Cross-Chain Bridges: How Value Moves And Why It Gets Stolen

You have ETH on Ethereum but you want to use it on Arbitrum. You need a bridge. That single sentence has cost users more than any category of DeFi exploit. Ronin, Wormhole, Nomad, Multichain, Poly Network, Chainport — the list of nine-figure bridge hacks is longer than any protocol category.

How a bridge actually works

The most common design is 'lock and mint'. You send ETH to a bridge contract on Ethereum. The bridge sees the deposit and instructs a corresponding contract on Arbitrum to mint an equal amount of 'wrapped ETH'. Wrapped ETH is backed 1:1 by the original ETH sitting in the bridge contract.

To go back, you burn wrapped ETH on Arbitrum, the bridge releases the original ETH on Ethereum.

Where the money gets stolen

The bridge contract on the source chain holds real ETH. If anyone can trick the bridge into releasing that ETH (or into minting fake wrapped tokens on the destination chain), they walk away with the entire treasury. Ronin lost $625M because 5 of 9 validator signing keys were compromised. Wormhole lost $325M because a smart contract signature verification was bypassed.

The attack surface is enormous:

The validator set (multi-sig or MPC) can be socially engineered
The signature verification contract can have bugs
The message-passing protocol between chains can be attacked
Wrapped tokens on the destination can be minted if the bridge is fooled

The four main bridge models

1. Canonical (native) bridges

Built by the L2 team. Arbitrum, Optimism, Base each have official bridges. These have the strongest security because they use the L2's own trust assumptions (which for optimistic rollups includes a 7-day fraud proof window).

2. Lock and mint (third-party)

Multichain, Portal (Wormhole for tokens), old Anyswap. Simple model, easiest to attack.

3. Liquidity network / atomic swap

Hop, Across, Stargate. Uses pools of pre-deposited liquidity on both sides. When you 'bridge', you swap on one chain and someone releases equivalent liquidity on the other. Faster but relies on liquidity depth.

4. Generic message-passing

LayerZero, Wormhole (messaging), Axelar. Instead of just moving tokens, they can pass arbitrary messages between chains. Powerful but complex, and each has its own trust model to evaluate.

How to bridge without losing your money

Safe bridging protocol

Rule	Why
Use native bridges when possible	Fewer trust assumptions
Test with small amount first	$10 to verify path works
Split large amounts across multiple bridges	Reduce single point of failure
Verify contract addresses independently	Phishing pages copy bridge UIs
Prefer bridges live for 2+ years	Time is the only real security proof

What to remember

Every bridge is a honeypot. The more value in it, the more attackers target it. Use canonical bridges when possible, test with small amounts, and never park your entire portfolio on a bridged token unless you understand exactly what backs it.