What is a Cross-Chain Bridge?

Detailed Instructions

The process begins when a user wants to move an asset, like 1 ETH, from Ethereum to Avalanche. They must first lock the original asset in a secure, audited bridge smart contract on the source chain. This contract acts as a custodian, taking custody of the funds. The user typically interacts with a bridge's front-end dApp, which connects their wallet (e.g., MetaMask).

• Sub-step 1: Connect Wallet & Select Assets: Connect your Ethereum wallet to the bridge interface. Select Ethereum as the source chain and Avalanche as the destination. Specify the exact amount, such as 1.0 ETH.
• Sub-step 2: Approve & Lock: The dApp will first request an ERC-20 approval transaction for the bridge contract address (e.g., 0x1234...abcd) to spend your ETH. After approval, you submit the lock transaction, sending your 1 ETH to the contract. This transaction includes critical data like your destination Avalanche address (0xC...).
• Sub-step 3: Confirmation: Wait for the required number of block confirmations on Ethereum (e.g., 12 blocks) to ensure the transaction is irreversible. The bridge validators monitor for this event.

Tip: Always verify the official contract address from the bridge's documentation to avoid scams. Gas fees on the source chain are required for this step.

Detailed Instructions

Once the lock transaction is confirmed, the bridge's validator network or off-chain relayers detect the event. These entities are responsible for consensus and attestation. They read the transaction log from the Ethereum blockchain, which contains the lock event emitted by the smart contract. The critical data extracted includes the user's address, the amount locked, and the destination address.

• Sub-step 1: Monitor the Chain: Validators run nodes for the source chain (Ethereum) and constantly scan for events from the bridge contract using specific event signatures like Locked(address indexed from, uint256 amount, string destChain, bytes destAddress).
• Sub-step 2: Reach Consensus: For a decentralized bridge, a threshold of validators (e.g., 8 out of 13) must sign off on the validity of the lock event. This prevents a single point of failure.
• Sub-step 3: Generate Proof: The validators collectively generate a cryptographic proof, such as a Merkle proof or a multi-signature, that attests to the lock. This proof is formatted for submission to the destination chain. For example, a relay might prepare a payload containing the transaction hash 0x789... and the validator signatures.

Tip: The security of the entire bridge hinges on the honesty and decentralization of this validator set. Some bridges use optimistic or zero-knowledge proofs for enhanced security.

Detailed Instructions

The cryptographic proof is now submitted to a corresponding smart contract on the destination chain (Avalanche). This contract, often called the mint or bridge handler, verifies the proof's validity. If the verification passes, it mints an equivalent amount of wrapped tokens (e.g., 1 avWETH) on Avalanche. These tokens are pegged 1:1 in value to the original locked asset.

• Sub-step 1: Submit the Proof: A relayer or validator posts the proof in a transaction to the Avalanche bridge contract at address 0xdef0.... This transaction includes the proof data and the user's destination address.
• Sub-step 2: On-Chain Verification: The destination contract runs a verification function. For a multi-signature bridge, this checks if the signatures meet the threshold. The contract might execute code like:

solidityCopy
function verifyProof(bytes calldata _proof, bytes32 _txHash) public view returns (bool) {
    return checkSignatures(_proof, _txHash);
}

• Sub-step 3: Mint and Transfer: Upon successful verification, the contract mints the wrapped tokens directly to the user's specified Avalanche address. The user's wallet (e.g., MetaMask connected to Avalanche) will now show a balance of 1 avWETH.

Tip: The minted token is a bridged representation (like avWETH) and not native AVAX. You may need to add its contract address to your wallet to see the balance.

Detailed Instructions

To move the asset back to the original chain (e.g., return avWETH to Ethereum as ETH), the user initiates a burn-and-release process. This is the reverse of lock-and-mint. The user must burn the wrapped tokens on the destination chain, providing proof of this burn to unlock the original asset on the source chain.

• Sub-step 1: Initiate Burn on Destination: The user calls a burn function on the Avalanche bridge contract, specifying the amount (e.g., 1 avWETH) and their Ethereum return address. This transaction destroys the wrapped tokens, emitting a Burned event.
• Sub-step 2: Proof Generation & Relay: Similar to Step 2, validators detect the burn event on Avalanche and generate a proof. This proof is relayed to the source chain (Ethereum).
• Sub-step 3: Unlock on Source Chain: The proof is submitted to the original locking contract on Ethereum. After verification, the contract releases the originally locked 1 ETH to the user's specified return address. The entire cycle is now complete, and the user holds native ETH again.

Tip: Always account for transaction fees on both chains during this process. The speed of the reverse transfer depends on the confirmation times and validator responsiveness of the destination chain (Avalanche in this case).

