Reentrancy attacks in Blockchain / Solidity

Introduction

Reentrancy attacks happen when a program lets another program call back into it before finishing its work. This can cause unexpected problems like losing money or data.

When writing smart contracts that handle money transfers.

When you want to protect your contract from being tricked by repeated calls.

When you need to make sure your contract finishes one task before starting another.

When you want to understand how hackers might try to steal funds from your contract.

When testing your contract for security before deploying it.

Syntax

Blockchain / Solidity

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

This example shows a vulnerable withdraw function in Solidity.

The call to send money happens before updating the balance, which can be exploited.

Examples

This fixes the problem by updating the balance before sending money.

Blockchain / Solidity

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

This uses a lock to prevent the function from being called again before it finishes.

Blockchain / Solidity

bool locked;

modifier noReentrancy() {
    require(!locked, "No reentrancy");
    locked = true;
    _;
    locked = false;
}

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

Sample Program

This contract lets users deposit and withdraw money. The withdraw function is vulnerable because it sends money before updating the balance.

Blockchain / Solidity

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

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

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

    function withdraw(uint amount) public {
        require(balances[msg.sender] >= amount, "Insufficient balance");
        (bool success, ) = msg.sender.call{value: amount}("");
        require(success, "Transfer failed");
        balances[msg.sender] -= amount;
    }
}

Important Notes

Always update balances before sending money to prevent reentrancy.

Use a reentrancy guard (lock) to block repeated calls during execution.

Test your contracts with attack simulations to find vulnerabilities.

Summary

Reentrancy attacks happen when a contract calls back into itself before finishing.

Update state before sending money to avoid this attack.

Use locks or modifiers to protect your functions.

Practice

1. What is a reentrancy attack in blockchain smart contracts?

easy

A. An attack where a contract is called repeatedly before finishing its first call

B. An attack that steals private keys from a wallet

C. An attack that floods the network with transactions

D. An attack that changes the blockchain consensus rules

Solution

1. Step 1: Understand the concept of reentrancy

   Reentrancy happens when a contract allows another call to itself before the first call finishes, causing unexpected behavior.

2. Step 2: Identify the correct description

   An attack where a contract is called repeatedly before finishing its first call correctly describes repeated calls before finishing, which is the core of reentrancy attacks.

3. Final Answer:

   An attack where a contract is called repeatedly before finishing its first call -> Option A

4. Quick Check:

   Repeated calls before finish = Reentrancy attack [OK]

Hint: Look for repeated calls before state update to spot reentrancy [OK]

Common Mistakes:

• Confusing reentrancy with key theft
• Thinking it floods network instead
• Mixing with consensus attacks

2. Which Solidity code snippet correctly prevents reentrancy by updating state before sending Ether?

easy

A. payable(msg.sender).transfer(amount); balances[msg.sender] = 0;

B. require(balances[msg.sender] > 0); payable(msg.sender).transfer(amount);

C. balances[msg.sender] = 0; payable(msg.sender).transfer(amount);

D. balances[msg.sender] += amount; payable(msg.sender).transfer(amount);

Solution

1. Step 1: Identify the safe order of operations

   To prevent reentrancy, update the user's balance before sending Ether to avoid repeated withdrawals.

2. Step 2: Check each option's order

   balances[msg.sender] = 0; payable(msg.sender).transfer(amount); updates balance first, then sends Ether, which is the correct pattern.

3. Final Answer:

   balances[msg.sender] = 0; payable(msg.sender).transfer(amount); -> Option C

4. Quick Check:

   Update state before transfer = Safe pattern [OK]

Hint: Always update balances before sending Ether to avoid reentrancy [OK]

Common Mistakes:

• Sending Ether before updating balance
• Not checking balance before transfer
• Increasing balance instead of resetting

3. Consider this Solidity function snippet:

function withdraw(uint amount) public {
  require(balances[msg.sender] >= amount);
  payable(msg.sender).transfer(amount);
  balances[msg.sender] -= amount;
}

What is the likely outcome if this contract is attacked by a reentrancy exploit?

medium

A. The attacker can withdraw more than their balance multiple times

B. The contract will reject the withdrawal due to require failing

C. The contract will revert due to syntax error

D. The attacker cannot withdraw more than their balance

Solution

1. Step 1: Analyze the order of operations in withdraw

   The contract sends Ether before reducing the balance, allowing reentrant calls to withdraw repeatedly.

2. Step 2: Understand the attack effect

   Because balance is reduced after transfer, attacker can call withdraw again before balance updates, draining funds.

3. Final Answer:

   The attacker can withdraw more than their balance multiple times -> Option A

4. Quick Check:

   Transfer before balance update = Vulnerable to reentrancy [OK]

Hint: Check if balance updates happen after transfer to spot vulnerability [OK]

Common Mistakes:

• Assuming require protects fully
• Thinking syntax error occurs
• Believing attacker can't withdraw extra

4. This Solidity function is vulnerable to reentrancy. What is the best fix?

function withdraw(uint amount) public {
  require(balances[msg.sender] >= amount);
  payable(msg.sender).transfer(amount);
  balances[msg.sender] -= amount;
}

medium

A. Add a loop to call transfer multiple times

B. Remove the require statement

C. Increase balances[msg.sender] after transfer

D. Move balances[msg.sender] -= amount; before transfer call

Solution

1. Step 1: Identify vulnerability cause

   Balance is updated after sending Ether, allowing reentrant calls to exploit the contract.

2. Step 2: Apply fix by reordering

   Moving balance update before transfer prevents reentrancy by blocking repeated withdrawals.

3. Final Answer:

   Move balances[msg.sender] -= amount; before transfer call -> Option D

4. Quick Check:

   Update state before transfer = Fix reentrancy [OK]

Hint: Update balances before sending Ether to fix reentrancy [OK]

Common Mistakes:

• Removing require weakens security
• Adding loops increases risk
• Increasing balance after transfer worsens issue

5. You want to secure a smart contract against reentrancy attacks. Which combination of techniques is best practice?

hard

A. Allow multiple reentrant calls to improve performance

B. Update state before external calls and use a reentrancy guard modifier

C. Use only require statements without updating state

D. Send Ether first, then update state, and avoid using modifiers

Solution

1. Step 1: Recall best practices for reentrancy protection

   Update contract state before external calls and use reentrancy guard modifiers to block repeated calls.

2. Step 2: Evaluate options

   Update state before external calls and use a reentrancy guard modifier combines both safe state update order and reentrancy guard, providing strong protection.

3. Final Answer:

   Update state before external calls and use a reentrancy guard modifier -> Option B

4. Quick Check:

   State update + guard = Best reentrancy defense [OK]

Hint: Combine state update and guard modifier for strong defense [OK]

Common Mistakes:

• Sending Ether before state update
• Ignoring reentrancy guard usage
• Allowing reentrant calls for speed