Replay attack occurs when transactions that have been made ọn the PoS chain are copied and replayed on the PoW chain (or vice versa). It only happens when two blockchains have the same chainID and transactions were made with the same nonce from the same address.
Before The Merge took place, both Ethereum and Ethereum PoW’s development team confirmed that the two blockchain would have different chainID. The latest update of Ethereum PoW after The Merge has also added some features, including the ability to change the Network ID to reduce the risks related to replay attacks.
The Ethereum PoW team seems to have prepared for the worst possible scenarios, so how did this attack happen? According to BlockSec, a firm dedicated to building blockchain security infrastructure, hackers transferred 200 WETH from OmniBridge of Gnosis Chain and replayed them on ETH PoW to withdraw another 200 ETHW. By this way, the attacker could drain the balance of the contract on the PoW chain. The reason is probably because the bridge of the Gnosis Chain did not correctly verify the chainID of cross-chain messages between the PoW and PoS chains, BlockSec added.
The Ethereum PoW team also confirmed this attack was caused by the bridge’s fault and not on chain level as the network enforced EIP-155 before the hard fork. This means that transactions on the ETH proof-of-stake chain cannot be re-enacted on the PoW chain or vice versa.
However, Martin Köppelmann, the founder of Gnosis, commented on BlockSec’s tweet that their bridge was not at fault as they did not announce support for the ETH PoW chain and so they are not responsible for this attack.
“To be clear – the bridge on Gnosis Chain and Ethereum is in no way effected. We do not support the Eth-POW chain and do not see us responsible for what is happening on that chain”, Martin Köppelmann said.
Martin also added that replaying transactions on the PoW chain can lead to a situation where people create fake bridge activity to drain funds on the ETH PoW chain. Even if the number of transactions reaches the limit that a bridge can allow per day, transactions will continue to be conducted the next day. Therefore, he suggested that the bridge on the ETH PoW chain should be temporarily shut down to prevent similar attacks.
After the news was released, ETHW price dropped sharply and now is trading around $5.08 on CoinMarketCap.
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