No more than 14 blocks were reorganized, CoinMetrics said.
UPDATE (August 4, 04:00 UTC): Added quote from Lucas Nuzzi and information on hashrate and block reorgs.
Bitcoin SV suffered a 51% attack Tuesday morning, according to an analyst from the crypto intelligence firm CoinMetrics.
- Around 11.45 a.m. "some serious hashing power" was unleashed on the network, tweeted Lucas Nuzzi, network data product manager at CoinMetrics.
- The intelligence firm later corroborated his report, tweeting that its blockchain security monitoring tool FARUM saw a 14-block reorg of the network.
- CoinMetrics said that the attack had ended but that there was still confusion between mining pools. Pools are mining on "completely different" block heights at the moment, Nuzzi told CoinDesk in a message late on Tuesday.
- A 51% attack occurs when malicious miners take control of more than 50% of a Proof-of-Work blockchain. The attackers can take advantage of block reorganization, a feature meant to clear up incidences when two miners have mined the same block.
- Broadly speaking, when two versions of the same blocks are mined, the system defaults to the longest chain. By mining faster than the honest miners during a 51% attack, the assailants can create a longer chain, such that the system will render the honest miners' blocks invalid.
- The Bitcoin Association recommended that node operators invalidate the fraudulent chain.
- Bitcoin SV, which was born through a hard fork of Bitcoin Cash, suffered four attacks in July.
- There is no "clear indication" of who the attacker was and CoinMetrics is still looking into the attacker's chain for "traces of double spends targeting exchanges," Nuzzi said.
- Major BSV mining pools are still struggling to align on the same blockchain, he said.
- The BSV hashrate fell by almost 50% from Monday to Tuesday, according to Bitinfocharts. But Nuzzi said that due to the confusion after the reorg, it's hard to estimate how much the hashrate has dropped.