A possible 51% attack is one of the setbacks of blockchain that operate a proof of work (PoW). A 51% attack occurs when an individual or coordinated group of individual controls more than half of the computing power of a blockchain network. This gives them the power to double spend, as well as manipulate the blockchain network. Smaller coins like Bitcoin Gold, Monacoin, Litecoin Cash, Zencash, and Verge, amongst others have all been hit by 51% attacks.

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Source Medium Post

Cryptocurrency project, Pirl, which at some point were victims to a 51% attack, have unveiled a new security protocol aimed at fighting a possible 51% attack. The new protocol is called the PirlGuard.

According to a Medium post by Guy Fawkes,

The PirlGuard security protocol greatly deters attackers from attempting malicious peering giving the main network a much-needed boost in security. This new security mechanism reduces the chances to approximately 0.03%.

Once a malicious miner tries to overrun the network by quickly broadcasting a longer chain, their nodes will be peered to other nodes on the network, and their malicious intent will be noticed and they will be penalized accordingly.

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Source Medium Post

In addition to PirlGuard, the team has hinted that they will increase the number of confirmations required for exchanges. Additionally, masternodes will be saddled with the responsibility of notarizing the blockchain and penalizing malicious nodes.