Majority Attack
A potential attack on a blockchain network where a single entity gains control of the majority of the network's hash rate.
- Also known as
- 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. The primary motivation is often financial gain through double-spending: spending the same coins multiple times by reversing previously confirmed transactions.
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. Major cryptocurrencies with substantial mining infrastructure like Bitcoin and Bitcoin Cash have significant protection due to their large mining communities.
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. The economics of attempting such an attack often make it financially impractical on large networks.
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 networks also implement delayed confirmations for high-value transactions.
Throughout cryptocurrency history, several successful 51% attacks have occurred, primarily targeting smaller networks with relatively low hash rates. These incidents have led to financial losses and decreased confidence in the affected cryptocurrencies.
The resistance to 51% attacks is often viewed as a key indicator of a blockchain's security and health. Networks with more distributed mining or staking power are considered more secure against such attacks, highlighting the importance of decentralization in blockchain security.
Related terms
7 linkedExplore connected entries beyond the alphabetical index.
Proof of Work (PoW)
→A consensus algorithm where computing power is used to solve complex problems, verify transactions, and create new blocks.
Hash rate
→The number of proof-of-work hash attempts performed per second by mining hardware or an entire network.
Double Spend
→A double spend is an attempt to spend the same coins twice by getting one conflicting transaction accepted over another.
Confirmations
→The count of blocks confirming a transaction, usually including the block that first contains it.
Block
→A batch of valid transactions added to a blockchain, linked to the previous block by a cryptographic hash.
Decentralization
→Decentralization distributes control across many participants, reducing single points of failure, censorship, or unilateral rule changes.
Difficulty Adjustment Algorithm (DAA)
→A DAA is the consensus rule that recalibrates mining difficulty so average block time stays near target as hash rate changes.
All terms and definitions may update as the Cryptionary improves.
