What happened
On August 6, 2020, Ethereum Classic (ETC) suffered its second 51% attack in under a week. An attacker who had privately mined a longer chain released it and reorganized 4,236 blocks, rewriting about 15 hours of history. Blockchain data firm Bitquery found that the attacker tried to double-spend 465,444 ETC (about $3.3 million) but succeeded with 238,306 ETC (about $1.68 million), and also collected about 14,234 ETC in block rewards on the replacement chain.
According to Bitquery, most of the double-spent coins came from deposits to Bitfinex (about 143,000 ETC) and an unidentified service (about 95,650 ETC), and the hashpower was probably rented from NiceHash's DaggerHashimoto market, as in the first attack. Bitfly and Binance reported the reorg; Binance suspended ETC deposits and withdrawals, and ETC developers urged exchanges and pools to sharply raise confirmation requirements. ETC Labs hired law firm Kobre & Kim to pursue the attackers behind both attacks.
How it happened
- The attacker obtained more hashpower than the honest ETC network, which Bitquery believes was bought on NiceHash's DaggerHashimoto market.
- From about 11:50 UTC on August 5, 2020, the attacker mined a private fork that did not include their own deposit transactions seen on the public chain, while sending ETC to exchanges on the public chain.
- The exchanges credited those deposits after their confirmation thresholds, and the attacker could trade or withdraw the credited funds.
- The attacker then released the longer private chain, which was mined as 4,236 consecutive blocks. Nodes switched to it, the original deposits disappeared, and the attacker kept both the coins and what the exchanges had paid out.
Protocol details
Evidence
- report Ethereum Classic Attacker Successfully Double-Spends $1.68M in Second Attack: Report coindesk.com
- analysis DeFiLlama defillama.com
- analysis Ethereum Classic Attack, 8 August: Catch me if you can bitquery.io
- analysis Ethereum Classic Suffers Second 51% Attack in a Week coindesk.com
- analysis ethereumclassic.org page ethere….org ethereumclassic.org
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