A bridge lets you move value from one blockchain to another, such as from Ethereum to Base. This article explains how bridges work, why some withdrawals take a week, where the risks are, and what to check before you bridge.
Chains are separate ledgers that cannot read each other directly. A bridge is the extra layer of software that connects them. That extra layer is also the reason bridges deserve care.
What a bridge actually does
A bridge does not move coins like a package. Each chain has its own ledger, so the bridge creates a matching balance on the destination chain and takes control of the original on the source chain. There are three common designs.
Lock and mint
You send tokens to a bridge contract on chain A, which locks them. The bridge then mints a wrapped token on chain B that represents the locked one. When you want to go back, the wrapped token is destroyed and the original is unlocked.
The wrapped token is only worth as much as the locked funds backing it. If the locked funds are stolen, the wrapped tokens may become worth little or nothing. This is why the safety of the bridge contract matters so much.
Burn and mint
Some tokens, such as native USDC, are issued on many chains by the same issuer. A burn and mint bridge destroys the tokens on chain A and the issuer creates the same amount on chain B. No locked pile of funds exists, so the "pile gets stolen" risk is lower. You rely instead on the issuer and the messaging system between the chains. See stablecoins for native vs bridged USDC.
Liquidity networks
A liquidity network keeps pools of tokens on each chain. When you send 100 USDC from chain A, a market maker or pool on chain B pays you 100 USDC (minus a fee) right away, and is repaid later from the funds you deposited. This is how many fast bridges feel instant. You rely on the network's pools, pricing and reimbursement logic.
Canonical bridges vs third-party bridges
The canonical bridge of a rollup is the official bridge built into the rollup's design and run by smart contracts on Ethereum. Arbitrum, Base and OP Mainnet each have one. Its security is tied to the rollup itself, and it does not add a new group of trusted parties.
Third-party bridges are built by other teams. They often offer more chains, lower fees and faster speed, but you add that team's contracts, validators or relayers to the list of things that must work correctly.
| Feature | Canonical bridge | Third-party bridge |
|---|---|---|
| Run by | The rollup's own contracts | Independent team |
| Trust added | None beyond the rollup | The bridge's code and operators |
| Speed to L2 | Usually minutes | Often seconds to minutes |
| Speed from optimistic L2 | About 7 days | Often minutes to hours |
| Chains covered | That rollup and Ethereum | Often many |
Why withdrawals take about 7 days
Depositing into an optimistic rollup, from Ethereum to the L2, is fast. Withdrawing back to Ethereum through the canonical bridge is not. Optimistic rollups accept new state as correct unless someone proves it wrong, so the bridge has a challenge window of about 7 days. Only after it ends can the withdrawal be finalized on Ethereum. The reasons are in rollups: optimistic vs zk.
The long wait protects the funds: it gives watchers time to catch a wrong claim before money leaves.
How fast bridges shortcut the wait
Fast bridges use a liquidity network. A provider on Ethereum pays you right away, for example 99.9 of 100 USDC, and in return takes your 7-day claim on the L2 funds. The provider waits for the window, collects the full amount, and earns the fee as payment for waiting and carrying risk.
You get your funds in minutes, at the cost of the fee and of trusting the bridge's contracts and the provider.
The risks
Bridges hold or control large pools of value and join different systems, which makes them attractive targets. Bridge hacks have been among the largest thefts in crypto history. The main risks are:
- Smart-contract bugs. A flaw in the bridge contract can let an attacker mint tokens that were never backed, or drain the locked funds.
- Compromised keys. Some bridges are controlled by a group of signers. If enough of their keys are stolen, attackers can approve fake transfers.
- Faulty verification. The bridge may fail to check messages from the other chain correctly.
- Wrapped token risk. If a bridge fails, wrapped tokens like USDC.e can lose their value even though the original token is fine.
- Operator and upgrade risk. Teams can pause or upgrade contracts. This can protect users, and it can also be a point of control.
- User mistakes. Choosing the wrong chain, the wrong token version or a fake bridge website.
Audits help but do not remove risk. A bridge that has operated for a long time with a lot of value and a clear security record tells you more than a new one with a high advertised yield.
Checklist before bridging
- Do you need to bridge at all? Sometimes you can withdraw from an exchange straight to the chain you want, and avoid a bridge.
- Use the official link. Open the bridge from the project's own documentation, not from an ad or a message.
- Compare the options. Check the fee, the speed, and whether the bridge is canonical or third-party.
- Check the token. Confirm whether you will receive native or bridged tokens, and whether your destination app accepts them.
- Send a small test first. Especially for new routes or large sums.
- Keep gas on the destination. You need a bit of the destination chain's gas token to do anything after arriving.
- Review approvals. Revoke token approvals you gave to a bridge once done.
- Don't bridge more than you can afford to lose into a bridge you do not understand.
You can compare what a transfer will cost on each chain on the gas tracker, and learn how to keep your keys safe in wallets and keys.
Frequently asked questions
Is it safe to use a crypto bridge?
Some are safer than others, but all add risk beyond staying on one chain. Canonical bridges rely only on the rollup's own security. Third-party bridges add their own code and operators. Use well-established ones and avoid sending more than you can afford to lose.
How long does a bridge take?
Depositing to an L2 usually takes minutes. Withdrawing from Arbitrum, Base or OP Mainnet through the official bridge takes about 7 days. Fast third-party bridges can cut that to minutes or hours for a fee.
What is a canonical bridge?
It is the official bridge of a rollup, built into the rollup's design and operated by contracts on Ethereum. It is usually the baseline for security.
Why have so many bridges been hacked?
Bridges hold large amounts of locked value, connect two systems with different rules, and depend on code and keys that must be right in both places. A single weak point can be enough for an attacker.
What is the difference between bridged and native tokens?
A bridged token is a wrapped copy backed by funds locked elsewhere. A native token is issued directly on the chain by its issuer. See the stablecoins article for the example of USDC and USDC.e.