Common vulnerability patterns in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to prevent reentrancy by using a mutex lock.

Blockchain / Solidity

bool locked = false;

function withdraw(uint amount) public {
    require(![1], "Reentrant call detected");
    locked = true;
    // send funds
    locked = false;
}

Drag options to blanks, or click blank then click option'

Alocked

Btrue

Camount

Dmsg.sender

Attempts:

3 left

💡 Hint

Common Mistakes

Using a constant true value in the require statement.

Checking the amount or sender instead of the lock variable.

The variable locked is used as a mutex to prevent reentrancy. The require checks that locked is false before proceeding.

2fill in blank

medium

Complete the code to check for integer overflow before addition.

Blockchain / Solidity

function add(uint a, uint b) public pure returns (uint) {
    uint c = a + b;
    require(c [1] a, "Overflow detected");
    return c;
}

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A<

B!=

C==

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using < which would allow overflow.

Using equality which does not detect overflow.

The sum c should be greater than or equal to a to ensure no overflow occurred.

3fill in blank

hard

Fix the error in the access control modifier to restrict function calls to the owner only.

Blockchain / Solidity

address owner;

modifier onlyOwner() {
    require(msg.sender [1] owner, "Not owner");
    _;
}

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A!=

B==

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using inequality operator which blocks the owner.

Using greater or less than which is invalid for addresses.

The modifier should check that the caller is exactly the owner using ==.

4fill in blank

hard

Fill both blanks to safely update a mapping after checking conditions.

Blockchain / Solidity

mapping(address => uint) balances;

function updateBalance(address user, uint amount) public {
    require(balances[user] [1] 0, "No balance");
    balances[user] [2] amount;
}

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A>

B=

C<

D+=

Attempts:

3 left

💡 Hint

Common Mistakes

Using less than in the require which allows zero balances.

Using += which adds instead of replacing the balance.

The require checks that the user has a positive balance using >. Then the balance is set to the new amount using =.

5fill in blank

hard

Fill all three blanks to implement a safe withdrawal function avoiding reentrancy and updating state correctly.

Blockchain / Solidity

bool locked = false;

function safeWithdraw(uint amount) public {
    require(![1], "Reentrant call");
    locked = true;
    require(balances[msg.sender] [2] amount, "Insufficient balance");
    balances[msg.sender] [3] balances[msg.sender] - amount;
    // send funds
    locked = false;
}

Drag options to blanks, or click blank then click option'

Alocked

B>=

C=

D-=

Attempts:

3 left

💡 Hint

Common Mistakes

Using -= which may cause underflow in older Solidity versions.

Not checking balance before withdrawal.

Not using the lock variable correctly.

The locked variable prevents reentrancy. The balance is checked to be at least the amount with >=. The balance is updated by assigning the new value with =.