If `msg.sender` is a contract or multisig, the `safeAmount` may result into loss of the funds.

[c4-bot-5]

Lines of code

https://github.com/code-423n4/2024-06-thorchain/blob/main/chain/ethereum/contracts/THORChain_Router.sol#L196

Vulnerability details

Impact

When transferring ETH, use call() instead of transfer().

The use of payable.transfer() is heavily frowned upon because it can lead to the locking of funds. The transfer() call requires that the recipient has a payable callback, only provides 2300 gas for its operation. This means the following cases can cause the transfer to fail:

1. The contract does not have a payable callback
2. The contract's payable callback spends more than 2300 gas (which is only enough to emit something)
3. The contract is called through a proxy which itself uses up the 2300 gas

Additionally, using higher than 2300 gas might be mandatory for some multisig wallets.

Proof of concept

The function THORChain_Router::transferOut() is used to transfer out any asset to any recipient but the issue arises when the first send() fails it goes tries to transfer the funds back to the msg.sender, the transfer() here doesn't check if it is successful or not may lead to loss of funds if the explained conditions are met.

function transferOut(
    address payable to,
    address asset,
    uint amount,
    string memory memo
  ) public payable nonReentrant {
    uint safeAmount;
    if (asset == address(0)) {
      safeAmount = msg.value;
      bool success = to.send(safeAmount); // Send ETH.
      if (!success) {
@>       payable(address(msg.sender)).transfer(safeAmount); // For failure, bounce back to vault & continue. // @audit issue 
      }
    } else {
      _vaultAllowance[msg.sender][asset] -= amount; // Reduce allowance
      (bool success, bytes memory data) = asset.call(
        abi.encodeWithSignature("transfer(address,uint256)", to, amount)
      );
      require(success && (data.length == 0 || abi.decode(data, (bool))));
      safeAmount = amount;
    }
    emit TransferOut(msg.sender, to, asset, safeAmount, memo);
  }

Tools Used

Manual Review

Recommendation

Check if the the transfer() is successful otherwise revert with an error.

Assessed type

call/delegatecall