Solidity Security: Defeating Smart Contract Reentrancy

In decentralized finance (DeFi), reentrancy represents the most destructive smart contract vulnerability, leading to multi-million dollar drainage of token staking pools.

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The Reentrancy Exploit Flow

Reentrancy happens when a smart contract executes an external call (e.g. sending Ether to an untrusted address) before updating its internal ledger state.

// VULNERABLE METHOD
function withdraw() public {
    uint amount = balances[msg.sender];
    (bool success, ) = msg.sender.call{value: amount}(""); // External call
    require(success);
    balances[msg.sender] = 0; // State update after external call!
}

An attacker contracts a fallback function that calls withdraw() again when receiving the transaction. Because the balance has not been set to 0, the withdrawal executes repeatedly, draining the contract pool.

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Mitigations

1. Checks-Effects-Interactions Pattern

Always update states before triggering interactions with external contracts:

function withdrawPatched() public {
    uint amount = balances[msg.sender];
    balances[msg.sender] = 0; // Update state FIRST (Effect)
    (bool success, ) = msg.sender.call{value: amount}(""); // Interaction LAST
    require(success);
}

2. Use ReentrancyGuard

Import and inherit OpenZeppelin's ReentrancyGuard, utilizing the nonReentrant modifier for withdrawal functions.