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51% Attack Tutorial! Ethereum Classic (ETC) Under Threat with Two Attacks in Five Days

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51% Attack Tutorial! Ethereum Classic (ETC) Under Threat with Two Attacks in Five Days

Ethereum Classic (ETC) has once again fallen victim to a 51% attack today (6th), resulting in over 4000 blocks being reorganized. This is the second blockchain reorganization that the blockchain has experienced in the past five days.

The First Attack Encountered by ETC

The previous attack occurred on August 1st, when Bitfly, the parent company of Ethermine mining pool, released a tweet claiming that due to node desynchronization, the block time of Ethereum Classic was longer than usual. However, things seemed more complicated than that.

According to blockchain data company Bitquery, it was not just a simple case of node desynchronization causing abnormal block production that day; it was a malicious 51% attack on the blockchain. According to Bitquery's event analysis report, within 12 hours from August 1st to August 2nd, a miner with the address starting with 0x75d1e quietly produced over 3500 blocks on a secretly operated forked chain (from 10904146 to 10907740) and broadcasted this forked chain to other miners. Due to the large number of blocks, based on the blockchain's longest (more precisely, the "most difficult" or "most work" chain) consensus, other miners had to accept this forked chain, ultimately allowing the attacker's version of the forked chain to successfully replace the original blockchain.

According to Bitquery, the attacker only spent $204,000 (the cost of renting hash power) and profited 807,260 ETC (valued at around $5.6 million) in this attack.

The Second Encounter with a 51% Attack

Less than a week after the previous attack, unfortunate news surfaced again as Bitfly tweeted that the ETC blockchain was attacked once more, with over 4000 blocks being reorganized. Binance exchange confirmed this news in a tweet and temporarily suspended ETC deposits and withdrawals to prevent further losses from occurring. It is unclear how much the attacker profited from this incident, but a simple calculation based on the block rewards obtained by the attacker in over 4000 blocks estimates that the attacker could profit at least $90,000.

The consecutive attacks have severely compromised the security of the ETC blockchain, making the next few weeks crucial for the ETC community. ETC Core, the core development organization of ETC, has urged miners to use whitelist identifiers to collectively resist malicious attackers. Additionally, Bitfly has recommended all exchanges, mining pools, and other ETC service providers significantly increase the transaction confirmation times for all deposits and withdrawals.

In response to this event, Ethereum's co-founder Vitalik Buterin believes that the ETC blockchain should follow Ethereum's shift to a Proof of Stake (PoS) consensus mechanism to reduce the possibility of double spend attacks.

However, the ETC community has been strongly opposed to this idea in the past and wishes to continue using Proof of Work. It is currently unclear whether the consecutive attacks will change the community's stance.