced
- Consensus validity rules
Common questions
Can the attacker steal any wallet’s coins?
Not simply by acquiring hashpower. Spending still requires satisfying the transaction’s authorization conditions.
Does 51% refer to nodes?
No. The term refers to a majority of mining power.
Are fewer confirmations more exposed?
Recent history generally requires less accumulated work to replace than deeply buried history. Confirmations reduce risk; they are not an absolute guarantee.
Sources and further reading
Bitcoin developer guide: blockchain and attacks
Bitcoin Core validation explanation
This definition explains the term; it is not investment or legal advice.