# 51% Attack > A 51% attack is a potential threat to blockchains, where a group controls over 50% of network hash power, theoretically allowing blockchain modification. - Canonical URL: https://iq.wiki/wiki/51percent-attack - Categories: Glossary - Created: 2023-08-11T04:49:43.035Z - Last updated: 2026-06-01T01:44:47.829Z - Source: IQ.wiki — the world's largest blockchain and crypto encyclopedia (https://iq.wiki) --- A **51% attack** occurs when a [blockchain](https://iq.wiki/wiki/blockchain) is targeted by a coalition of miners who possess more than 50% of the total [mining ](https://iq.wiki/wiki/mining)hash rate on the network. If this group gains ownership of 51% of the network's nodes, it theoretically grants them the authority to modify the [blockchain](https://iq.wiki/wiki/blockchain).[\[1\]](#cite-id-wltt2pcif1m)\ # Overview A [blockchain](https://iq.wiki/wiki/blockchain) achieves decentralization by ensuring that no single individual or specific group holds control over the network. This decentralized structure is crucial because all participants within the blockchain ecosystem must reach a consensus on the present state of the blockchain. This requirement for unanimous agreement among the widely distributed network participants establishes a high level of certainty regarding the authenticity of the block's state.[\[2\]](#cite-id-48l4ylloarn) A 51% attack refers to a deliberate attack on a blockchain in which a group gains control over more than 50% of the network's hashing power, which is the computational capability used to solve cryptographic puzzles. This group subsequently introduces a modified version of the blockchain into the network at a precise juncture. Theoretically, the network might accept this altered blockchain due to the attackers' majority control over the network.[\[1\]](#cite-id-wltt2pcif1m)[\[2\]](#cite-id-48l4ylloarn)[\[3\]](#cite-id-45gt55lheed) # Impacts of 51% Attack ## Reduced Miner Rewards In a 51% attack, a transaction might be reversed even before it is verified. This results in [double spending](https://iq.wiki/wiki/double-spending-problem). As the attackers take their shares, legitimate miners also make less money updating the network.[\[4\]](#cite-id-brvnnevnzr5)[\[2\]](#cite-id-48l4ylloarn) A 51% assault on a blockchain could adversely impact a [cryptocurrency](https://iq.wiki/wiki/cryptocurrency)'s reputation, which would cause investors to sell their crypto, depreciating its value.[\[5\]](#cite-id-gxk09x4yuy)[\[6\]](#cite-id-xqcmssv2ur8) ## Network Disruption As long as a blockchain employs a [Proof-of-Work (PoW)](https://iq.wiki/wiki/proof-of-work-pow) mechanism to validate transactions, perpetrators can intentionally disrupt the network by impeding the confirmation and sequential arrangement of blocks.[\[4\]](#cite-id-brvnnevnzr5) A 51% assault has a significant negative effect on the miner's computational capabilities. As a result, it takes longer for the transaction to be approved and recorded in a [block](https://iq.wiki/wiki/block). As a result, the [blockchain](https://iq.wiki/wiki/blockchain) network becomes compromised, enabling the attackers to execute transactions faster than the miner.[\[4\]](#cite-id-brvnnevnzr5) # 51% Attack Prevention Blockchains employ several strategies to prevent 51% attacks. One of such strategies is to incentivize a greater number of participants to join the network and operate their own nodes, thereby bolstering a strong network community. A higher number of participants contributing resources makes it more challenging for a sole entity to exert dominance over the network.[\[3\]](#cite-id-45gt55lheed)[\[4\]](#cite-id-brvnnevnzr5) In a small blockchain network, a single miner could potentially dominate as the majority participant. Blockchain networks employing the [Proof of Work](https://iq.wiki/wiki/proof-of-work-pow) (PoW) consensus mechanism typically implement a requirement that miners regularly update their equipment. Neglecting this can result in them forfeiting block rewards and falling behind other miners within the network. [\[4\]](#cite-id-brvnnevnzr5) To mitigate the risk of a 51% attack, blockchain networks can opt for the [Proof of Stake (PoS)](https://iq.wiki/wiki/proof-of-stake) consensus mechanism, which is generally considered a more secure alternative to [Proof of Work](https://iq.wiki/wiki/proof-of-work-pow) (PoW) because it is more challenging for an attacker to acquire the larger majority of the staked cryptocurrency compared to most of the mining power in a [PoW](https://iq.wiki/wiki/proof-of-work-pow) [blockchain](https://iq.wiki/wiki/blockchain).[\[4\]](#cite-id-brvnnevnzr5)[\[5\]](#cite-id-gxk09x4yuy)[\[8\]](#cite-id-apado95xybl)\ ![1671478532822.jpeg](https://ipfs.everipedia.org/ipfs/Qmd5wWFUVcUL1p6kVD5tUbUzihjdmnnpfPJh8qUdh4SugC) ## Past Examples of 51% Attacks The majority of 51% attacks are directed towards smaller cryptocurrencies. Experts suggest that major cryptocurrencies are unlikely to fall victim to a successful 51% attack due to the substantial cost associated with gaining control of over half of the mining power. The following are blockchain networks that have experienced a 51% attack in the past: * In 2018, an attack on [Bitcoin Gold](https://iq.wiki/wiki/bitcoin-gold) (BTG) led to the [double spending](https://iq.wiki/wiki/double-spending-problem) of over $18 million worth of the currency. In 2020, BTG encountered another 51% attack. Within a span of two days, the [blockchain](https://iq.wiki/wiki/blockchain) underwent two reorganizations, enabling double-spending activities that resulted in substantial financial losses. However, this attack was quickly stopped to prevent further loss. [\[4\]](#cite-id-brvnnevnzr5)[\[6\]](#cite-id-xqcmssv2ur8) * [Ethereum Classic](https://iq.wiki/wiki/ethereum-classic) (ETC), a cryptocurrency that emerged from a hard fork of [Ethereum](https://iq.wiki/wiki/ethereum) in 2016, fell victim to a 51% attack in 2020. This incident resulted in the reported theft of several million dollars.[\[6\]](#cite-id-xqcmssv2ur8) * [Bitcoin SV](https://iq.wiki/wiki/bitcoin-sv) (BSV) faced three attacks in 2021. These attacks enabled hackers to manipulate or erase the most recent blocks by gaining control over the network.[\[6\]](#cite-id-xqcmssv2ur8)