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Last updated: February 06, 2026

SCWE-034: Insecure Cross-Chain Messaging

Stable Version v1.0

This content is in the version-(v1.0) and still under active development, so it is subject to change any time (e.g. structure, IDs, content, URLs, etc.).

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Description

Insecure cross-chain messaging refers to vulnerabilities that arise when communicating between different blockchains. This can lead to: - Unauthorized actions by malicious actors. - Loss of funds or data. - Exploitation of vulnerabilities in cross-chain logic.

  • Economic inconsistencies due to unvalidated msg.value in message handlers.

Remediation

  • Validate messages: Ensure all cross-chain messages are properly validated.
  • Use secure protocols: Leverage secure cross-chain communication protocols.

  • Test thoroughly: Conduct extensive testing to ensure cross-chain logic is secure.

  • Validate msg.value: Decode expected value from the message payload or protocol parameters and require that msg.value matches (or meets) this expectation; revert on mismatch.

Examples

  • Insecure Cross-Chain Messaging 
    pragma solidity ^0.8.0;

contract InsecureCrossChain {
    event MessageProcessed(bytes message);

    function processMessage(bytes memory message) public {
        // ❌ No validation of the sender (anyone can call this function!)
        // ❌ No signature verification (attackers can inject fake messages!)
        // 🚨 No relayer authorization
        // 🚨 No replay protection
        emit MessageProcessed(message);
    }
}
    🔴 Issue: The function accepts messages without validation, allowing unauthorized transactions.

Unauthorized Relayers (Anyone Can Call!) - Issue: The function allows any msg.sender to call processMessage(), meaning an attacker can send arbitrary messages. - Impact: Attackers can forge transactions, trigger unintended actions, or drain funds if the function is connected to cross-chain asset transfers.

No Signature Verification (Fake Messages) - Issue: The contract doesn’t verify the authenticity of the message. - Impact: Attackers can inject fake messages and trick the contract into executing unauthorized actions.

Replay Attacks - Issue: The contract doesn’t track processed messages. - Impact: The same message can be replayed multiple times, leading to repeated execution of sensitive operations.

  • Secure Cross-Chain Messaging 
    pragma solidity ^0.8.0;

contract SecureCrossChain {
    mapping(address => bool) public trustedRelayers;
    mapping(bytes32 => bool) public processedMessages;

    event MessageProcessed(bytes32 indexed messageHash, address indexed sender);
    event RelayerUpdated(address relayer, bool status);

    address public owner;

    modifier onlyOwner() {
        require(msg.sender == owner, "Not the owner");
        _;
    }

    constructor(address[] memory initialRelayers) {
        owner = msg.sender;
        for (uint i = 0; i < initialRelayers.length; i++) {
            trustedRelayers[initialRelayers[i]] = true;
        }
    }

    function setRelayer(address relayer, bool status) external onlyOwner {
        trustedRelayers[relayer] = status;
        emit RelayerUpdated(relayer, status);
    }

    function processMessage(
        bytes memory message, 
        uint8 v, bytes32 r, bytes32 s
    ) public {
        require(trustedRelayers[msg.sender], "Unauthorized relayer");

        bytes32 messageHash = keccak256(abi.encodePacked("\x19Ethereum Signed Message:\n32", keccak256(message)));
        address signer = ecrecover(messageHash, v, r, s);
        require(signer != address(0), "Invalid signature");

        require(!processedMessages[messageHash], "Message already processed");
        processedMessages[messageHash] = true;

        emit MessageProcessed(messageHash, signer);

        // ✅ Securely process the message
    }
}

Fix: Implements signature verification, relayer validation, and replay protection. Why is this better? ✅ Verifies Signatures Properly: Uses ecrecover() with Ethereum Signed Message hashing. ✅ Admin Can Manage Relayers: Allows dynamic relayer updates via setRelayer(). ✅ Prevents Replay Attacks: Tracks processed messages in processedMessages mapping. ✅ Ensures Message Authenticity: Only validly signed messages are accepted.

  • Unvalidated msg.value in Cross-Chain Message Handling 

    // SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract VulnerableBridgedGovernor {
    address public endpoint;
    uint256 public _lastNonce;
    uint32 public ownerEid;
    address public owner;

    struct Origin {
        uint32 srcEid;
        address sender;
        uint256 nonce;
    }

    struct Call {
        address to;
        uint256 value;
        bytes data;
    }

    modifier onlyProxy() {
        _;
    }

    function runCalls(Call[] memory calls) internal {
        for (uint i = 0; i < calls.length; i++) {
            (bool success, ) = calls[i].to.call{value: calls[i].value}(calls[i].data);
            require(success, "Call failed");
        }
    }

    function lzReceive(
        Origin calldata origin,
        bytes32, /* guid */
        bytes calldata message,
        address, /* executor */
        bytes calldata /* extraData */
    ) public payable onlyProxy {
        require(msg.sender == endpoint, "Must be called by the endpoint");
        require(origin.srcEid == ownerEid, "Invalid message source chain");
        require(origin.sender == owner, "Invalid message sender");
        require(origin.nonce == _lastNonce + 1, "Invalid message nonce");
        _lastNonce = origin.nonce;
        runCalls(abi.decode(message, (Call[])));
        // <-- No check on msg.value!
    }
}
    🔴 Issue: The handler accepts arbitrary msg.value, enabling front-running or unintended value-carrying calls that desync protocol accounting.

  • Validated msg.value in Cross-Chain Message Handling 

    // SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract SafeBridgedGovernor {
    address public endpoint;
    uint256 public _lastNonce;
    uint32 public ownerEid;
    address public owner;

    struct Origin {
        uint32 srcEid;
        address sender;
        uint256 nonce;
    }

    struct Call {
        address to;
        uint256 value;
        bytes data;
    }

    modifier onlyProxy() {
        _;
    }

    function runCalls(Call[] memory calls) internal {
        for (uint i = 0; i < calls.length; i++) {
            (bool success, ) = calls[i].to.call{value: calls[i].value}(calls[i].data);
            require(success, "Call failed");
        }
    }

    function lzReceive(
        Origin calldata origin,
        bytes32, /* guid */
        bytes calldata message,
        address, /* executor */
        bytes calldata /* extraData */
    ) public payable onlyProxy {
        require(msg.sender == endpoint, "Must be called by the endpoint");
        require(origin.srcEid == ownerEid, "Invalid message source chain");
        require(origin.sender == owner, "Invalid message sender");
        require(origin.nonce == _lastNonce + 1, "Invalid message nonce");
        _lastNonce = origin.nonce;

        (uint256 expectedMsgValue, Call[] memory calls) = abi.decode(message, (uint256, Call[]));
        require(msg.value >= expectedMsgValue, "Invalid message value");
        runCalls(calls);
    }
}
    ✅ Fix: Decode expected value from the payload and enforce that msg.value meets it before executing downstream calls.