What happened
On 23 and 24 January 2020, the Bitcoin Gold (BTG) blockchain was hit by two 51% attacks. Someone with temporary control of most of the network's hashrate mined a hidden chain in private, then published it to replace blocks that were already confirmed. That reversed payments and redirected the same coins to addresses the attacker controlled.
The first reorganization, on 23 January, removed 14 blocks and added 13, double-spending about 1,900 BTG (around $19,000). Just over six hours later, early on 24 January, a second reorg removed 15 blocks and added 16, double-spending about 5,267 BTG (around $53,000) across several redirected transactions. The total was about 7,167 BTG, reported as roughly $70,000-$72,000 at the time.
Developer James Lovejoy found and documented the attacks. He estimated that each attack cost only about $1,200-$1,700 in rented hashrate. Reorgs this deep look aimed at exchanges such as Binance, which at the time allowed BTG withdrawals after 12 confirmations. Binance later raised the requirement to 20. Bitcoin Gold had already lost about $18M to a similar 51% attack in May 2018.
How it happened
- The attacker got enough hashrate to outpace the honest Bitcoin Gold network. Lovejoy's cost estimate was based on hashrate rental prices.
- The attacker sent BTG to a recipient (likely an exchange deposit) on the public chain. At the same time, they privately mined a competing chain in which those coins went to the attacker's own addresses instead.
- After the public-chain payment had enough confirmations to be credited, the attacker released the longer private chain. Nodes switched to it, reversing 14 blocks on 23 January and 15 blocks on 24 January.
- The original payments disappeared from the canonical chain while the redirected transactions stayed. That left about 7,167 BTG double-spent across the two reorgs.
Protocol details
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