blockchains-security-toolkit/vulnerabilities/reentrancy-notes.md

🥛 reeentrancy

TL; DR

• When a contract calls an external function, that external function may itself call the calling function.
• A reentrancy attack may occur when a function makes an external call to another untrusted contract. Then, the unstrusted contract makes a recursive callback to the vulnerable contract function to steal funds.
• To prevent this attack, a contract can implement a lock in storage that prevents re-entrant calls.

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Example of re-entrancy attack

For example, suppose this method:

function withdrawBalance() public {
      uint amountToWithdraw = userBalances[msg.sender];
      (bool success, ) = msg.sender.call.value(amountToWithdraw)("");
      requires(success);
      userBalances[msg.sender] = 0;
}

and this exploit:

function() public payable {
      if(msg.sender == address(vulnContract)) {
        vulnContract.withdrawBalance();
      }
}

How to fix?

Option 1: Adding a mutex locking:

modifier noReentrant() {
  require(!locked, "nooooope");
  locked = true;
  _;
  locked = false;
}

so

function withdrawBalance() public noReentrant {
    ...
}

Option 2: CEI (checks effects interaction) pattern

function withdrawBalance() public {
    uint amountToWithdraw = userBalances[msg.sender];
    userBalances[msg.sender] = 0; // update state first
    (bool success, ) = msg.sender.call.value(amountToWithdraw)("");
    requires(success);
}

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Resources

• Solidity docs
• DASP
• SWC
• Not so smart contract
• reentrancy patterns