Blockchain Security Training Smart Contract Exploit Analysis 2

Question 1

In a reentrancy attack, what must the vulnerable contract do BEFORE updating its internal state?

Accepted Answer

Make an external call that transfers value

Answer

Emit an event

Answer

Increment a nonce

Answer

Pause the contract

Question 2

Which coding pattern most directly prevents reentrancy attacks?

Accepted Answer

Checks-Effects-Interactions

Answer

Lazy initialization

Answer

Proxy delegation

Answer

Factory pattern

Question 3

The 2016 DAO hack on Ethereum primarily exploited which vulnerability class?

Accepted Answer

Reentrancy

Answer

Integer overflow

Answer

Front-running

Answer

Access control misconfiguration

Question 4

A read-only reentrancy attack abuses what specifically?

Accepted Answer

A view function returning inconsistent state mid-execution

Answer

An overflowing loop counter

Answer

A missing return value check

Answer

An unbounded gas refund

Question 5

Which mechanism does OpenZeppelin's ReentrancyGuard use to block reentry?

Accepted Answer

A status flag locked during execution (nonReentrant modifier)

Answer

A gas stipend cap

Answer

A timelock on every call

Answer

A signature nonce per transaction

Question 6

Cross-function reentrancy is dangerous because the attacker re-enters through:

Accepted Answer

A different function sharing the same state variables

Answer

The constructor

Answer

A pure helper function

Answer

The fallback of an unrelated contract

Question 7

Why does using `transfer()` (2300 gas) no longer reliably prevent reentrancy?

Accepted Answer

Gas costs can change via hard forks, breaking the fixed stipend assumption

Answer

It now forwards all gas by default

Answer

It silently swallows reverts

Answer

It bypasses the receive function