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Vitalik Buterin called cross-chain bridges vulnerable to 51% attacks

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Ethereum’s co-founder Vitalik Buterin stated that he supports the concept of a multi-chain ecosystem, but is “pessimistic” about cross-chain bridges. The latter are vulnerable to attacks like 51%, according to him.

“The fundamental security limits of bridges are actually a key reason why while I am optimistic about a multi-chain blockchain ecosystem (there really are a few separate communities with different values and it’s better for them to live separately than all fight over influence on the same thing), I am pessimistic about cross-chain applications,” he wrote on Reddit.

According to Buterin, compared to first-level networks, cross-chain bridges are more vulnerable to 51% attacks. He explained that even if an attacker manages to seize most of the blockchain’s capacity; he will not be able to take away their cryptocurrency from users since this “violates protocol rules.”

Censoring or reversing a decentralized application for a certain time

Ethereum’s co-founder added that a 51% attack is capable of censoring or reversing a decentralized application for a certain time. But then the network will return to a “consistent state.”

“If you had 100 ETH, but sold it for 320000 DAI on Uniswap, even if the blockchain gets attacked in some arbitrary crazy way, at the end of the day you still have a sensible outcome – either you keep your 100 ETH or you get your 320000 DAI. The outcome where you get neither (or, for that matter, both) violates protocol rules and so would not get accepted,” Buterin said.

He stressed that cross-chain bridges do not provide such guarantees, which means that users may lose funds. As an example, he cited the situation with the hypothetical Ethereum-Solana bridge. The attacker invests a large number of his own funds in wrapped ETH on Solana; attacks the Ethereum network and, after waiting for confirmation of the transaction on the Solana side, cancels it; the SOL-WETH contract ceases to be fully secured. As a result, the price of the wrapped assets of other users falls. In fact, they lose money.

The more usage of cross-chain bridges and apps, the worse the problem becomes

“Even if there’s a perfect ZK-SNARK-based bridge that fully validates consensus, it’s still vulnerable to theft through 51% attacks like this,” he said. According to Buterin, the pairing of “various” of blockchains through cross-chain bridges will lead to the emergence of many interdependent dApps. In such a situation, a 51% attack on even one network creates a “systemic contagion”; that threatens the economy of the entire ecosystem.

“I don’t expect these problems to show up immediately. 51% attacking even one chain is difficult and expensive. However, the more usage of cross-chain bridges and apps there is, the worse the problem becomes,” he concluded.

Recall that at the beginning of 2022, Vitalik Buterin reviewed his forecasts, demonstrating a proclivity towards being correct about abstract notions rather than ongoing software development challenges.

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