Checks-Effects-Interactions pattern in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Checks-Effects-Interactions pattern in smart contract programming?

It is a design pattern that helps prevent security issues by first checking conditions, then updating the contract's state, and finally interacting with other contracts or external addresses.

beginner

Why do we perform checks before effects and interactions in the Checks-Effects-Interactions pattern?

Performing checks first ensures that the function only proceeds if conditions are met, preventing wasted work or vulnerabilities. It helps avoid unexpected behavior or attacks.

beginner

In the Checks-Effects-Interactions pattern, what does 'effects' mean?

Effects refer to updating the contract's internal state, like changing balances or flags, after all checks pass but before interacting with external contracts.

intermediate

What kind of vulnerabilities does the Checks-Effects-Interactions pattern help prevent?

It helps prevent reentrancy attacks, where an external contract calls back into the original contract before the state is updated, causing inconsistent or malicious behavior.

beginner

Show the correct order of steps in the Checks-Effects-Interactions pattern.

1. Checks: Verify conditions (e.g., require statements). 2. Effects: Update contract state variables. 3. Interactions: Call external contracts or send funds.

What is the first step in the Checks-Effects-Interactions pattern?

A Send funds to users

B Update the contract's state

C Interact with external contracts

D Perform checks to validate conditions

The pattern starts by performing checks to ensure conditions are met before any state changes or interactions.

Why should state changes happen before interactions in this pattern?

A To save gas fees

B To prevent reentrancy attacks

C To make the contract faster

D To allow multiple interactions

Updating state before interactions prevents attackers from exploiting reentrancy by calling back before state updates.

Which vulnerability is most directly mitigated by the Checks-Effects-Interactions pattern?

A Reentrancy attack

B Denial of service

C Phishing attack

D Integer overflow

The pattern is designed to prevent reentrancy attacks by controlling the order of operations.

What happens if you interact with external contracts before updating your state?

A It can cause inconsistent state or vulnerabilities

B It saves gas

C Nothing special

D It improves security

Interacting first can allow external contracts to reenter and exploit the contract before state updates.

Which of these is NOT part of the Checks-Effects-Interactions pattern?

A Interactions

B Effects

C Loops

D Checks

Loops are not a step in this pattern; the pattern focuses on checks, effects, and interactions.

Explain the Checks-Effects-Interactions pattern and why it is important in smart contract security.

Think about the order of operations and how attackers might exploit the contract.

Describe a simple example scenario where following the Checks-Effects-Interactions pattern prevents a security problem.

Imagine sending money to a user and what could go wrong if you don't update balances first.

Practice

1.

What is the main purpose of the Checks-Effects-Interactions pattern in smart contracts?

easy

A. To reduce the gas cost of transactions

B. To prevent reentrancy attacks by ordering operations safely

C. To increase the speed of contract execution

D. To allow contracts to call external APIs

Solution

1. Step 1: Understand the pattern's purpose

   The Checks-Effects-Interactions pattern is designed to prevent security issues by ordering operations carefully.

2. Step 2: Identify the main security benefit

   By checking conditions first, updating state second, and interacting last, it prevents reentrancy attacks.

3. Final Answer:

   To prevent reentrancy attacks by ordering operations safely -> Option B

4. Quick Check:

   Pattern purpose = Prevent reentrancy [OK]

Hint: Remember: check first, update second, interact last [OK]

Common Mistakes:

• Thinking it speeds up contract execution
• Believing it reduces gas costs
• Assuming it enables external API calls

2.

Which of the following Solidity code snippets correctly follows the Checks-Effects-Interactions pattern?

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

easy

A. Yes, it checks, then interacts, then effects

B. Yes, it updates state before checks

C. No, it interacts before updating state

D. No, it updates state before interaction

Solution

1. Step 1: Analyze the order of operations

   The code first checks the balance, then calls the external address, then updates the balance.

2. Step 2: Compare with pattern order

   The pattern requires checks, then effects (state update), then interactions. Here, interaction happens before effects.

3. Final Answer:

   No, it interacts before updating state -> Option C

4. Quick Check:

   Interaction before effect = Incorrect order [OK]

Hint: State update must come before external calls [OK]

Common Mistakes:

• Ignoring the order of state update and interaction
• Thinking checks alone are enough
• Confusing effects with interactions

3.

What will be the output of the following Solidity function if balances[msg.sender] is 100 and amount is 50?

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

Assume the call succeeds.

medium

A. The balance becomes 50 and 50 ether is sent

B. The balance remains 100 and 50 ether is sent

C. The balance becomes 50 but no ether is sent

D. Transaction reverts due to incorrect order

Solution

1. Step 1: Check balance and update state

   The require passes since 100 >= 50, then balance is reduced to 50.

2. Step 2: Send ether and confirm success

   The call sends 50 ether and returns success, so require(success) passes.

3. Final Answer:

   The balance becomes 50 and 50 ether is sent -> Option A

4. Quick Check:

   Balance updated then ether sent = Correct [OK]

Hint: State updates before sending ether means balance changes [OK]

Common Mistakes:

• Thinking balance stays the same after withdrawal
• Assuming call fails without reason
• Confusing order causing revert

4.

Identify the bug in this Solidity function violating the Checks-Effects-Interactions pattern:

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

medium

A. It correctly follows the pattern

B. It updates balance before checking conditions

C. It does not check for call success

D. It calls external address before checking balance

Solution

1. Step 1: Review order of operations

   The function calls the external address first, then checks balance, then updates state.

2. Step 2: Compare with pattern rules

   Pattern requires checks first, then effects, then interactions. Here, interaction is first, violating the pattern.

3. Final Answer:

   It calls external address before checking balance -> Option D

4. Quick Check:

   Interaction before checks = Bug [OK]

Hint: Always check before calling external contracts [OK]

Common Mistakes:

• Ignoring the order of require and call
• Assuming call success check is missing
• Thinking balance update order is wrong

5.

You want to write a Solidity function that sends a reward to a user only if they have enough points. Which code snippet correctly applies the Checks-Effects-Interactions pattern?

function rewardUser(address user, uint points) public {
  // Option A
  require(userPoints[user] >= points);
  userPoints[user] -= points;
  (bool sent, ) = user.call{value: points * 1 ether}('');
  require(sent);

  // Option B
  (bool sent, ) = user.call{value: points * 1 ether}('');
  require(sent);
  require(userPoints[user] >= points);
  userPoints[user] -= points;

  // Option C
  userPoints[user] -= points;
  require(userPoints[user] >= 0);
  (bool sent, ) = user.call{value: points * 1 ether}('');
  require(sent);

  // Option D
  require(userPoints[user] >= points);
  (bool sent, ) = user.call{value: points * 1 ether}('');
  require(sent);
  userPoints[user] -= points;
}

hard

A. Option A

B. Option B

C. Option C

D. Option D

Solution

1. Step 1: Analyze the order of operations

   The correct snippet first requires userPoints[user] >= points (checks), then subtracts points (effects), then sends ether via call (interactions).

2. Step 2: Identify violations in others

   The second snippet interacts before checks; the third updates state before checks; the fourth interacts before effects.

3. Final Answer:

   Option A -> Option A

4. Quick Check:

   Checks -> Effects -> Interactions = Correct snippet [OK]

Hint: Check first, update second, call last for safety [OK]

Common Mistakes:

• Calling external before checking conditions
• Updating state before checks
• Mixing order of effects and interactions