msgvalue-loop.md

Using msg.value in a Loop

The value of msg.value in a transaction’s call never gets updated, even if the called contract ends up sending some or all of the ETH to another contract. This means that using msg.value in for or while loops, without extra accounting logic, will either lead to the transaction reverting (when there are no longer sufficient funds for later iterations), or to the contract being drained (when the contract itself has an ETH balance).

contract depositer {
    function deposit(address weth) payable external {
        for (uint i = 0; i < 5; i ++) {
            WETH(weth).deposit{value: msg.value}();
        }
    }
}

In the above example, first iteration will use all the msg.value for the external call and all other iterations can:

• Drain the contract if enough ETH balance exists inside the contract to cover all the iterations.
• Revert if enough ETH balance doesn't exist inside the contract to cover all the iterations.
• Succeed if the external implementation succeeds with zero value transfers.

Also, if a function has a check like require(msg.value == 1e18, "Not Enough Balance"), that function can be called multiple times in a same transaction by sending 1 ether once as msg.value is not updated in a transaction call.

function batchBuy(address[] memory addr) external payable{
    mapping (uint => address) nft;

    for (uint i = 0; i < addr.length; i++) {
         buy1NFT(addr[i])
    }

    function buy1NFT(address to) internal {
         if (msg.value < 1 ether) { // buy unlimited times after sending 1 ether once
            revert("Not enough ether");
            } 
         nft[numero] = address;
    }
}

Thus, using msg.value inside a loop is dangerous because this might allow the sender to re-use the msg.value.

Reuse of msg.value can also show up with payable multicalls. Multicalls enable a user to submit a list of transactions to avoid paying the 21,000 gas transaction fee over and over. However, If msg.value gets re-used while looping through the functions to execute, it can cause a serious issue like the Opyn Hack.

Sources

• Rare Skills - Smart Contract Security
• TrustChain - Ethereum msg.value Reuse Vulnerability