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

Reentrancy occurs when an external call is made before state changes, letting the callee re-enter and exploit stale state.

Question 2

Which coding pattern most directly prevents reentrancy attacks?

Accepted Answer

Checks-Effects-Interactions

Answer

Checks-Effects-Interactions updates state before external calls, removing the reentrancy window.

Question 3

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

Accepted Answer

Reentrancy

Answer

The DAO's split function made an external call before zeroing the balance, allowing recursive withdrawals.

Question 4

A read-only reentrancy attack abuses what specifically?

Accepted Answer

A view function returning inconsistent state mid-execution

Answer

Read-only reentrancy exploits view functions that report stale or inconsistent values while a callback is in progress.

Question 5

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

Accepted Answer

A status flag locked during execution (nonReentrant modifier)

Answer

ReentrancyGuard sets a lock variable that reverts any nested call into a guarded function.

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