Reentrancy attacks in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a payable fallback function in Solidity.

Blockchain / Solidity

fallback() external [1] {}

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Ainternal

Bview

Cpure

Dpayable

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'view' or 'pure' which do not allow Ether reception.

Forgetting to mark fallback as payable.

The fallback function must be marked payable to receive Ether.

2fill in blank

medium

Complete the code to update the user's balance after withdrawal.

Blockchain / Solidity

balances[msg.sender] [1] 0;

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A=

B==

C+=

D-=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' which is a comparison, not assignment.

Using '+=' or '-=' which modify the value instead of setting it.

We assign zero to the user's balance after withdrawal using the '=' operator.

3fill in blank

hard

Fix the error in the withdrawal function to prevent reentrancy by updating state before sending Ether.

Blockchain / Solidity

function withdraw() public {
    uint amount = balances[msg.sender];
    require(amount > 0, "No balance");
    [1];
    (bool success, ) = msg.sender.call{value: amount}("");
    require(success, "Failed to send Ether");
}

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Abalances[msg.sender] -= amount

Bbalances[msg.sender] += amount

Cbalances[msg.sender] = 0

Damount = 0

Attempts:

3 left

💡 Hint

Common Mistakes

Updating balance after sending Ether.

Using '+=' or '-=' which do not reset the balance properly.

Setting the balance to zero before sending Ether prevents reentrancy attacks.

4fill in blank

hard

Fill both blanks to create a mapping and a function that checks the balance of an address.

Blockchain / Solidity

mapping(address => uint) public [1];

function getBalance(address user) public view returns (uint) {
    return [2][user];
}

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Abalances

Cusers

Daccounts

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for mapping and return statement.

Using unrelated names like 'users' or 'accounts'.

The mapping is named balances, and the function returns the balance for the given user.

5fill in blank

hard

Fill all three blanks to implement a safe withdrawal pattern using a mutex lock.

Blockchain / Solidity

bool private [1];

function withdraw() public {
    require(![2], "Reentrant call");
    [3] = true;
    uint amount = balances[msg.sender];
    require(amount > 0, "No balance");
    balances[msg.sender] = 0;
    (bool success, ) = msg.sender.call{value: amount}("");
    require(success, "Failed to send Ether");
    [3] = false;
}

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Alocked

Dactive

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for the lock.

Not resetting the lock to false after withdrawal.

The boolean locked is used as a mutex to prevent reentrancy by blocking repeated calls.