Cross Chain

Cross-Chain Vulnerability Patterns

This skill provides comprehensive knowledge for identifying cross-chain and bridge vulnerabilities in smart contracts.

Overview

Cross-chain communication introduces trust assumptions at chain boundaries. Message verification is the entire security model - if messages can be forged or replayed, billions can be stolen.

Historical Context

Vulnerability Categories

1. Missing Source Chain Validation

Vulnerable Pattern:

// DANGEROUS: Accepts from any chain
function receiveMessage(bytes calldata payload) external {
    // No srcChainId check!
    (address recipient, uint256 amount) = abi.decode(payload, (address, uint256));
    token.mint(recipient, amount);
}

Secure Pattern:

function receiveMessage(uint16 srcChainId, bytes calldata payload) external {
    require(msg.sender == trustedEndpoint, "Invalid endpoint");
    require(srcChainId == ALLOWED_SOURCE_CHAIN, "Invalid source chain");
    // Process...
}

Detection:

Grep("receiveMessage|onMessage|handleMessage", glob="**/*.sol")

Check if srcChainId is validated.

2. Missing Source Address Validation

Vulnerable Pattern:

// DANGEROUS: Accepts from any address on source chain
function lzReceive(
    uint16 _srcChainId,
    bytes calldata _srcAddress,
    uint64 _nonce,
    bytes calldata _payload
) external {
    require(msg.sender == lzEndpoint);
    require(_srcChainId == trustedChain);
    // Missing: _srcAddress validation!
    _processPayload(_payload);
}

Secure Pattern:

function lzReceive(
    uint16 _srcChainId,
    bytes calldata _srcAddress,
    uint64 _nonce,
    bytes calldata _payload
) external {
    require(msg.sender == lzEndpoint, "Invalid endpoint");
    require(_srcChainId == trustedChain, "Invalid chain");
    require(
        keccak256(_srcAddress) == keccak256(abi.encodePacked(trustedRemote)),
        "Invalid source"
    );
    _processPayload(_payload);
}

3. Replay Attacks

Vulnerable Pattern:

// DANGEROUS: No nonce tracking
function processMessage(bytes32 messageId, bytes calldata data) external {
    // Can be replayed!
    _execute(data);
}

Secure Pattern:

mapping(bytes32 => bool) public processedMessages;

function processMessage(bytes32 messageId, bytes calldata data) external {
    require(!processedMessages[messageId], "Already processed");
    processedMessages[messageId] = true;
    _execute(data);
}

Detection:

Grep("messageId|nonce|processed", glob="**/*.sol")

Check if there's uniqueness tracking.

4. Message Ordering Issues

Vulnerable Pattern:

// DANGEROUS: Assumes sequential processing
function receiveDeposit(uint256 depositId, uint256 amount) external {
    require(depositId == lastDepositId + 1, "Out of order"); // Can block!
    lastDepositId = depositId;
}

Risk: One failed message blocks all subsequent messages.

Secure Pattern:

// Allow out-of-order processing with idempotency
function receiveDeposit(bytes32 depositHash, uint256 amount) external {
    require(!processed[depositHash], "Already processed");
    processed[depositHash] = true;
    _processDeposit(amount);
}

5. Chain-Specific Code Assumptions

block.number Differences

Vulnerable Pattern:

// DANGEROUS: Different meaning on L2
uint256 blockAge = block.number - startBlock;
require(blockAge >= REQUIRED_BLOCKS, "Too early");

Chain Differences:

PUSH0 Opcode (Solidity 0.8.20+)

Risk: Contracts compiled with Solidity >=0.8.20 use PUSH0, not supported on:

• zkSync Era
• Arbitrum (until Dencun)
• Some other L2s

Detection:

Grep("pragma solidity", glob="**/*.sol")

Check if version >= 0.8.20 and deploying to chains without PUSH0.

6. Bridge Lock/Mint Attacks

Vulnerable Pattern:

// Source chain: Lock
function bridgeOut(uint256 amount) external {
    token.transferFrom(msg.sender, address(this), amount);
    emit BridgeOut(msg.sender, amount);
}

// Destination chain: Mint
function bridgeIn(address user, uint256 amount) external onlyRelayer {
    // If relayer is compromised or message forged...
    wrappedToken.mint(user, amount);
}

Attack Vectors:

1. Forge message without locking
2. Replay lock message
3. Lock on chain A, mint on chains B and C

Mitigations:

• Multi-sig relayers
• Optimistic verification with fraud proofs
• Zero-knowledge proofs
• Message hash verification

7. Finality Assumptions

Vulnerable Pattern:

// DANGEROUS: Assumes instant finality
function onMessageReceived(bytes calldata proof) external {
    // Process immediately after 1 confirmation
    _execute(proof);
}

Risk: Chain reorganizations can invalidate processed messages.

Chain Finality:

LayerZero Specific Checks

// Required validations for LayerZero
function lzReceive(
    uint16 _srcChainId,
    bytes calldata _srcAddress,
    uint64 _nonce,
    bytes calldata _payload
) external override {
    // 1. Endpoint check
    require(msg.sender == address(lzEndpoint), "Invalid endpoint");

    // 2. Chain check
    require(_srcChainId == trustedRemoteChain, "Invalid chain");

    // 3. Source address check
    bytes memory trustedRemote = trustedRemoteLookup[_srcChainId];
    require(
        _srcAddress.length == trustedRemote.length &&
        keccak256(_srcAddress) == keccak256(trustedRemote),
        "Invalid source"
    );

    // 4. Process payload
    _nonblockingLzReceive(_srcChainId, _srcAddress, _nonce, _payload);
}

Cross-Chain Audit Checklist

• Source chain ID validated
• Source address validated
• Message nonce/ID tracked for replay protection
• Endpoint address verified (msg.sender check)
• Chain-specific code identified (block.number, PUSH0)
• Finality assumptions appropriate
• Out-of-order messages handled
• Failed message recovery possible

Severity Classification

Critical

• Missing source chain validation
• Missing source address validation
• No replay protection
• Signature verification bypass

High

• Insufficient finality wait
• Message ordering DoS
• Cross-chain reentrancy

Medium

• Chain-specific code assumptions
• Incomplete trusted remote setup
• Missing event emission