How cross-chain bridges work and why $4 billion has been stolen from them

Introduction Blockchains do not talk to each other. Ethereum cannot read Solana’s state. Arbitrum cannot verify a transaction on Avalanche. Each chain maintains its own ledger, its own consensus, and its own finality rules. This isolation is a feature of security design, but it creates a practical problem: users hold assets on one chain and want to use them on another. Bridges exist to solve this. A bridge is a system that lets a user deposit assets on chain A and receive corresponding assets on chain B. The concept sounds simple. The implementation is where billions of dollars have been lost. The core difficulty is verification. When a user claims to have deposited 100 ETH on Ethereum and asks for 100 ETH on Arbitrum, someone or something must verify that the deposit actually happened. The mechanism chosen for this verification determines the bridge’s security model, its speed, its cost, and its attack surface. As a Coinbase analysis of bridge hacks noted, bridge security failures consistently stem from the gap between the trust assumptions a bridge claims and the trust assumptions it actually enforces. This guide covers how the major bridge architectures work, why each of the lar...