Common vulnerability patterns in Blockchain / Solidity - Mini Project: Build & Apply

Common Vulnerability Patterns in Blockchain Smart Contracts

📖 Scenario: You are a blockchain developer learning about common security issues in smart contracts. Understanding these vulnerabilities helps you write safer code and protect users' assets.

🎯 Goal: Build a simple smart contract example that demonstrates a common vulnerability pattern and then fix it step-by-step.

📋 What You'll Learn

Create a smart contract with a vulnerable function

Add a configuration variable to control access

Implement the core logic that shows the vulnerability

Print or return the result demonstrating the vulnerability and its fix

💡 Why This Matters

🌍 Real World

Smart contracts control valuable assets on blockchains. Understanding vulnerabilities helps prevent hacks and loss of funds.

💼 Career

Blockchain developers must know common security patterns to write safe contracts and pass security audits.

1

Create a vulnerable smart contract with a balance mapping

Create a Solidity contract named VulnerableContract with a public mapping balances from address to uint. Initialize the contract with a constructor that sets the deployer's balance to 1000.

Blockchain / Solidity

contract VulnerableContract {
    mapping(address => uint) public balances;
    
    constructor() {
        // Your code here
    }
}

Hint

Use mapping(address => uint) public balances; and set balances[msg.sender] = 1000; inside the constructor.

2

Add an owner variable to control access

Add a public address variable called owner to the contract. Set owner to msg.sender inside the constructor.

Blockchain / Solidity

contract VulnerableContract {
    mapping(address => uint) public balances;
    address public owner;
    
    constructor() {
        balances[msg.sender] = 1000;
        // Your code here
    }
}

Hint

Declare address public owner; and assign owner = msg.sender; in the constructor.

3

Add a withdraw function vulnerable to reentrancy

Add a public function withdraw(uint amount) that sends amount of ether to msg.sender using call. Subtract amount from balances[msg.sender] after sending. This function is vulnerable to reentrancy attacks.

Blockchain / Solidity

contract VulnerableContract {
    mapping(address => uint) public balances;
    address public owner;
    
    constructor() {
        owner = msg.sender;
        balances[msg.sender] = 1000;
    }

    function withdraw(uint amount) public {
        // Your code here
    }
}

Hint

Use require to check balance, then send ether with call, then subtract amount from balances[msg.sender].

4

Fix the reentrancy vulnerability by updating balance before sending

Modify the withdraw function to subtract amount from balances[msg.sender] before calling msg.sender.call. Then print a message "Withdrawal successful".

Blockchain / Solidity

contract VulnerableContract {
    mapping(address => uint) public balances;
    address public owner;
    
    constructor() {
        owner = msg.sender;
        balances[msg.sender] = 1000;
    }

    function withdraw(uint amount) public {
        // Your code here
    }
}

Hint

Subtract balance before sending ether. Use an event WithdrawalSuccessful to print the success message.

Practice

1. Which of the following is a common vulnerability in blockchain smart contracts that allows attackers to repeatedly call a function before the first call finishes?

easy

A. Phishing attack

B. Integer overflow

C. Timestamp dependency

D. Reentrancy attack

Solution

1. Step 1: Understand reentrancy attack

   Reentrancy happens when a contract calls another contract and the called contract calls back before the first call finishes.

2. Step 2: Identify the vulnerability type

   This allows attackers to exploit the contract by repeatedly calling a function, draining funds.

3. Final Answer:

   Reentrancy attack -> Option D

4. Quick Check:

   Repeated calls before completion = Reentrancy attack [OK]

Hint: Repeated calls before finish? It's reentrancy [OK]

Common Mistakes:

• Confusing integer overflow with repeated calls
• Thinking timestamp dependency allows repeated calls
• Mixing phishing attack with smart contract vulnerabilities

2. Which of the following Solidity code snippets correctly prevents integer overflow using Solidity 0.8+ built-in checks?

easy

A. uint256 x = a + b; // no extra check needed

B. uint256 x = SafeMath.add(a, b);

C. uint256 x = a + b; require(x >= a);

D. uint256 x = a - b; // prevents overflow

Solution

1. Step 1: Recall Solidity 0.8+ overflow behavior

   From Solidity 0.8, arithmetic operations revert on overflow automatically.

