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Blockchain Smart Contracts with Solidity · Aula

Vulnerabilidades comuns (reentrância etc.)

Entenda e minimize vulnerabilidades comuns em contratos inteligentes, como reentrância, estouros e subestouros de inteiros e ataques de front-running.

Vulnerabilidades comuns (reentrância etc.) é uma aula grátis de Blockchain Smart Contracts with Solidity no CoddyKit. Esta é a aula 1 de 4. Você pode ler a aula completa abaixo gratuitamente — depois pratica ao vivo no navegador com um editor de código integrado e um tutor de IA 24/7. Faz parte do caminho de aprendizado de Blockchain Smart Contracts with Solidity, e seu progresso é sincronizado entre a web e o app CoddyKit. O curso de Blockchain Smart Contracts with Solidity inclui 4 aulas no total.

Partes desta aula ainda não foram traduzidas e aparecem em inglês.

Smart Contract Security: Overview

Welcome to this crucial lesson on smart contract security! Unlike traditional software, bugs in smart contracts can lead to irreversible loss of funds.

Because contracts on the blockchain are often immutable, fixing vulnerabilities after deployment is incredibly difficult, if not impossible. Security must be a top priority from day one.

Understanding Reentrancy Attacks

Reentrancy is a critical vulnerability where an external call to an untrusted contract can 're-enter' the original contract before the first function call has completed its execution.

This allows the attacker to repeatedly drain funds or manipulate state by calling the vulnerable function multiple times.

Reentrancy: A Vulnerable Example

Consider this simplified withdrawal contract. Can you spot the potential issue?

The state (balances[msg.sender]) is updated *after* the external call to msg.sender.call. This delay creates a window for attack.

pragma solidity ^0.8.0;

contract VulnerableWithdraw {
  mapping(address => uint) public balances;

  constructor() payable {
    // Fund contract for demo purposes
  }

  function deposit() public payable {
    balances[msg.sender] += msg.value;
  }

  function withdraw(uint _amount) public {
    require(balances[msg.sender] >= _amount, "Insufficient balance");

    // External call FIRST, state update LATER
    (bool success, ) = msg.sender.call{value: _amount}("");
    require(success, "Transfer failed");

    balances[msg.sender] -= _amount; // This line is vulnerable!
  }

  function getBalance() public view returns (uint) {
    return address(this).balance;
  }
}

The Reentrancy Attack Flow

Here's how an attacker exploits the previous contract:

  • 1. Deposit: Attacker deposits funds into VulnerableWithdraw.
  • 2. Withdraw: Attacker calls withdraw(amount).
  • 3. Re-enter: When msg.sender.call transfers Ether to the attacker, their malicious fallback function is triggered.
  • 4. Repeat: The fallback function immediately calls withdraw(amount) again, before the original call updates the balance. This repeats until funds are drained.

Mitigating Reentrancy: The Fix

The most effective way to prevent reentrancy is to follow the Checks-Effects-Interactions (CEI) pattern:

  • 1. Checks: Verify all conditions (e.g., require statements).
  • 2. Effects: Update all state variables (e.g., balances[msg.sender] -= _amount).
  • 3. Interactions: Make external calls (e.g., msg.sender.call).

This ensures state is updated *before* any untrusted external code can execute.

Reentrancy: The Fixed Contract

Here's the corrected version of the withdrawal contract, applying the CEI pattern. Notice the order of operations.

Now, the balance is decremented *before* the external call, closing the reentrancy window.

pragma solidity ^0.8.0;

contract SafeWithdraw {
  mapping(address => uint) public balances;

  constructor() payable {
    // Fund contract for demo purposes
  }

  function deposit() public payable {
    balances[msg.sender] += msg.value;
  }

  function withdraw(uint _amount) public {
    // 1. Checks
    require(balances[msg.sender] >= _amount, "Insufficient balance");

    // 2. Effects: Update state BEFORE external call
    balances[msg.sender] -= _amount;

    // 3. Interactions: Make external call LAST
    (bool success, ) = msg.sender.call{value: _amount}("");
    require(success, "Transfer failed");
  }

  function getBalance() public view returns (uint) {
    return address(this).balance;
  }
}

Integer Overflows & Underflows

Integer overflows occur when an arithmetic operation results in a value larger than the maximum that the variable type can hold. It 'wraps around' to its minimum value.

Integer underflows are the opposite: when a result is smaller than the minimum value, wrapping around to the maximum.

