Over the past weekend, Ethereum hard fork Ethereum Classic (ETC) experienced a “huge 51% attack” where one miner generated a series of over 3500 blocks, as a huge fork from block 10904146 till 10907740.

It only took 17.5 BTC worth about $170k for the attacker to “fool all the network” for 12 hours.

The attack by the miner was intentional as he bought hash rate power from external sources and mined these blocks for many hours and then broadcasted them to other miners. The Bitquery report reads,

“As this sequence of the block had more weight than the chain, built by all other miners, they had to accept these blocks, effectively replacing the blockchain history with attacker’s one.”

However, not all nodes equally processed this event; those based on Open Ethereum didn’t accept those blocks, as such resulting in a fork.

“Previously at July Open Ethereum announced that they skip support for ETC, but we did not expect that the software will break in such a weird and unpredictable way. Especially when nobody make any changes to it.”

This event probably has miners losing “a large amount of ETC mined during the attack,” and some miners spent more hash power, mining on the wrong “old” fork after the attack.

The attacker was not publishing his “forked” blocks after mining as such, nobody knew it was happening, and when users sent transactions to the chain, it went to the wrong chain. The attacker was only active since July 29 for three days and generated a “modest” number of blocks, 30 per day.

On August 1st, there was a period of time, when several miners continued to mine the “old” chain, and the transfers made by users of that wallet weren’t reflected in the main ETC chain.

Interestingly, a spike in ETC price was seen at the same time as the attack and “matches the attack period exactly.” Also, the report points out the matching data says this jump in price can be related to the ETC attack.

Currently, ETC is trading at $7.24, up 58% YTD, still down 88% from its all-time high of nearly $58.

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