Ethereum Classic
$6.91 -0.45%
ETC · 1w

Ethereum Classic 51% Chain Attack July 31, 2020

This article is based on excellent exploration by Yaz Khoury, Sam Johnson, Austin Roberts and on on-chain data from

Update: Read our recent article showing how the attacker stole 807K ETC in Ethereum Classic 51% attack.

What happened

Ethereum Classic yesterday experienced a huge ‘51% attack’. One miner, 0x75d1e5477f1fdaad6e0e3d433ab69b08c482f14e generated a series of more than 3500 blocks, as a huge fork from block 10904146 till 10907740. He mined these blocks for many hours and broadcasted them afterward to other miners. 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.

What else happened

It appeared, that not all nodes equally processed this event. Nodes based on Open Ethereum ( and we at among them ) did not do accept attacker series of blocks. This resulted in fork on 2 blockchains depending on what software installed on nodes:

Geth and Hyperledger Besu — based nodes accepted attacker blocks and selected the longest ( heaviest ) forked chain Open Ethereum — based nodes remained on the “old” forked chain

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 🙂

What it means for users

Some transfers of ETC that happened during 12 hours, starting from 16:30 UTC July, 31, till 3:30 AM August, 1st, are not included in the new blockchain. If anyone transferred money at this time, now it will not appear in her or his wallet.

There was a period of time at August, 1st, when there was a number of miners ( probably based on open ethereum ) continuing to mine the ‘old’ chain. Users of that wallets made transfers, and they were not reflected in the main ETC chain.

What it means for miners


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