For example, a uint8 can hold values from 0 to 255. If it's 255 and you add 1, it becomes 0 (overflow). If it's 0 and you subtract 1, it becomes 255 (underflow).

Overflow/Underflow Example

Prior to Solidity 0.8.0, these operations would silently wrap around. Since Solidity 0.8.0, arithmetic operations default to checking for overflows/underflows and will revert if one occurs.

However, understanding the concept is vital, especially when working with older codebases or using unchecked blocks for gas optimization.

pragma solidity ^0.8.0;

contract MathVulnerabilities {
  uint8 public smallNumber = 255; // Max for uint8

  function triggerOverflow() public {
    // In Solidity 0.8.0+, this transaction will revert.
    // In older versions, smallNumber would become 0.
    smallNumber = smallNumber + 1;
  }

  function triggerUnderflow() public {
    smallNumber = 0; // Reset for demo
    // In Solidity 0.8.0+, this transaction will revert.
    // In older versions, smallNumber would become 255.
    smallNumber = smallNumber - 1;
  }
}

Front-Running Attacks

Front-running is an attack where a malicious actor observes a pending transaction and submits their own transaction with a higher gas fee to have it executed first.

This is common in DeFi (Decentralized Finance) where transactions like large swaps or liquidations can be anticipated and exploited for profit.

Mitigating Front-Running

Preventing front-running is challenging due to the public nature of the mempool (pending transaction pool). However, some strategies exist:

  • Commit-Reveal Schemes: Users submit a hashed version of their intent (commit), then later reveal the actual data.
  • Batching: Grouping transactions together to reduce individual transaction visibility.
  • Decentralized Sequencers/L2s: Using solutions that offer more private or controlled transaction ordering.
  • Slippage Control: Users setting maximum acceptable price slippage for swaps.

Vulnerability Check

You've learned about three major smart contract vulnerabilities. Let's test your understanding!

Recap: Security First

In this lesson, we explored critical smart contract vulnerabilities: reentrancy, integer overflows/underflows, and front-running.

  • We saw how reentrancy exploits external calls and how the Checks-Effects-Interactions pattern provides a robust defense.
  • We understood how integer arithmetic can lead to unexpected values and the importance of compiler checks (Solidity 0.8.0+).
  • Finally, we discussed front-running and methods like commit-reveal to mitigate it.

Always prioritize security in your smart contract development!

Perguntas Frequentes

A aula “Vulnerabilidades comuns (reentrância etc.)” é grátis?

Sim — o texto completo de “Vulnerabilidades comuns (reentrância etc.)” é grátis para ler aqui na web. Para praticá-la interativamente (um editor de código integrado e um tutor de IA 24/7) e desbloquear o restante do curso de Blockchain Smart Contracts with Solidity, atualize para CoddyKit PRO. O curso de Blockchain Smart Contracts with Solidity inclui 4 aulas no total.

O que vou aprender em “Vulnerabilidades comuns (reentrância etc.)”?

Entenda e minimize vulnerabilidades comuns em contratos inteligentes, como reentrância, estouros e subestouros de inteiros e ataques de front-running. Você pratica Blockchain Smart Contracts with Solidity com código prático que executa diretamente no navegador, e um tutor de IA 24/7 responde suas dúvidas enquanto trabalha na aula.

Preciso ter experiência prévia para começar Blockchain Smart Contracts with Solidity?

Nenhuma experiência prévia é necessária. Blockchain Smart Contracts with Solidity no CoddyKit é estruturado para alunos iniciantes até avançados, então você pode começar aqui ou desde o início e aprender no seu ritmo. Esta é a aula 1 de 4.

Quanto tempo leva a aula “Vulnerabilidades comuns (reentrância etc.)”?

A maioria das aulas CoddyKit leva cerca de 5–10 minutos. Cada uma é compacta e interativa, então você faz progresso constante e retoma exatamente de onde parou entre web e app.

Posso escrever e executar código nesta aula de Blockchain Smart Contracts with Solidity?

Sim. Cada aula de Blockchain Smart Contracts with Solidity inclui um editor de código integrado, então você escreve e executa código real direto no navegador e recebe feedback de IA instantaneamente — nenhuma configuração local necessária.

Todas as aulas deste curso

  1. Vulnerabilidades comuns (reentrância etc.)
  2. Padrões de controle de acesso
  3. Programação segura com SafeMath
  4. Auditoria, Testes e Recompensas por Bugs
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