In plain English
What 51% attack means
On a proof-of-work blockchain, consensus power is commonly measured by hash rate. An attacker with a majority can often build an alternative chain faster than honest miners and attempt to replace recent blocks. On proof-of-stake systems, an analogous attack concerns control of sufficient staked voting power, but the exact capabilities and penalties differ by protocol.
Why it matters
The risk affects finality and settlement confidence, particularly for smaller networks with concentrated mining or staking power. Exchanges and merchants may respond by requiring more confirmations before crediting deposits. A majority attack is especially relevant to double-spend risk, where an attacker spends coins, receives goods or another asset, then tries to replace the transaction history.
Example
A buyer sends 10 coins to a merchant, and the merchant accepts after one confirmation. The buyer secretly mines a conflicting chain in which that payment does not exist. If the buyer’s chain becomes the accepted chain, the original payment can disappear from recent history while the buyer keeps the goods. This simplified example assumes the attacker has sufficient consensus power.
Quick answers
Common questions
Can a 51% attack steal coins from any wallet?+
No. Controlling consensus power does not reveal private keys or authorize spending someone else’s funds. The main concern is manipulating the ordering or confirmation of recent transactions, including an attacker’s own payments.
Why are more confirmations used for some deposits?+
Each additional block generally makes a proof-of-work chain reorganization more difficult and costly. Platforms set confirmation policies based on their assessment of a network’s security, transaction size, and operational risk.
Sources