Re-entrancy

Steal all the funds from the contract using a re-entrancy attack.

Vulnerable Code

Analyze the Solidity code below to find the vulnerability.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import '@openzeppelin/contracts/utils/math/SafeMath.sol';

contract Reentrance {

  using SafeMath for uint256;
  mapping(address => uint) public balances;

  function donate(address _to) public payable {
    balances[_to] = balances[_to].add(msg.value);
  }

  function balanceOf(address _who) public view returns (uint balance) {
    return balances[_who];
  }

  function withdraw(uint _amount) public {
    if(balances[msg.sender] >= _amount) {
      (bool result,) = msg.sender.call{value:_amount}(""); // Unchecked external call
      if(result) {
        balances[msg.sender] -= _amount; // State change AFTER external call
      }
    }
  }

  receive() external payable {}
}

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Re-entrancy

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