What can happen if attackers control more than 50% of a blockchain network’s mining power? Learn how a 51% attack enables transaction reversal and double-spending—critical knowledge for CompTIA Security+ (Plus) SY0-701 exam success.
Table of Contents
Question
A group of malicious miners gain control of more than 50% of the mining power of a blockchain network. What can they do?
A. Reverse completed transactions
B. Create new private keys
C. Modify smart contracts
D. Expose users’ identities
E. Encrypt stored transactions
Answer
A. Reverse completed transactions
Explanation
With 51% of mining power, attackers can double spend coins and revert transactions, breaking the integrity of the blockchain.
If a group of malicious miners gains control of more than 50% of a blockchain network’s mining power, they can reverse completed transactions.
This scenario is known as a 51% attack. When attackers control a majority of the network’s computational power, they can manipulate the blockchain ledger in several ways, most notably by reversing transactions that were completed while they were in control.
Reversing transactions enables double-spending, where the same coins can be spent more than once, undermining the integrity of the blockchain.
Attackers can also prevent new transactions from being confirmed, halt payments, censor specific transactions, and disrupt the normal operation of the blockchain network.
However, they cannot create new private keys, modify smart contracts, expose users’ identities, or encrypt stored transactions—their power is limited to manipulating transaction confirmations and blockchain history for the period they control the majority.
The primary risk and most damaging effect is the ability to reverse completed transactions, leading to financial losses and a loss of trust in the blockchain network.
With more than 50% control, attackers can reverse transactions, enabling double-spending and severely compromising the blockchain’s security and trustworthiness.
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