Reentrancy Attacks

A reentrancy attack occurs when a contract makes an external call before updating its state, allowing the called contract to re-enter the calling function and drain funds.

How It Works

The attacker deploys a malicious contract that calls a vulnerable withdraw function. When the victim contract sends ETH, the attacker's fallback function triggers another withdrawal before the balance is updated, draining funds repeatedly.

Real-World Examples

The DAO Hack

2016

$60M

The most famous reentrancy attack drained 3.6M ETH from The DAO, leading to the Ethereum hard fork.

Cream Finance

2021

$130M

A reentrancy exploit in Cream Finance's lending protocol drained $130M in assets.

Code Examples

Vulnerable Code

// VULNERABLE: State updated after external call
function withdraw() external {
    uint256 balance = balances[msg.sender];
    (bool success, ) = msg.sender.call{value: balance}("");
    require(success);
    balances[msg.sender] = 0; // Too late!
}

Secure Code

// FIXED: Checks-Effects-Interactions pattern
function withdraw() external nonReentrant {
    uint256 balance = balances[msg.sender];
    balances[msg.sender] = 0; // Update state first
    (bool success, ) = msg.sender.call{value: balance}("");
    require(success);
}

Prevention

Use the Checks-Effects-Interactions pattern: update state before external calls
Use OpenZeppelin's ReentrancyGuard modifier
Prefer pull over push payment patterns
Minimize external calls and use transfer() or send() when possible

Related Vulnerabilities

Flash Loan Attack Vectors

critical

Cross-Contract Interaction Bugs

high

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// VULNERABLE: State updated after external call function withdraw() external { uint256 balance = balances[msg.sender]; (bool success, ) = msg.sender.call{value: balance}(""); require(success); balances[msg.sender] = 0; // Too late! }

// FIXED: Checks-Effects-Interactions pattern function withdraw() external nonReentrant { uint256 balance = balances[msg.sender]; balances[msg.sender] = 0; // Update state first (bool success, ) = msg.sender.call{value: balance}(""); require(success); }