A flaw in Ethereum Classic’s code makes it very vulnerable to a specially engineered type of blockchain network attack called a 51% attack.
A 51% attack is a way to compromise the control of token production by controlling 51% or more of mining activity through a single node or group of nodes. In the legacy version of Ethereum, attackers have been able to gain control of 51% of Ethereum network mining activity and steal large amounts of tokens.
In each attack, millions of dollars worth of Ethereum Classic tokens have been stolen. Attackers can “double spend”, essentially sending two transactions at the same time and doubling their tokens at no cost. In August and September of 2020, Ethereum Classic suffered three consecutive 51% attacks.
These attacks are what caused a hard fork of the Ethereum network and the creation of Ethereum Classic. In order to secure the network from such attacks, a new version of the Ethereum blockchain was developed. The old version persisted and is known today as Ethereum Classic.
These attacks have been used as an argument by Ethereum’s original founders as a demonstration of the superiority of Proof-of-Stake systems over Proof-of-Work systems like Ethereum Classic and Bitcoin.