Detailed Instructions

First, you must classify the bridge's fundamental architecture. Is it a trusted (custodial) bridge, where a single entity or federation controls the locked assets, or a trust-minimized bridge that relies on cryptographic proofs and decentralized networks? For trust-minimized bridges, determine if it uses light clients, optimistic verification, or zero-knowledge proofs. This initial classification is critical because the trust assumptions directly dictate the primary attack surface. A custodial bridge centralizes risk on the operator's security, while a decentralized bridge's risk is in its consensus mechanism and proof validity.

• Sub-step 1: Research the validator set. For a bridge like Wormhole, check the on-chain Guardian set size and governance. The multisig threshold is a key value.
• Sub-step 2: Analyze the message passing protocol. Determine if it uses a lock-and-mint (assets locked on Chain A, minted on Chain B) or a liquidity pool model (like Synapse).
• Sub-step 3: Map the smart contracts. Identify the core bridge contract addresses on each chain. For example, the Polygon PoS bridge uses 0x8484Ef722627bf18ca5Ae6BcF031c23E6e922B30 on Ethereum.

Tip: Always consult the project's official documentation and audited smart contract repositories on GitHub as your primary source.

Detailed Instructions

This step involves a deep dive into the smart contract code that handles asset custody, minting, burning, and relayer logic. The most critical vulnerabilities often reside here. Focus on identifying reentrancy attacks, improper access controls, integer overflows/underflows, and logic errors in validation. For bridges, a specific and devastating flaw is in the signature verification logic for validating cross-chain messages. An attacker could spoof a valid message if the contract doesn't correctly verify the sender's chain ID or nonce.

• Sub-step 1: Review privilege escalation. Ensure only the designated relayer or oracle can submit proofs. Check for functions with onlyOwner modifiers that could rug-pull funds.
• Sub-step 2: Test edge cases in amount handling. Look for places where user-supplied input controls token amounts. Use a tool like Slither or MythX for static analysis.
• Sub-step 3: Examine upgradeability mechanisms. If the contract uses a proxy pattern (e.g., OpenZeppelin's TransparentUpgradeableProxy), assess the timelock and governance of the admin address.

solidityCopy
// Example of a critical validation check in a bridge receiver
function receiveMessage(bytes calldata _proof, bytes calldata _message) external {
    require(verifiedSenders[msg.sender], "Unauthorized relayer");
    (uint256 chainId, address recipient, uint256 amount) = abi.decode(_message, (uint256, address, uint256));
    require(chainId == expectedSourceChainId, "Invalid source chain"); // CRITICAL CHECK
    _mint(recipient, amount);
}

Tip: Compare the live contract code on a block explorer like Etherscan with the audited version to detect any unauthorized changes.

Detailed Instructions

Most bridges depend on oracles or relayers to observe events on one chain and submit proof transactions to another. This creates a centralized failure point. You must assess the decentralization, incentive alignment, and liveness of these network participants. A 51% attack on the source chain can allow double-spending by fooling the oracles. Similarly, a Sybil attack could compromise a permissionless relayer network. Analyze the economic security: are relayers heavily bonded, and what is the slashing condition for malicious behavior?

• Sub-step 1: Determine oracle data freshness. Check for mechanisms like heartbeat transactions or challenge periods to ensure liveness.
• Sub-step 2: Evaluate the consensus threshold. For a multi-signature bridge like Multichain, if 8 out of 15 signers are compromised, funds are lost.
• Sub-step 3: Stress-test the message delay. A slow or halted relayer can cause liquidity issues and enable time-bandit attacks where an attacker reorganizes a chain after a withdrawal.

Tip: Monitor the bridge's status page and relay transaction histories on subgraphs to identify patterns of delay or failure.

Detailed Instructions

Beyond code, bridges face economic attacks that exploit the system's financial design. A primary risk is liquidity insolvency, where the bridge cannot honor withdrawals because its wrapped tokens on the destination chain are not fully backed. This can happen in liquidity pool bridges if an attacker executes a flash loan attack to drain pools asymmetrically. Another critical vector is market manipulation of the wrapped asset's price relative to the native asset, creating arbitrage opportunities that destabilize the peg. Also, consider the governance token risk; if bridge parameters are controlled by a token vote, a token whale could maliciously change fees or pause the bridge.

• Sub-step 1: Audit liquidity pool balances. For Anyswap v3, check the underlyingBalance() in the pool contract against total supply of the wrapped token.
• Sub-step 2: Model slippage and fee impacts. High withdrawal fees can make the bridge economically non-viable and are a soft rug-pull vector.
• Sub-step 3: Analyze governance controls. Review proposals on Snapshot or Tally to see who holds voting power. A proposal with a quorum of 5% and 50M tokens could be dangerous.

bashCopy
# Example command to check a pool's balance on Ethereum using cast (Foundry)
cast call 0x1234...abcd "underlyingBalance()(uint256)" --rpc-url $MAINNET_RPC

Tip: Use DeFi Llama or similar dashboards to track the TVL and cross-chain composition of major bridges over time to spot liquidity trends.