Common vulnerability patterns in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Reentrancy Attack Output

What is the output of this simplified Solidity-like pseudocode when a reentrancy attack occurs?

Blockchain / Solidity

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

// Attacker calls withdraw repeatedly before balances[msg.sender] is updated.

A The contract locks funds permanently due to a deadlock in withdraw.

B The attacker drains all funds by repeatedly calling withdraw before balance updates.

C The contract rejects the withdrawal due to insufficient balance immediately.

D The contract throws a compile-time error due to missing return statement.

💡 Hint

Think about what happens if the external call happens before the balance is updated.

Because the balance is updated after sending funds, the attacker can call withdraw recursively before the balance decreases, draining all funds.

❓ Predict Output

intermediate

Integer Overflow Result

What is the value of variable `total` after running this Solidity-like pseudocode?

Blockchain / Solidity

uint8 a = 250;
uint8 b = 10;
uint8 total = a + b; // uint8 max is 255

A 4

B 260

C OverflowError at runtime

D 250

💡 Hint

Remember uint8 can only hold values from 0 to 255 and wraps around on overflow.

Adding 250 + 10 equals 260, but uint8 max is 255, so it wraps around: 260 - 256 = 4.

🔧 Debug

advanced

Fix the Timestamp Dependency Bug

This smart contract uses block.timestamp to decide a winner. What is the main vulnerability here?

Blockchain / Solidity

contract Lottery {
    address public winner;

    function pickWinner() public {
        require(block.timestamp % 2 == 0, "Not even timestamp");
        winner = msg.sender;
    }
}

A Function pickWinner is missing a return statement causing compile error.

B The contract will always revert because block.timestamp is never even.

C Miner can manipulate block.timestamp to influence winner selection.

D There is no vulnerability; the code is safe.

💡 Hint

Think about who controls the block timestamp and how it affects randomness.

Miners can slightly manipulate block.timestamp, allowing them to influence the outcome unfairly.

📝 Syntax

advanced

Identify the Syntax Error in Access Control

Which option contains the correct Solidity code to restrict a function to the contract owner?

A

modifier onlyOwner() {
    require(msg.sender == owner)
    _;
}

B

modifier onlyOwner() {
    if(msg.sender != owner) {
        revert();
    }
}

C

modifier onlyOwner() {
    require(msg.sender == owner);
    _;
}

D

modifier onlyOwner() {
    require(msg.sender == owner, "Not owner");
    _;
}

💡 Hint

Look for correct syntax including semicolons and the placeholder _; in modifiers.

Option D correctly uses require with an error message and includes the _; placeholder properly.

🚀 Application

expert

Detecting Front-Running Vulnerability

Which of the following code snippets is vulnerable to front-running attacks in a blockchain environment?

A

function bid() public payable {
    require(msg.value > highestBid);
    highestBidder = msg.sender;
    highestBid = msg.value;
}

B

function bid() public payable {
    require(msg.value >= highestBid);
    highestBid = msg.value;
    highestBidder = msg.sender;
}

C

function bid() public payable {
    require(msg.value > highestBid);
    emit NewBid(msg.sender, msg.value);
    highestBid = msg.value;
    highestBidder = msg.sender;
}

D

function bid() public payable {
    require(msg.value > highestBid);
    highestBid = msg.value;
    highestBidder = msg.sender;
}

💡 Hint

Consider the order of state changes and how miners or attackers can exploit it.

Option A updates highestBidder before highestBid, allowing an attacker to front-run and submit a higher bid before state is fully updated.

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