Blockchain Security Training Consensus Mechanism Vulnerabilities Questions and Answers

Question 1

A mining pool in a Proof-of-Work network secretly finds several blocks but deliberately withholds them from the rest of the network.
The pool's objective is to build a private, longer chain and then release it strategically to invalidate blocks found by honest miners, thereby increasing its own share of block rewards.
Which consensus vulnerability does this scenario describe?

Accepted Answer

Selfish Mining

Answer

51% Attack

Answer

Sybil Attack

Answer

Nothing-at-Stake Problem

Question 2

In a Proof-of-Stake blockchain, a vulnerability exists where validators have no economic penalty for voting on multiple competing forks simultaneously. This can destabilize the network by preventing a clear consensus from being reached, as rational validators may support all forks to maximize their potential rewards. What is this specific vulnerability called?

Accepted Answer

Nothing-at-Stake Problem

Answer

Grinding Attack

Answer

Long-Range Attack

Answer

Timejacking Attack

Question 3

An attacker gains control of the private keys from several early-stage validators on a Proof-of-Stake network who are no longer active. Using these keys, the attacker creates an entirely new, valid-looking blockchain history starting from an early block (e.g., the genesis block). This new chain is then presented to new nodes joining the network. Which of the following attacks is being executed?

Accepted Answer

Long-Range Attack

Answer

Sybil Attack

Answer

51% Attack

Answer

Selfish Mining

Question 4

Which of the following describes a Sybil attack in the context of a blockchain's consensus mechanism?

Accepted Answer

An adversary creates a large number of pseudonymous nodes or identities to gain a disproportionate influence over the network.

Answer

An attacker with over 50% of the network's hashing power reorganizes the blockchain to double-spend funds.

Answer

A validator attempts to influence the block creation lottery by iterating through potential block parameters.

Answer

A miner withholds a newly found block to gain a secret advantage over the rest of the network.

Question 5

A malicious validator in a Proof-of-Stake system notices that the process for selecting the next block producer is deterministic and based on knowable variables from the current block (e.g., timestamp, previous block hash). The validator repeatedly alters their own proposed block's contents to "grind" through possible outcomes until they find a combination that ensures they are selected as the producer for a future block. This vulnerability is known as a:

Accepted Answer

Grinding Attack

Answer

Nothing-at-Stake Problem

Answer

Censorship Attack

Answer

51% Attack

Question 6

A malicious actor plans to launch a 51% attack on a Proof-of-Work blockchain. To succeed, which of the following resources is MOST critical for the attacker to accumulate?

Accepted Answer

A superior amount of computational (hashing) power

Answer

A large number of staked coins

Answer

Control over the majority of network nodes

Answer

Access to the private keys of other major miners