Moving crypto between blockchains is useful, but it comes with risks. Cross-chain bridges can contain software bugs, rely on trusted validators, or use systems that attackers can exploit.
Before using a bridge, you need to understand how it works, what can go wrong, and how to check its security. No bridge is completely risk-free, but a few practical checks can help you make a better decision.
If you are new to bridging, start with this beginner's guide to moving crypto between blockchains. You should also learn how to recognize crypto phishing attacks, which can put your wallet and funds at risk.
How Cross-Chain Bridges Work
Blockchains usually operate separately. A cross-chain bridge helps move assets or messages between them. Different bridges use different methods, so understanding the design is important before you deposit funds.
Lock-and-Mint Bridges
One common method is called lock-and-mint. It works like this:
- You send your tokens to a smart contract on the source blockchain.
- The bridge verifies the deposit.
- The destination blockchain issues an equivalent amount of wrapped tokens.
- When you move back, the wrapped tokens are burned and the original assets are released.
Example: You deposit 1 ETH into a bridge on Chain A. After the bridge verifies the deposit, it issues a corresponding wrapped asset on Chain B. When you return, the wrapped asset is burned and the original ETH is released, according to the bridge's rules.
This process depends on the bridge correctly verifying deposits and withdrawals. A flaw in its smart contracts or verification system could put the locked assets at risk.
Liquidity-Based Bridges
Other bridges use liquidity pools on connected blockchains. Instead of locking your specific assets and minting wrapped tokens, the bridge may send assets from a pool on the destination chain.
These systems can make transfers faster, but they have different risks:
- Smart contract bugs: A flaw may allow unauthorized withdrawals or disrupt transfers.
- Insufficient liquidity: A pool may not have enough assets to complete a transfer as expected.
- Pool-related losses: Liquidity providers may face impermanent loss, depending on how the pool works.
Neither model is automatically safer. The important questions are how the bridge verifies transactions, who controls it, and what happens if a component fails.
Common Cross-Chain Bridge Security Risks
Bridge exploits often involve weaknesses in code, compromised keys, or failures in transaction verification. Understanding these risks helps you know what to look for when evaluating a bridge.
Smart Contract Bugs
Smart contracts control important bridge functions, such as deposits, withdrawals, and token issuance. If a contract does not check transactions correctly, an attacker may be able to take funds without authorization.
Common weaknesses include:
- Access control flaws: Unauthorized users may gain access to restricted functions.
- Reentrancy bugs: An attacker may exploit the order in which a contract processes external calls and updates its state.
- Incorrect validation: A contract may accept a withdrawal without properly verifying the required conditions.
Example: Imagine a bridge releases tokens after receiving a withdrawal request but fails to verify that the request is valid. An attacker could exploit that flaw to withdraw assets they do not own.
This is a simplified hypothetical example. The exact attack depends on the bridge's code and design.
Stolen Validator Keys
Some bridges rely on validators or multisignature wallets to approve transfers. These systems depend on private keys that authorize important actions.
If attackers steal enough keys to meet the bridge's approval requirements, they may be able to authorize fraudulent withdrawals or messages.
The Ronin Bridge exploit involved compromised validator keys that were used to authorize unauthorized withdrawals. It shows why key security and the number of approvals required matter.
When evaluating a bridge, check:
- How many validators or signers approve transfers.
- How many approvals are required for a transaction.
- Whether control is concentrated among a small number of parties.
- How the project protects keys and responds to suspected compromise.
Message Verification and Oracle Failures
Bridges must verify information coming from another blockchain. Some also rely on external data sources, known as oracles.
If verification fails, a bridge may accept a false message, process the same message twice, or act on incorrect data.
Example: A bridge should only issue wrapped tokens after confirming that the required deposit occurred. If an attacker can bypass that check, they may create tokens without depositing the assets that should back them.
The 2022 Wormhole incident involved a verification flaw that allowed an attacker to create wrapped assets without the required backing. It illustrates why proof and signature checks must work correctly.
These risks are not limited to token transfers. Bridges that support general cross-chain messages must also prevent forged messages and repeated execution.
How to Evaluate Bridge Safety
No single metric can prove that a bridge is safe. Instead, review several aspects of its design, controls, and history before transferring a significant amount.
Check Audits and Smart Contract Controls
Look for independent security audits and read the reports when they are available. Do not rely only on a website's claim that a bridge is "audited."
Check whether the reports cover important areas, such as:
- Deposit and withdrawal logic.
- Signature and message verification.
- Access controls and privileged functions.
- Contract upgrades and administrative permissions.
An audit can identify weaknesses, but it cannot guarantee that every vulnerability has been found. Also check whether identified issues were fixed and whether later changes have been reviewed.
Understand Who Controls the Bridge
A bridge may depend on a small group of validators, a multisignature wallet, or a broader verification system. These designs have different trust assumptions.
If only a few parties can approve transfers, pause operations, or change the contract, their keys and decisions become important security risks.
Before using the bridge, find out:
- Who can approve or reject transfers.
- Who can pause the bridge or change its settings.
- Who can upgrade its smart contracts.
- Whether upgrades have a time lock or another review process.
Upgrade permissions deserve particular attention. They can help a team fix bugs, but compromised or misused administrative access can also expose users to risk.
Review Liquidity and Security History
Total value locked (TVL) shows how much value is held in a protocol. It can provide useful context, but it is not proof that a bridge is secure.
A large TVL may attract more attackers because more funds are available to target. A smaller TVL does not automatically mean a bridge is unsafe, either.
Instead, review several signals together:
- Incident history: Has the bridge suffered exploits or major service disruptions?
- Security updates: Does the team explain vulnerabilities and fixes clearly?
- Bug bounty: Is there a clear program that rewards researchers for responsibly reporting vulnerabilities?
- Liquidity: Does the bridge have enough available liquidity for the transfer you intend to make?
- Transparency: Can you find reliable information about its design, controls, and risks?
Before You Transfer: A Quick Safety Checklist
Use this checklist before sending assets through a bridge:
- Confirm that you are using the bridge's legitimate website.
- Check the source network, destination network, token, and recipient address.
- Review the fees and the amount you expect to receive.
- Check the bridge's security documentation and known incident history.
- Understand whether the transfer relies on validators, liquidity pools, or another verification system.
- For a new bridge or an unfamiliar transfer route, consider starting with a small amount if the fees make this practical.
- Do not assume that an audit, high TVL, or popular reputation guarantees safety.
Remember that a small test transfer can help confirm that you selected the correct network and destination. It cannot prove that the bridge itself is secure.
Conclusion
Cross-chain bridges make it possible to move assets between blockchains, but they introduce risks involving smart contracts, validators, private keys, and message verification.
Before using one, understand how it works, review its security controls, and check its history. Pay particular attention to who can authorize transfers and upgrade contracts.
No checklist can remove every risk. However, careful research and checking transaction details before signing can help you avoid common mistakes and make more informed decisions when moving crypto between networks.