A bridge is a protocol that lets assets, messages, or data cross between independent blockchains.
The fundamental problem: blockchains can't natively read each other's state, so a bridge needs trusted (or trust-minimized) attestation that an event happened on chain A before unlocking funds on chain B. The attestation can be done by:
- A multi-sig of validators the bridge operator runs.
- A light client of one chain implemented as a contract on the other.
- A zk-proof of state on chain A verified on chain B.
- An optimistic system with a fraud-proof window.
Bridges are the most exploited category of DeFi protocol by total dollars lost. The reason is structural: a successful bridge holds the locked supply of every asset that's been bridged through it, and a bug anywhere in the trust pipeline drains the lot. Ronin ($625M), Wormhole ($320M), Nomad ($190M), Harmony ($100M), all bridge incidents.
If you operate a bridge, your security posture is not "audit and ship." It is continuous monitoring, capped flows per epoch, time-locked governance, and economic security proportional to the value at stake.