Blockchain Security and Attacks Questions and Answers

Question 1

An attacker creates thousands of pseudonymous nodes on a peer-to-peer blockchain network. The goal is to gain a disproportionately large influence, potentially overpowering honest nodes in consensus or disrupting network routing. What is this type of attack called?

Accepted Answer

Sybil Attack

Answer

A Sybil attack is characterized by a single entity creating multiple fake identities (Sybil nodes) to undermine the authority or reputation system of a network. By controlling a large number of these nodes, an attacker can out-vote honest participants or disrupt network operations.

Question 2

A smart contract function is designed to transfer a user's remaining token balance. The code is written as `balances[user] -= amount;`. If a user with a balance of 100 tokens calls this function with an `amount` of 101, and the contract is running on an older, unprotected version of Solidity, the user's balance could wrap around to a very large number. This vulnerability is known as:

Accepted Answer

Integer Underflow

Answer

Integer underflow occurs when an arithmetic operation results in a value smaller than the minimum value the data type can hold, causing it to wrap around to the maximum possible value. In this case, subtracting 101 from 100 would result in -1, which for an unsigned integer wraps around to the largest possible value, potentially allowing the user to claim an enormous balance. Modern Solidity versions (0.8.0+) have built-in checks to prevent this.

Question 3

A user notices they have received several unsolicited transactions, each for a minuscule amount of cryptocurrency (e.g., 0.00000546 BTC), in their wallet. What is the most likely purpose of this 'dusting attack'?

Accepted Answer

To de-anonymize the wallet owner by tracking how and when this 'dust' is spent with other funds.

Answer

The primary goal of a dusting attack is to compromise user privacy. Attackers send tiny amounts of crypto ('dust') to many addresses and then use blockchain analysis to track when this dust is combined with other funds in a transaction. This allows the attacker to link different addresses to a single entity, de-anonymizing the user.

Question 4

A malicious miner on a Proof-of-Work blockchain successfully mines a block but chooses not to broadcast it to the network. Instead, they immediately start mining the next block on top of their secret one. The goal is to build a longer private chain and release it strategically to orphan blocks found by honest miners. This strategy is known as:

Accepted Answer

Selfish Mining

Answer

Selfish mining is a strategy where a miner or pool keeps newly found blocks secret, aiming to get a head start on mining subsequent blocks. By releasing their longer private chain at an opportune moment, they can invalidate the work of honest miners, thereby increasing their own relative share of mining rewards.

Question 5

An attacker wants to isolate a single, specific node from the rest of the blockchain network. They do this by monopolizing all of the target node's incoming and outgoing peer connections with nodes they control. This allows the attacker to feed the victim false transaction data and a counterfeit view of the blockchain. Which network-level attack is being described?

Accepted Answer

Eclipse Attack

Answer

An eclipse attack is a targeted network-level attack where a malicious actor takes control of all of a victim node's peer connections, effectively isolating it from the honest network. This 'eclipses' the node's view of the real blockchain, making it vulnerable to deception, such as accepting double-spent transactions.

Blockchain Practice Test

Blockchain Practice Test

Blockchain Security and Attacks Questions and Answers