Learn About Reentracny Attacks and how to Prevent them

Learn About Reentracny Attacks and how to Prevent them

What is a Solidity Reentrancy Attack?

In Solidity smart contracts, a reentrancy attack occurs when an external contract is called, allowing the function to be recursively called before its initial execution is complete. This can enable the external contract to manipulate the state of the original contract before it finishes executing.

Reentrancy attacks exploit state synchronization issues, occurring when the state is not updated before making an external call. For instance, if a function checks a condition, updates the state, and then makes an external call, an attacker can re-enter the function in the middle of its execution, bypassing the updated state and potentially draining funds.

Types of Smart Contract Reentrancy Attacks

1. Single Function Reentrancy

This basic form of reentrancy attack involves a single function within a contract being re-entered. This typically happens when the function modifies the contract’s state and then calls an external contract or sends Ether without first updating its internal state variables.

2. Cross-Function Reentrancy

In cross-function reentrancy, one function performs an external call before updating the state, and the external contract calls another function that depends on this state. This can lead to unexpected interactions, allowing an attacker to manipulate the contract’s state across multiple functions.

3. Cross-Contract Reentrancy

Cross-contract reentrancy involves interactions between functions in multiple contracts where the state is shared. If the shared state in the first contract is not updated before an external call, other contracts that depend on the shared state can be re-entered.

5. Cross-Chain Reentrancy

Cross-chain reentrancy, though less common, involves interactions between smart contracts on different blockchain networks. This scenario arises in interoperability protocols or decentralized exchanges (DEXs) facilitating transactions across multiple blockchains.

6. Read-Only Reentrancy

Read-only reentrancy occurs when an external call is made to another contract, but the called contract’s function does not modify its state. Instead, the called function reads data from the calling contract and then reenters it, potentially causing unexpected behavior.

How to prevent Solidity Reentrancy Attacks

Use the Checks-Effects-Interactions Pattern

Ensure that state changes are made before interacting with external contracts or sending Ether. Modify the execution order to follow the Checks-Effects-Interactions pattern:

  1. Checks: Verify the state of the caller.
  2. Effects: Update the global state.
  3. Interactions: Perform the external call.


mapping(address => uint) public balance;

function withdraw(uint amount) public {
    // 1. Checks
    require(balance[msg.sender] >= amount);
    // 2. Effects
    balance[msg.sender] -= amount;
    // 3. Interactions
    msg.sender.call{value: amount}("");
    emit Withdrawal(msg.sender, amount);
Implement Mutexes or Locks

A mutex (mutual exclusion) mechanism prevents a function from being executed multiple times within the same transaction. This is typically done using a boolean flag.

Example Using Reentrancy Guard:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.18;
import {ReentrancyGuard} from "@openzeppelin/contracts/security/ReentrancyGuard.sol";

contract ReentracyProtected is ReentrancyGuard {
    mapping(address => uint) public balances;

    function withdraw() external nonReentrant {
        uint balance = balances[msg.sender];
        require(balance > 0, "Insufficient balance");
        balances[msg.sender] = 0;
        (bool success, ) = address(msg.sender).call{ value: balance }("");
        require(success, "Failed to withdraw");

Perform Extensive Code Review and Testing

  • Audits: Multiple rounds of smart contract audits can significantly decrease the chance of a reentrancy attack.
  • Thorough Testing: Beyond audits, thorough testing, including invariant testing, is crucial to ensure smart contract security.

Examples of Smart Contract Reentrancy Attacks

The DAO Hack: In 2016, the DAO, a decentralized investment fund, was exploited due to a reentrancy vulnerability. The attacker was able to repeatedly withdraw funds before the contract could update its balance, resulting in the theft of approximately $6 million worth of Ether.

Curve Finance: On July 30th, 2023, Curve Finance, a decentralized finance (DeFi) protocol, was attacked due to a Vyper compiler bug, leading to a loss of nearly $70 million.

Reentrancy Example

A vulnerability is found in the HypercertMinter::splitValue function, which calls _mintBatch() before updating the storage. This violates the Checks-Effects-Interactions pattern, making it vulnerable to a reentrancy attack.

Vulnerable Code:

function _splitValue(address _account, uint256 _tokenID, uint256[] calldata _values) internal {
    uint256 valueLeft = tokenValues[_tokenID];
    for (uint256 i; i < len;) {
        valueLeft -= values[i];
        tokenValues[toIDs[i]] = values[i];
        unchecked { ++i; }
    _mintBatch(_account, toIDs, amounts, "");
    tokenValues[_tokenID] = valueLeft;
    emit BatchValueTransfer(typeIDs, fromIDs, toIDs, values);

The above code is susceptible to reentrancy attacks as the state is not updated before the external call to _mintBatch(), allowing an attacker to exploit this vulnerability.

Performing Audits Using Armur

Reentrancy attacks are a significant vulnerability in Solidity smart contracts, enabling attackers to manipulate contract states and potentially drain funds. With Armur, you can easily catch attacks like reentrancy by conducting smart contract audits. This tool will give you a detailed audit report with all the vulnerabilities in you code before you can eveing deploy your protocols.

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