uncheckedsSend.sol

pragma solidity>=0.4.24;

contract vulnerable{
    uint256 defaultAmount = 1 ether;
    address payable public nominee ;
    mapping (address => uint256) balance;
    uint256 _amount;

    function() external payable{
        
    }
    
    function addNominee(address payable _nominee) public {
        nominee =_nominee;
    }
    
    // function add(uint256 _amountToBeAdded){
    //     balance[msg.sender] += 
    // }
    
    function deposit() payable public{
        address(this).transfer(_amount);
    }
    
    function transact() public{
        nominee.send(defaultAmount);
    }
    
    function getTargetBalance() public view returns(uint256){
        return address(this).balance;
    }
}

contract attacker{
    
    bool private flag;
    // vulnerable v = vulnerable(0xef55BfAc4228981E850936AAf042951F7b146e41);
    uint256 _amount = 3 ether;
    
    constructor(address payable _vulnerableContract) public{
        flag = false;
        vulnerable v = vulnerable(_vulnerableContract);
    }
    
    function deposit(address payable _addr) payable public{
        _addr.transfer(msg.value);
    }
    
    function() external payable{
        if(!flag){
            // then do some computationally heavy stuff
            // address(v).transfer(_amount/1000);
        }
        else{
            // Do some computationally lighter stuff
        }
    }
    
    function getTargetBalance() public view returns(uint256){
        return address(this).balance;
    }
    
}

contract echidna{
    vulnerable v = new vulnerable();
    attacker a = new attacker(address(v));
         
     constructor() public payable{
        
        //  a.deposit(address(v));     // transferring 3 ethers to vulnerable contract and rest to the attacker contract
         v.addNominee(address(a));
     }
     
     function action() public payable{
         a.deposit(address(v));     // transferring 3 ethers to vulnerable contract and rest to the attacker contract
         address(v).send(msg.value);
     }
     
    //  function transactToV() public payable{
    //      address(v).send(msg.value);
    //  }
     
     function getTargetBalance() public view returns(uint256, uint256){
        return (address(v).balance , address(this).balance);
    }
     
     function attack() public payable{
         v.transact();
     }
     
    //  function() external payable{
    //      address(a).transfer(msg.value);
    //  }
}