51% attack
A Majority Attack, also known as a 51% attack, occurs when a single entity gains control of over 51% of a blockchain network's hash power. This control allows the entity to manipulate the network by orphaning confirmed blocks and selecting which transactions get confirmed or left in the mempool.
This control can be used to selectively confirm transactions, potentially leading to no transactions being confirmed, specific transactions being manipulated, or executing double spends.
"In a 51% attack, an attacker with majority control could potentially reverse transactions that they send while they are in control, leading to a double-spending problem."
The risk of a Majority Attack is low on networks with significant hash power due to the high cost and coordination required. However, blockchains with low hash power are more vulnerable.
The risk of a 51% attack decreases as the hash power and the number of stakeholders for a blockchain increase, making the network more secure.
"Shortly after the BCH / BCHA hard fork, the BCHA network was subjected to a 51% attack over multiple days. During this time, confirmation times reached up to 12 hours. The attack was mitigated when the BCHA node released an update to their mining client."
Various strategies can be employed to prevent a Majority Attack. These include increasing the network's hash power, implementing a different consensus mechanism, or using checkpointing to limit the depth of a blockchain reorganization.
"Some cryptocurrencies use a consensus mechanism called Proof of Stake (PoS) instead of Proof of Work (PoW) to reduce the risk of a 51% attack."
* All terms and definitions may update as the Cryptionary improves.
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