2. Step 2: Identify correct prevention method

   So, just using uint256 x = a + b; is safe without extra checks or libraries.

3. Final Answer:

   uint256 x = a + b; // no extra check needed -> Option A

4. Quick Check:

   Solidity 0.8+ auto checks overflow = uint256 x = a + b; // no extra check needed [OK]

Hint: Solidity 0.8+ auto checks overflow, no SafeMath needed [OK]

Common Mistakes:

• Using SafeMath in Solidity 0.8+ unnecessarily
• Adding manual require checks that are redundant
• Confusing subtraction with addition overflow

3. Consider this Solidity function snippet:

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

What is the main vulnerability in this code?

medium

A. Integer overflow in balances update

B. Reentrancy attack due to external call before state update

C. Timestamp dependency in withdrawal timing

D. Unchecked call return value

Solution

1. Step 1: Analyze call order in function

   The external call to msg.sender.call happens before updating balances[msg.sender].

2. Step 2: Identify vulnerability type

   This allows reentrancy: attacker can call withdraw again before balance is reduced.

3. Final Answer:

   Reentrancy attack due to external call before state update -> Option B

4. Quick Check:

   External call before balance update = Reentrancy risk [OK]

Hint: External call before state update? Watch for reentrancy [OK]

Common Mistakes:

• Thinking integer overflow is the issue here
• Ignoring call return value is not the main problem
• Confusing timestamp dependency with this pattern

4. This Solidity code snippet is intended to prevent reentrancy. What is the bug?

mapping(address => uint) public balances;

function withdraw(uint amount) public {
  require(balances[msg.sender] >= amount);
  balances[msg.sender] -= amount;
  (bool success, ) = msg.sender.call{value: amount}('');
  require(success);
  balances[msg.sender] = 0;
}

medium

A. Balance is set to zero after sending funds, causing loss of remaining balance for partial withdrawals

B. No require check on call success

C. Balances mapping is not initialized

D. Function lacks payable modifier

Solution

1. Step 1: Check balance update order

   Balance is reduced before sending funds (good for reentrancy prevention), but then reset to zero after sending.

2. Step 2: Identify bug impact

   For partial withdrawals (e.g., balance=20, amount=10), after -=10 and send 10, setting to 0 discards the remaining 10 without sending it, stealing user funds.

3. Final Answer:

   Balance is set to zero after sending funds, causing loss of remaining balance for partial withdrawals -> Option A

4. Quick Check:

   Post-call balance=0 after partial -= = remaining balance theft [OK]

Hint: Extra balance=0 after partial -= and send? Steals remainder [OK]

Common Mistakes:

• Ignoring that balance reset happens after external call
• Thinking missing payable modifier causes this bug
• Assuming call success check is missing

5. You want to write a Solidity function that safely transfers tokens and prevents both reentrancy and integer overflow. Which approach is best?

hard

A. Call external contracts first, then update balances with SafeMath

B. Use timestamp checks to limit withdrawals and SafeMath for math

C. Update state variables before external calls and use Solidity 0.8+ arithmetic

D. Avoid updating balances and rely on external contract trust

Solution

1. Step 1: Prevent reentrancy by state update order

   Update all internal state variables before making any external calls to avoid reentrancy.

2. Step 2: Prevent integer overflow using Solidity 0.8+

   Use Solidity 0.8+ built-in overflow checks instead of external libraries.

3. Final Answer:

   Update state variables before external calls and use Solidity 0.8+ arithmetic -> Option C

4. Quick Check:

   State update first + Solidity 0.8+ overflow checks = Safe transfer [OK]

Hint: Update state before calls; Solidity 0.8+ handles overflow [OK]

Common Mistakes:

• Calling external contracts before state update
• Relying on timestamp checks for safety
• Ignoring overflow checks in older Solidity versions