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51% Attack Meaning

A 51% attack is a type of majority-control attack on a blockchain where a single actor or coordinated group gains control of more than half of the network’s consensus power. On Proof-of-Work (PoW) chains, this means controlling more than 50% of the total hashrate; on Proof-of-Stake (PoS) chains, it typically involves owning or controlling a majority of the staked native token. With this level of influence, the attacker can interfere with the normal operation of the network’s consensus mechanism.

They may prevent some or all new transactions from being confirmed, selectively reorder recent blocks, or build an alternative chain that invalidates previously confirmed transactions. This ability can be used to double-spend funds (reversing their own payments after receiving goods or services) or disrupt services that rely on the blockchain’s settlement guarantees. Importantly, even a successful 51% attack does not allow an attacker to arbitrarily create new coins or seize assets directly from other users’ wallets.

Instead, the primary risk is loss of transaction finality and erosion of trust in the network’s reliability. Exchanges, payment processors, and other infrastructure providers are particularly exposed if they accept deposits with too few confirmations.

Executing a 51% attack on a large, well-secured chain is economically difficult because of the high cost of acquiring sufficient hashrate or staked tokens. Smaller or less decentralized networks are more vulnerable and have historically been the main targets of such attacks.

Some analytics sites estimate the theoretical cost of attacking different networks based on current hashrate and cloud-mining prices, highlighting the critical role of decentralization and economic security in protecting consensus systems. Overall, the 51% attack model underlines why distributing control among many independent participants is essential for a resilient blockchain.

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