Common vulnerability patterns in Blockchain / Solidity

Introduction

Knowing common vulnerability patterns helps keep blockchain programs safe. It stops bad actors from stealing or breaking things.

When writing smart contracts to protect user funds.

When reviewing blockchain code for security before deployment.

When learning how hackers might attack blockchain apps.

When fixing bugs that could cause loss of tokens.

When designing new blockchain features that handle money.

Syntax

Blockchain / Solidity

No single syntax applies; these are patterns to watch for in blockchain code.

Vulnerabilities often come from how code is written, not special commands.

Patterns include reentrancy, integer overflow, and improper access control.

Examples

This code lets attackers call withdraw repeatedly before balance updates, stealing funds.

Blockchain / Solidity

// Reentrancy vulnerability example in Solidity
contract Vulnerable {
    mapping(address => uint) public balances;

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

Adding 1 to 255 overflows and resets count to 0, causing wrong behavior.

Blockchain / Solidity

// Integer overflow example in Solidity
contract Overflow {
    uint8 public count = 255;

    function increment() public {
        count += 1; // Overflows back to 0
    }
}

Anyone can call setSecret and change the secret, which should be restricted.

Blockchain / Solidity

// Missing access control example
contract OpenAccess {
    string public secret;

    function setSecret(string memory _secret) public {
        secret = _secret; // Anyone can change secret
    }
}

Sample Program

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

Blockchain / Solidity

// Safe withdrawal example preventing reentrancy
pragma solidity ^0.8.0;

contract Safe {
    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");
        balances[msg.sender] -= _amount; // Update balance first
        (bool success, ) = msg.sender.call{value: _amount}("");
        require(success, "Transfer failed");
    }
}

Important Notes

Always update state variables before calling external contracts to avoid reentrancy.

Use Solidity version 0.8.0 or higher to get built-in overflow checks.

Restrict sensitive functions with access control like 'onlyOwner' modifiers.

Summary

Common vulnerabilities include reentrancy, integer overflow, and missing access control.

Fix reentrancy by updating balances before external calls.

Use modern Solidity versions to prevent overflow bugs automatically.

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