Network Layer, Consensus, Smart Contract, and Node Security Best Practices BDC Practice Questions (Page 2)

Questions 6–10

1. 6application · medium

   During a threat modeling exercise for a public blockchain network, the team identifies the risk of a 51% attack. Which layer of the blockchain stack does this threat primarily target?

   • AApplication layer
   • BSmart contract layer
   • CConsensus layer
   • DNetwork layer
   Reveal Solution Discussion Select an answer first
2. 7application · medium

   A blockchain startup runs a small network of validator nodes on cloud VMs. They are concerned about eclipse attacks where an attacker monopolizes all of a node's peer connections. Which combination of measures would most effectively mitigate this threat?

   • ARestrict each node to a fixed list of trusted peers and disable dynamic peer discovery.
   • BIncrease the maximum number of outbound connections per node to allow more peers.
   • CUse a public DNS seed list to discover peers randomly at startup.
   • DDeploy a firewall that only allows inbound connections from known data-center IP ranges.
   Reveal Solution Discussion Select an answer first
4. 8expert · hard

   A smart contract uses an external oracle to fetch price data. The oracle is compromised and returns manipulated prices, causing the contract to execute unfair trades. What is the most effective way to mitigate this risk?

   • AUse multiple independent oracles and take the median value.
   • BHardcode the price data in the contract.
   • CIncrease the frequency of oracle updates.
   • DTrust the oracle because it is from a reputable company.
   Reveal Solution Discussion Select an answer first
5. 9expert · hard · select all that apply

   A security team is conducting a threat model for a blockchain-based supply chain system. The system uses a permissioned blockchain with PoA consensus, smart contracts for asset tracking, and nodes hosted on cloud VMs. Select all the attack vectors that are relevant to this system and should be considered in the threat model.

   • AA smart contract vulnerability allows an attacker to manipulate the asset tracking data.
   • BAn attacker compromises a cloud VM and steals the node's signing key, allowing them to sign unauthorized transactions.
   • CA DDoS attack targets the cloud VMs hosting the nodes, disrupting consensus.
   • DAn attacker launches a Sybil attack by creating many fake identities to gain a majority in the PoA consensus.
   Check answers Discussion Select an answer first
6. 10application · medium

   A developer is writing a smart contract that allows users to withdraw Ether. To prevent reentrancy attacks, which coding pattern is most appropriate?

   • AMake the external call first, then update the user's balance.
   • BUse a low-level call and ignore the return value.
   • CRely on the Ethereum gas limit to prevent reentrancy.
   • DUpdate the user's balance to zero before making the external call.
   Reveal Solution Discussion Select an answer first