Common vulnerability patterns in Blockchain / Solidity - Time & Space Complexity

Time Complexity: Common vulnerability patterns

O(n)

Understanding Time Complexity

When we look at common vulnerability patterns in blockchain code, we want to understand how the cost of running the code changes as inputs grow.

We ask: How does the program's work increase when it faces more data or more users?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

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

This code allows a user to withdraw funds from their balance, sending Ether to their address.

Identify Repeating Operations

Look for loops or repeated actions that affect time.

• Primary operation: Single balance check and transfer call per withdrawal.
• How many times: Runs once per withdrawal request, no loops inside.

How Execution Grows With Input

The work done is mostly fixed per withdrawal, no matter how many users or transactions happen.

Input Size (n)	Approx. Operations
10	10 operations (one per withdrawal)
100	100 operations
1000	1000 operations

Pattern observation: The cost grows linearly with the number of withdrawal calls.

Final Time Complexity

Time Complexity: O(n)

This means the total work grows directly with the number of withdrawal requests made.

Common Mistake

[X] Wrong: "The withdrawal function runs in constant time no matter how many users withdraw."

[OK] Correct: Each withdrawal call costs time, so more calls mean more total work, even if each call alone is simple.

Interview Connect

Understanding how vulnerabilities affect time helps you explain risks clearly and shows you can think about code efficiency and safety together.

Self-Check

"What if the withdraw function included a loop over all users to update balances? How would the time complexity change?"

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