Blockchain Security Training Smart Contract Exploit Analysis 3

Question 1

Before Solidity 0.8.0, an unchecked addition that exceeds uint256 max would:

Accepted Answer

Wrap around to a small number (overflow)

Answer

Pre-0.8.0 arithmetic wrapped silently on overflow, enabling balance manipulation exploits.

Question 2

How does Solidity 0.8.0+ handle integer overflow and underflow by default?

Accepted Answer

Reverts the transaction on over/underflow

Answer

Solidity 0.8.0 introduced built-in checked arithmetic that reverts on overflow/underflow.

Question 3

What is the security risk of wrapping arithmetic in an `unchecked { }` block in Solidity 0.8+?

Accepted Answer

It disables overflow/underflow protection inside the block

Answer

The `unchecked` block restores wrapping behavior, reintroducing overflow risk if used carelessly.

Question 4

An underflow in a token balance subtraction (balance -= amount) before checks could let an attacker:

Accepted Answer

Obtain a massive balance from a small one

Answer

Underflowing a subtraction below zero wraps to near uint256 max, granting an enormous fake balance.

Question 5

Which library was historically used to add overflow protection before Solidity 0.8?

Accepted Answer

SafeMath

Answer

OpenZeppelin's SafeMath provided checked add/sub/mul/div that reverted on overflow.

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