Vitalik Buterin, creator of Ethereum, wrote a long Reddit comment I read in January about the high risk of bridges, and why he doesn’t support them.

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.

To understand why bridges have these limitations, we need to look at how various combinations of blockchains and bridging survive 51% attacks. Many people have the mentality that "if a blockchain gets 51% attacked, everything breaks, and so we need to put all our force on preventing a 51% attack from ever happening even once". I really disagree with this style of thinking; in fact, blockchains maintain many of their guarantees even after a 51% attack, and it's really important to preserve these guarantees.

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. Even if 99% of the hashpower or stake wants to take away your ETH, everyone running a node would just follow the chain with the remaining 1%, because only its blocks follow the protocol rules. More generally, if you have an application on Ethereum, then a 51% attack could censor or revert it for some time, but what comes out at the end is 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.

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 this reason, it's always safer to hold Ethereum-native assets on Ethereum or Solana-native assets on Solana than it is to hold Ethereum-native assets on Solana or Solana-native assets on Ethereum. And in this context, "Ethereum" refers not just to the base chain, but also any proper L2 that is built on it. If Ethereum gets 51% attacked and reverts, Arbitrum and Optimism revert too, and so "cross-rollup" applications that hold state on Arbitrum and Optimism are guaranteed to remain consistent even if Ethereum gets 51% attacked. And if Ethereum does not get 51% attacked, there's no way to 51% attack Arbitrum and Optimism separately. Hence, holding assets issued on Optimism wrapped on Arbitrum is still perfectly safe.

The problem gets worse when you go beyond two chains. If there are 100 chains, then there will end up being dapps with many interdependencies between those chains, and 51% attacking even one chain would create a systemic contagion that threatens the economy on that entire ecosystem. This is why I think zones of interdependency are likely to align closely to zones of sovereignty (so, lots of Ethereum-universe applications interfacing closely with each other, lots of Avax-universe applications interfacing with each other, etc etc, but NOT Ethereum-universe and Avax-universe applications interfacing closely with each other)

This incidentally is also why a rollup can't just "go use another data layer". If a rollup stores its data on Celestia or BCH or whatever else but deals with assets on Ethereum, if that layer gets 51% attacked you're screwed. The DAS on Celestia providing 51% attack resistance doesn't actually help you because the Ethereum network isn't reading that DAS; it would be reading a bridge, which would be vulnerable to 51% attacks. To be a rollup that provides security to applications using Ethereum-native assets, you have to use the Ethereum data layer (and likewise for any other 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. No one will 51% attack Ethereum just to steal 100 Solana-WETH (or, for that matter, 51% attack Solana just to steal 100 Ethereum-WSOL). But if there's 10 million ETH or SOL in the bridge, then the motivation to make an attack becomes much higher, and large pools may well coordinate to make the attack happen. So cross-chain activity has an anti-network-effect: while there's not much of it going on, it's pretty safe, but the more of it is happening, the more the risks go up.

https://www.reddit.com/r/ethereum/comments/rwojtk/ama_we_are...