Free Blockchain Developer Blockchain Security and Vulnerabilities Questions and Answers

Question 1

An attacker creates thousands of pseudonymous nodes on a peer-to-peer network to gain a disproportionately large influence.
By controlling a significant number of these fake identities, the attacker aims to out-vote honest participants or disrupt network operations.
Which type of attack does this scenario describe?

Accepted Answer

Sybil Attack

Answer

Eclipse Attack

Answer

51% Attack

Answer

Timejacking Attack

Question 2

A smart contract written in a pre-0.8.0 version of Solidity contains a function to process batch transfers. An attacker discovers that by sending a carefully crafted array of transfer amounts, they can cause the internal loop's counter variable, a `uint8`, to exceed its maximum value of 255. This causes the variable to wrap around to 0, leading to unintended contract behavior. What is this vulnerability called?

Accepted Answer

Integer Overflow

Answer

Reentrancy

Answer

Denial of Service

Answer

Short Address Attack

Question 3

An attacker targets a specific merchant's node on a blockchain network. The attacker floods the node's connection table with IP addresses of malicious nodes they control. As a result, the merchant's node is completely isolated from honest peers and only receives information, such as blocks and transactions, curated by the attacker. This allows the attacker to feed the merchant a fraudulent transaction, leading to a double-spend. Which attack has taken place?

Accepted Answer

Eclipse Attack

Answer

Sybil Attack

Answer

51% Attack

Answer

Reentrancy Attack

Question 4

Which of the following describes the primary mechanism of a Timejacking attack?

Accepted Answer

Manipulating a node's network time counter by connecting it to malicious peers with inaccurate timestamps, forcing it to accept an alternative blockchain.

Answer

Gaining control of over 50% of the network's hashing power to unilaterally control the creation of new blocks.

Answer

Creating a large number of fake identities to undermine a network's reputation system and gain disproportionate influence.

Answer

Repeatedly calling a function in a smart contract before the previous invocation has completed its state updates.

Question 5

A malicious actor gains control of more than 50% of a Proof-of-Work blockchain's total hashing power. With this majority control, they are able to mine a private version of the blockchain faster than the rest of the network. They then broadcast this longer chain, which causes the honest network to discard its version and accept the attacker's, allowing the attacker to reverse their own previous transactions. This vulnerability is known as a:

Accepted Answer

51% Attack

Answer

Reentrancy Attack

Answer

Sybil Attack

Answer

Eclipse Attack

Question 6

A DeFi lending protocol's smart contract has a `withdraw()` function that follows this sequence: 1. Checks the user's balance. 2. Sends the Ether to the user's address. 3. Updates the user's balance to zero. An attacker creates a malicious contract with a fallback function. When the attacker calls `withdraw()`, their fallback function is triggered by the incoming Ether and immediately calls the `withdraw()` function again before the original call can update the balance. What type of vulnerability is being exploited?

Accepted Answer

Reentrancy

Answer

Integer Underflow

Answer

Denial of Service

Answer

Timejacking