Hero Ctf V5

Mon, 15 May 2023 18:53:37 +0200

Intro

Hello voici de courts writeup du HeroCTF v5, histoire de garder une trace !

Un premier merci à ma team, owalid, kibatche, Perle et Sawyerf !

Blockchain

Challenge 01 : Classic one tbh (489)

ERC20, bad guy has money in it, prevent him from getting his money back.
This is a standalone ERC20, not available on a swap.
Block all the sells to get the flag

 1pragma solidity 0.8.17;
 2
 3contract  hero2303
 4 {
 5    mapping (address => uint256) private userBalances;
 6
 7    uint256 public constant TOKEN_PRICE = 1 ether;
 8    string public constant name = "UNDIVTOK";
 9    string public constant symbol = "UDK";
10    
11    uint8 public constant decimals = 0;
12
13    uint256 public totalSupply;
14
15    function buy(uint256 _amount) external payable {
16        require(
17            msg.value == _amount * TOKEN_PRICE, 
18            "Ether submitted and Token amount to buy mismatch"
19        );
20
21        userBalances[msg.sender] += _amount;
22        totalSupply += _amount;
23    }
24
25    function sell(uint256 _amount) external {
26        require(userBalances[msg.sender] >= _amount, "Insufficient balance");
27
28        userBalances[msg.sender] -= _amount;
29        totalSupply -= _amount;
30
31        (bool success, ) = msg.sender.call{value: _amount * TOKEN_PRICE}("");
32        require(success, "Failed to send Ether");
33
34        assert(getEtherBalance() == totalSupply * TOKEN_PRICE);
35    }
36
37    function transfer(address _to, uint256 _amount) external {
38        require(_to != address(0), "_to address is not valid");
39        require(userBalances[msg.sender] >= _amount, "Insufficient balance");
40        
41        userBalances[msg.sender] -= _amount;
42        userBalances[_to] += _amount;
43    }
44
45    function getEtherBalance() public view returns (uint256) {
46        return address(this).balance;
47    }
48
49    function getUserBalance(address _user) external view returns (uint256) {
50        return userBalances[_user];
51    }
52}

Après avoir compris le concept d’ERC20 ainsi que le but du challenge (empêcher tout appelle de la fonction sell()), la façon de procéder venait d’elle même, faire en sorte qu’un des assert / require de la fonction sell() soit faux afin d’empêcher toute éxecution de ladite fonction.

Web3 on ./ircgod.com

Hero Ctf V5

Intro

Blockchain

Challenge 01 : Classic one tbh (489)