A blockchain bridge is a system that moves tokens or messages from one blockchain to another, for example from Ethereum to a layer 2 or to Solana. Blockchains can't read each other's state directly, so a bridge has to verify that something happened on the source chain and then act on the destination chain. Bridges are essential for a multi-chain world, but they have also been the target of some of the largest hacks in crypto.
How do bridges work?
- Lock and mint: tokens are locked in a contract on the source chain, and a wrapped version is minted on the destination chain. Returning burns the wrapped tokens and unlocks the originals.
- Burn and mint: the token issuer burns tokens on one chain and mints them natively on another, so no wrapped version is needed. Circle's CCTP for USDC works this way.
- Liquidity networks: liquidity providers hold funds on both chains; you deposit on one side and receive from a pool on the other, which is fast but depends on available liquidity.
- Canonical rollup bridges: the official bridge of a rollup, secured by the rollup's own proof system and Ethereum.
- Cross-chain messaging: protocols that pass arbitrary messages between chains, so a smart contract on one chain can trigger actions on another.
Who verifies the transfer?
The security of a bridge depends on how it verifies events on the source chain. Trusted bridges rely on a group of validators or a multisig to sign off on transfers; if enough of those keys are compromised, the bridge can be drained. Trust-minimised bridges verify the source chain cryptographically, with light clients or zero-knowledge proofs, or inherit security from Ethereum, as canonical rollup bridges do. Many bridges combine approaches, for example with separate verifier networks and rate limits.
Bridge risks
Bridges hold large amounts in a few contracts, which makes them attractive targets. In 2022 alone, the Ronin, Wormhole and Nomad bridges lost hundreds of millions of dollars through compromised keys and contract bugs. Wrapped tokens are only worth as much as the bridge behind them. When choosing or building a bridge, look at the verification model, audits, rate limits, pause mechanisms and monitoring. See our cross-chain bridges and multi-chain tooling.
Frequently Asked Questions about Bridges
What is a wrapped token?
A token on one chain that represents a token locked on another chain, such as wrapped BTC on Ethereum. Its value depends on the bridge or custodian holding the original.
Why are bridges hacked so often?
They hold large pools of funds, combine complex code on several chains, and often rely on a small set of keys. A single bug or compromised key set can expose everything.
What is the safest way to bridge to a layer 2?
The rollup's canonical bridge is generally the most secure, because it relies on Ethereum and the rollup's proof system. It can be slower, especially for withdrawals from optimistic rollups.
What is cross-chain interoperability?
The ability of blockchains to exchange tokens and messages with each other. Bridges and messaging protocols are the infrastructure that makes it possible.