🛡️ Security Best Practices: Prevent Exploits

Smart contracts hold billions in assets—and they're immutable. Once deployed, bugs become permanent vulnerabilities. The DAO hack ($60M), Parity wallet freeze ($150M), Poly Network exploit ($611M), Ronin bridge ($625M) weren't theoretical—they were production disasters. Unlike traditional software where you can patch bugs, blockchain code is final. A single reentrancy vulnerability, missing access control, or integer overflow can drain entire treasuries in seconds. Security isn't optional—it's existential. This module teaches you to think like an attacker, recognize common vulnerabilities, apply secure patterns, and build contracts that withstand adversarial conditions.

function withdraw(uint amount) public {
  require(balances[msg.sender] >= amount);
  (bool success,) = msg.sender.call{value: amount}("");
  require(success);
  balances[msg.sender] -= amount; // ❌ Too late!
}

function withdraw(uint amount) public {
  require(balances[msg.sender] >= amount);
  balances[msg.sender] -= amount; // ✅ Update first!
  (bool success,) = msg.sender.call{value: amount}("");
  require(success);
}