OverUnderflow.sol

// Deposit to the msg.sender
// send some to other 2 accounts ["0xAb8483F64d9C6d1EcF9b849Ae677dD3315835cb2","0x4B20993Bc481177ec7E8f571ceCaE8A9e22C02db"], 0x8000000000000000000000000000000000000000000000000000000000000000

pragma solidity 0.6.0;

contract Over{
    mapping(address=>uint) balances;
    uint _amount;
    
    function batchTransfer(address[] memory _receivers, uint _value) public payable returns(bool){
        uint cnt =  _receivers.length;
        uint amount = cnt * _value; // overflow happens here, when 0x8000000000000000000000000000000000000000000000000000000000000000 inserted, this will trigger and amount becomes 0
        _amount = amount;
        
        require(_value >0 && balances[msg.sender] > amount); // amount becomes 0, so the check is bypassed.
        balances[msg.sender] = balances[msg.sender] - amount;
        for(uint i; i<cnt; i++){
            balances[_receivers[i]] = balances[_receivers[i]] + _value;
        }
        return true;
    }
    
    function getBalance() public view returns(uint){
        return balances[msg.sender];
    }
    
    function getAmount() public view returns(uint){
        return _amount;
    }
    
    function deposit() public payable{
        balances[msg.sender] += msg.value;
    }
}