Exactly. Vitalik Buterin even shared his concerns on the fundamental security limits of cross-chain bridges earlier this year:
> For example, suppose that you have 100 ETH on Ethereum, and Ethereum gets 51% attacked, so some transactions get censored and/or reverted. No matter what happens, you still have your 100 ETH. Even a 51% attacker cannot propose a block that takes away your ETH, because such a block would violate the protocol rules and so it would get rejected by the network
> Now, imaging what happens if you move 100 ETH onto a bridge on Solana to get 100 Solana-WETH, and then Ethereum gets 51% attacked. The attacker deposited a bunch of their own ETH into Solana-WETH and then reverted that transaction on the Ethereum side as soon as the Solana side confirmed it. The Solana-WETH contract is now no longer fully backed, and perhaps your 100 Solana-WETH is now only worth 60 ETH. 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.
> For example, suppose that you have 100 ETH on Ethereum, and Ethereum gets 51% attacked, so some transactions get censored and/or reverted. No matter what happens, you still have your 100 ETH. Even a 51% attacker cannot propose a block that takes away your ETH, because such a block would violate the protocol rules and so it would get rejected by the network
> Now, imaging what happens if you move 100 ETH onto a bridge on Solana to get 100 Solana-WETH, and then Ethereum gets 51% attacked. The attacker deposited a bunch of their own ETH into Solana-WETH and then reverted that transaction on the Ethereum side as soon as the Solana side confirmed it. The Solana-WETH contract is now no longer fully backed, and perhaps your 100 Solana-WETH is now only worth 60 ETH. 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.
[1] https://twitter.com/vitalikbuterin/status/147950136619213209...