Solidity Security Vulnerabilities: Unchecked Return Values

We can use .transfer() to send Ether which is considered safe as it reverts in case the transfer fails. We can also use .send() and .call() to transfer Ether but they only return a value of false if they fail. This creates a security vulnerability where an attacker can target a transaction that should have reverted when send() or call() failed.

Below is an example provided at

contract Lotto {

    bool public payedOut = false;
    address public winner;
    uint public winAmount;

    // ... extra functionality here

    function sendToWinner() public {
        payedOut = true;

    function withdrawLeftOver() public {

If winner.send(winAmount) somehow fails, it will simply return false but the transaction will not be reverted – this results in payedOut being set to true. Now since payedOut is set to true, withdrawLeftOver() can be executed by anyone to steal all the funds.