How bridges work, and why they keep getting hacked
Bridges hold enormous balances behind code that must be perfect and validators that must be honest. That combination has made them the single most exploited component in crypto.
Reviewed by Mason Walker, Ethereum & Layer-2 · Last reviewed October 16, 2026
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A bridge moves value between blockchains that cannot talk to each other. Since a coin cannot literally leave its own ledger, bridges lock or burn on one side and issue a representation on the other. That representation is only as good as whatever guarantees the lock — which is why bridges have been the target of some of the largest exploits in the industry's history.
What are the main designs?
- Lock and mint: assets are locked in a contract or custodian on the source chain, and a wrapped version is minted on the destination.
- Burn and mint: the issuer destroys tokens on one chain and issues the same amount on another — common for stablecoins issued natively on many chains.
- Liquidity network: pools exist on both sides; you deposit into one and are paid out of the other, with no wrapping involved.
- Canonical rollup bridge: the layer 2's own bridge, where withdrawals are enforced by the base chain's rules rather than by an external committee.
Who guarantees the lock?
This is the question that determines a bridge's risk, and it has three broad answers. An external validator set signs off on transfers, meaning you trust that committee. A cryptographic proof is verified on the destination chain, meaning you trust the maths and the implementation. Or the base chain itself enforces the rules, as with a rollup's canonical bridge, which is the strongest arrangement available.
Why have so many been exploited?
Three structural reasons, none of which have gone away. Bridges accumulate large balances by design, so the prize is enormous. Their code is complex, spanning two chains with different assumptions, and complexity is where bugs live. And many rely on a small validator set or an upgradeable contract, which turns key management into the real security boundary.
- Compromised signer keys, letting an attacker authorise fraudulent withdrawals.
- Flawed proof or signature verification, letting a forged message pass as valid.
- Upgradeable contracts with weak access control, changed to permit theft.
- Mismatched accounting between chains, allowing more to be minted than was locked.
- Front-end compromises that route users to a malicious contract entirely.
How do you bridge more safely?
| Route | Trust | Note |
|---|---|---|
| Canonical rollup bridge | Base chain rules | Slowest for optimistic designs, strongest guarantees |
| Native issuer transfer | The issuer | Same asset on both sides; no wrapper |
| Liquidity network | Pool solvency and operators | Fast, no wrapping, capacity limits |
| Third-party lock-and-mint | External validator set | Convenient, historically the most exploited |
| Centralised exchange | The exchange | Deposit on one chain, withdraw on another |
The last row is worth stating plainly: for many people, moving funds through an exchange is a perfectly good bridge, with a trust assumption they have already accepted. It is often cheaper than a third-party bridge and it avoids holding a wrapped asset at all.
What should you check before using one?
- Reach the bridge through your own bookmark, never a link — bridge phishing is a standing industry, as covered in our guide to avoiding crypto scams.
- Confirm which asset you will receive: the canonical version or a third-party wrapper.
- Check whether the contracts are upgradeable and who holds the keys.
- Look up the withdrawal delay before you need the funds back, especially on optimistic rollups.
- Move a small test amount first, and avoid leaving value parked in a wrapper you do not need to hold.
For where bridges sit in the wider scaling picture, see our guide to layer 1 versus layer 2 — the canonical bridge is part of what makes a rollup a rollup rather than a separate chain.
How do you judge a bridge before using it?
Bridges publish more than most users read, and the useful information is usually one click from the landing page. Five questions separate the ones worth using from the ones worth avoiding.
- Who can move the funds? A named validator committee, a proof verified on-chain, or the base chain itself.
- Are the contracts upgradeable, and if so who controls the upgrade key and is there a timelock?
- How much value does it hold, and how long has it held it without incident?
- What happens in an emergency — is there a pause function, and who can trigger it?
- Which asset arrives: the canonical version recognised by the destination ecosystem, or a wrapper only that bridge honours?
If those answers are hard to find, that is itself the answer. The bridges with the strongest designs tend to document them at length, because the design is the product.
- 1. Blockchain bridges — ethereum.org
- 2. Layer 2 networks and canonical bridges — ethereum.org
- 3. Smart contract security considerations — ethereum.org
Frequently asked
What is a crypto bridge?
Infrastructure that moves value between blockchains by locking or burning assets on one chain and issuing a corresponding amount on another. Nothing physically crosses between ledgers.
Why are bridges hacked so often?
They hold large balances, run complex code across two environments, and frequently depend on a small validator set or upgradeable contracts. That combination makes them both valuable and fragile.
Is a wrapped token the same as the original?
No. It is a claim backed by whatever the bridge holds. If those reserves are compromised, the wrapper can depeg even though the underlying asset is fine.
What is the safest way to move assets between chains?
A canonical rollup bridge or a native issuer transfer carries the strongest guarantees. For many users, depositing on one chain and withdrawing on another via an exchange is a practical alternative.
Why do some bridge withdrawals take days?
Optimistic rollups include a challenge window during which a fraudulent withdrawal can be disputed. The delay is the security mechanism, not congestion.
Kayla Peterson is a DeFi Research Analyst at Crypto Almanac Daily, where she specializes in decentralized finance, lending protocols, decentralized exchanges (DEXs), liquidity markets, yield strategies, and tokenomics. Her work focuses on analyzing the mechanics behind DeFi ecosystems, helping readers understand how lending platforms, automated market makers, liquidity incentives, and governance models influence the broader digital asset economy. Kayla regularly covers major protocols, emerging trends in on-chain finance, and the evolution of decentralized financial infrastructure through data-driven research and in-depth market analysis. Before joining Crypto Almanac Daily, she researched blockchain-based financial systems and digital asset markets, building expertise in protocol design, token economics, and decentralized capital markets. Her reporting combines technical accuracy with clear explanations, making complex DeFi concepts accessible to both experienced investors and newcomers to the industry. At Crypto Almanac Daily, Kayla contributes daily market coverage, protocol analyses, educational guides, and long-form research articles. Her goal is to provide readers with reliable, objective insights into the rapidly changing world of decentralized finance while highlighting the opportunities and risks shaping the next generation of financial innovation.
This guide is educational and general in nature. It is not financial, investment, legal or tax advice, and it does not account for your circumstances. Crypto assets are volatile and you can lose the money you put in. See our editorial policy and methodology.
