what is a 51 attack 
a 51 attack is an attack on a cryptocurrency blockchain by an entity or group that controls more than 50 of the network if a party were to gain this much control of a network it would have the power to alter the blockchain the attackers would be able to prevent new transactions from gaining confirmations allowing them to halt payments between some or all users they would also be able to reverse non confirmed transactions that were completed while they were in control reversing transactions could allow them to double spend coins one of the issues mechanisms like proof of work were created to prevent understanding a 51 attacka blockchain is a distributed ledger essentially a database that records transactions and information about them the blockchain s network reaches a majority consensus about transactions through a validation process the blocks where the data is stored are sealed the blocks are linked together via cryptographic techniques where previous block information is recorded in each block this makes the blocks nearly impossible to alter once they are confirmed enough times the 51 attack is an attack on the blockchain where a group controls more than 50 of the hashing power the computing that solves the cryptographic puzzle of the network this group then introduces an altered blockchain to the network at a very specific point in the blockchain which is theoretically accepted by the network because the attackers would own most of it 1changing historical blocks transactions locked in before the start of the attack would be extremely difficult even in the event of a 51 attack the further back the transactions are the more difficult it is to change them it would be impossible to change transactions before a checkpoint where transactions become permanent in bitcoin s blockchain a 51 attack is a very difficult and challenging task on a blockchain network with a large participation rate in most cases the group of attackers would need to be able to control the necessary 51 and have created an alternate blockchain that can be inserted at exactly the right moment then they would need to out hash the main network the cost of doing this is one of the most significant factors that prevent a 51 attack for example one of the most advanced application specific integrated circuit asic miners is the whatsminer m63s it costs more than 10 000 new and has a hash rate of 406 terahashes per second th s 2 a single or smaller group of miners would not be able to alter and mine the bitcoin blockchain with only a few of these machines it would take thousands of these asics to get ahead of the bitcoin network smaller networks could be out hashed using these mining rigs but the benefits of doing so wouldn t outweigh the costs of funding the attack and setting it up hashing power rental services provide attackers with lower costs as they only need to rent as much hashing power as they need for the duration of the attack 3after ethereum transitioned to proof of stake a 51 attack on the ethereum blockchain became even more expensive to conduct this attack a user or group would need to own 51 of the staked eth on the network it is possible for someone to own that much eth but it s unlikely according to beaconchain more than 32 3 million eth were staked on may 8 2024 4 an entity would need to own and stake more than 16 5 million eth more than 49 billion as of may 8 2024 to attempt an attack 5once the attack started the consensus mechanism would likely recognize it and immediately slash the staked eth costing the attacker an extraordinary amount of money additionally the community can vote to restore the honest chain so an attacker would lose all of their eth just to see the damage repaired 6in addition to the costs a group attempting to attack the network using a 51 attack must not only control 51 of the network but also introduce the altered blockchain at a very precise time even if they own 51 of the network hashing rate they still might not be able to keep up with the block creation rate or get their chain inserted before valid new blocks are created by the honest blockchain network again this is possible on smaller cryptocurrency networks because there is less participation and lower hash rates large networks make it nearly impossible to introduce an altered blockchain despite the name it is not necessary to have 51 of a network s mining power to launch an attack however such an attack would have a much lower chance of success in the event of a successful attack the attackers could block other users transactions or reverse them and spend the same cryptocurrency again this vulnerability known as double spending is the digital equivalent of a perfect counterfeit it is also the basic cryptographic hurdle blockchain consensus mechanisms were designed to overcome successful 51 attackers may also implement a denial of service dos attack where they block the addresses of other miners for the period they control the network 1 this keeps the honest miners from reacquiring control of the network before the dishonest chain becomes permanent who is at risk of 51 attack the type of mining equipment is also a factor as asic secured mining networks are less vulnerable than those that can be mined with gpus they are much faster cloud services such as nicehash which considers itself a hash power broker theoretically make it possible to launch a 51 attack using only rented hash power especially against smaller gpu only networks 7bitcoin gold has been a common target for attackers because it is a smaller cryptocurrency by hashrate since june 2019 the michigan institute for technology s digital currency initiative has detected observed or been notified of more than 40 51 attacks also called chain reorganizations or reorgs on bitcoin gold litecoin and other smaller cryptocurrencies 3