Cross-chain bridges explained

Why do blockchains need bridges?

Each blockchain keeps its own ledger with its own rules. Bitcoin's nodes know nothing about Ethereum's balances, and Ethereum's smart contracts cannot read Bitcoin's blocks. So you cannot simply "send" a coin from one chain to another. ethereum.org compares bridges to physical bridges between two places: they connect two blockchain ecosystems by transferring information and assets between them.

Demand for this has grown with the number of chains. People want to use an asset where it is not native, reach applications on another network, or move funds to and from layer-2 networks. ethereum.org gives a simple example: to get exposure to bitcoin on Ethereum you can hold Wrapped Bitcoin (WBTC), but WBTC is an Ethereum token — an Ethereum version of bitcoin, not the original asset on the Bitcoin blockchain.

How does a cross-chain bridge move tokens?

ethereum.org's developer documentation lists three basic mechanisms.

1. Deposit. You send 1 token to the bridge contract on chain A, where it is locked.

2. Verify. The bridge's validators, oracles or a light-client proof confirm the deposit happened.

3. Mint. A contract on chain B mints 1 wrapped token to your address.

4. Return. To go back, you burn the wrapped token on chain B and the bridge releases the original on chain A.

The documentation also groups bridges by design: native bridges built by a chain to bring in liquidity, validator- or oracle-based bridges, generalized message-passing bridges that can carry arbitrary data, and liquidity networks that focus on swaps.

What is the difference between trusted and trustless bridges?

Step 2 above — confirming that something happened on another chain — is the hard part, and it is where bridge designs split.

• Trusted bridges depend on a central entity or an external group of verifiers. ethereum.org notes they carry trust assumptions about custody of funds and the security of the bridge; users give up control of their assets while they are bridged. In return they tend to be faster and connect more chains.
• Trustless (trust-minimized) bridges use smart contracts and algorithms so that, in ethereum.org's words, the security of the bridge is the same as that of the underlying blockchain.

The developer documentation puts the trade-off plainly: bridges secured by external validators are typically less secure than bridges secured locally or natively by the blockchain's own validators. Faster and more flexible usually means more trust in someone.

Why do bridges get hacked so often?

A lock-and-mint bridge concentrates value. All the deposits backing a wrapped token sit in one contract, guarded by code and a set of keys or validators. Break either, and an attacker can withdraw the locked funds or mint wrapped tokens with nothing behind them. ethereum.org's developer documentation states that bridges account for the top three biggest hacks in DeFi and are still in the early stages of development.

Two documented cases show the scale. ethereum.org cites the Wormhole bridge, where 120k wETH, about $325 million, was stolen. In January 2023 the FBI confirmed that the Lazarus Group, cyber actors linked to North Korea, was responsible for stealing $100 million from Harmony's Horizon bridge on June 24, 2022, and said some of the funds were later laundered through a privacy protocol.

ethereum.org also warns of systemic risk: many bridges mint wrapped versions of assets, and wrapped tokens have been exploited, so a failure can spread to every DeFi app that accepted those tokens as collateral.

What mistakes do people make when using bridges?

• Assuming a wrapped token is the original coin. It is a claim backed by whatever the bridge holds.
• Choosing a bridge only by speed and fees. Check whether it relies on a small group of external validators or on the chains' own security.
• Leaving large balances in wrapped form for long periods. Exposure to the bridge lasts as long as you hold its tokens.
• Clicking bridge links from ads or messages. Lookalike sites can imitate a real bridge; see phishing and address poisoning and check your token approvals afterwards.
• Sending to the wrong network. The same token name can exist on several chains; confirm the destination chain and contract before you send.

How can you judge a bridge's risk?

1. Find who verifies transfers. An operator, an external validator set, or the chains themselves?

2. Find where the locked funds sit and who can move them, including any upgrade keys.

3. Check whether you get a native asset or a wrapped one on the other side, and who else accepts it.

4. Look for published audits and incident history. ethereum.org points to independent bridge risk dashboards as a starting point.

5. Start small. A test transfer costs a fee but can save the whole amount.