What happens if function transferFrom is called by owner of _tokenId but with different _from parameter than msg.sender? require() is not checking that msg.sender is equal to _from so I can pass any address to _from and mess up ownerZombieCount mapping and emit false Transfer event.

Am I failing to observe something?

pragma solidity >=0.5.0 <0.6.0;

import "./zombieattack.sol";
import "./erc721.sol";

contract ZombieOwnership is ZombieAttack, ERC721 {

  mapping (uint => address) zombieApprovals;

  function balanceOf(address _owner) external view returns (uint256) {
    return ownerZombieCount[_owner];

  function ownerOf(uint256 _tokenId) external view returns (address) {
    return zombieToOwner[_tokenId];

  function _transfer(address _from, address _to, uint256 _tokenId) private {
    zombieToOwner[_tokenId] = _to;
    emit Transfer(_from, _to, _tokenId);

  function transferFrom(address _from, address _to, uint256 _tokenId) external payable {
    require (zombieToOwner[_tokenId] == msg.sender || zombieApprovals[_tokenId] == msg.sender);
    _transfer(_from, _to, _tokenId);

  function approve(address _approved, uint256 _tokenId) external payable onlyOwnerOf(_tokenId) {
    zombieApprovals[_tokenId] = _approved;
    emit Approval(msg.sender, _approved, _tokenId);

Edit: I just checked ERC721.sol from


and there is require(ERC721.ownerOf(tokenId) == from, "ERC721: transfer from incorrect owner"); check so i guess Cryptozombies' implementation is only for educational purposes and is lacking security and is possible to compromise

1 Answer 1


Yes, this contract is only for educational purposes.

But you can do use require if msg.sender is a owners of tokenId

require(ownerOf(tokenId) == from, "Transfer from incorrect owner");

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