Transaction reversal: Two mining pools launched a 51% attack on Bitcoin Cash

Bitcoin Cash reportedly suffered a 51% attack following its May 15 hard fork upgrade.

After Bitcoin Cash underwent a hard fork upgrade, an attacker exploited a vulnerability in the update and added invalid transactions to the unprocessed transaction pool (MemPool).

One source said that these deals prevented miners from participating in mining and could be called “poison” deals.

The attack paralyzed the Bitcoin Cash network for an hour and a half, preventing most miners from participating in mining, and the remaining miners were only able to mine empty blocks.

Sources said BTC.TOP chose to mine empty blocks in order to continue to earn profits.

According to podcast host Guy Swann, at the time most miners were offline due to the attack, so BTC.TOP controlled more than half of the Bitcoin Cash network's computing power, which also enabled them to decide which blocks of transactions could continue on the BCH network.

During this period, BTC.TOP underwent a “non-consensus reorganization” with another dominant mining pool, BTC.com.

Some claimed that the move constituted a 51% attack on the Bitcoin Cash blockchain.

Image source: BitMEX Research

The motivation for these measures is said to be to reclaim previously unusable BCH. Previous updates to Bitcoin Cash made digital coins sent to specific addresses unusable, however, the May 15 hard fork changed this rule:

According to the Bitcoin Cash team on GitHub, this upgrade will exempt these coins and return them to the state before they were locked, meaning that any miner can take these coins and trade them.

Aaron Van Wirdum, technical editor at Bitcoin Magazine, said an unidentified miner then tried to claim the previously unspendable coins.

When an unknown miner tried to claim the coins, the BTC.com and BTC.TOP mining pools immediately coordinated a 51% attack and returned the coins to their original owners.

If confirmed, this could also be a major blow to Bitcoin Cash.

“This basically eliminates any notion that Bitcoin Cash is ‘decentralized, censorship-resistant money,’” said Guy Swann.