How to Detect Reentrancy Vulnerabilities in Solidity

Reentrancy is the most exploited vulnerability in Ethereum smart contract history.

Classic Example: The DAO Hack (2016)

function withdraw(uint amount) external {
    require(balances[msg.sender] >= amount);
    msg.sender.call{value: amount}("");
    balances[msg.sender] -= amount;
}

How to Prevent Reentrancy

Method 1: Checks-Effects-Interactions

function withdraw(uint amount) external {
    require(balances[msg.sender] >= amount);
    balances[msg.sender] -= amount;
    (bool ok,) = msg.sender.call{value: amount}("");
    require(ok);
}

Method 2: ReentrancyGuard

import "@openzeppelin/contracts/security/ReentrancyGuard.sol";
contract Vault is ReentrancyGuard {
    function withdraw(uint amount) external nonReentrant {
        (bool ok,) = msg.sender.call{value: amount}("");
        require(ok);
    }
}

Free Automated Detection

Cipher Zero automatically detects reentrancy:

• Scans every external call
• Validates ReentrancyGuard usage
• Checks state-update ordering

Try: Web Scanner | Telegram | Dashboard

Context: Reentrancy dropped from #2 to #8 in the OWASP Smart Contract Top 10 2026 — access control and business logic are now the dominant threats