Security

Bitcoin Cash experienced a 2-block reorganization following its scheduled hard fork on May 15

May 25, 2019, 11:03AM EDT

Following the scheduled hard fork on May 15, Bitcoin Cash experienced a series of seemingly related problems, which ultimately resulted in a 2-block reorganization. Shortly after the hard fork, an attacker exploited a bug in Bitcoin ABC (which could have been exploited even before the hard fork) by sending transactions that fulfilled the requirements of a valid transaction but failed the consensus checks. This resulted in miners producing blocks that were nearly empty.

Nearly at the same time, there was a 2-block reorganization, in which not all the transactions from the orphaned blocks made it into the main chain. A 25-transaction double spend attack had taken place with the output value of 3,392 BCH (~$1.3 million at the time), according to BitMEX Research. It's not yet clear whether the reorganization was deliberate or rather accidental. Most of the outputs appeared to have been double spent around 7 blocks after the orphaned block.

BitMEX Research said that the success of the reorganization is not positive news for Bitcoin despite having more than 22 times higher hash rate. It said: "In some ways, these incidents contribute to setting a dangerous precedent. It shows that it may be possible in Bitcoin. Alternatively, this could just illustrate the risks Bitcoin Cash faces while being the minority chain."

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