UnprotectedWithdraw.sol

/* Bad */
contract SendEth{
	mapping(address => uint256) public balanceOf;

	function withdraw(address user, uint256 numTokens) public {
	    require(balanceOf[user] >= numTokens);
	    balanceOf[user] -= numTokens;
	    user.transfer(numTokens * 1 ether);
	}
}

/* Better */
contract SendEth{
	mapping(address => uint256) public balanceOf;

	function withdraw(address user, uint256 numTokens) public {
	    require(user == msg.sender); // add**
	    require(balanceOf[user] >= numTokens);
	    balanceOf[user] -= numTokens;
	    user.transfer(numTokens * 1 ether);
	}
}