Newer cryptocurrencies have been shown to be vulnerable to a 51% attack. A cryptocurrency with a lower hashrate could be overwhelmed by a miner who adds blocks quickly. The attacker could then direct the users of the cryptocurrency to their own cryptocurrency. In some cases, even after a cryptocurency declines in value after a 51% attack an attacker can gain short positions of the cryptocurrency while the depreciation in value occurs.

Outcomes of a 51% Attack

- 2012 attack on CoiledCoin cryptocurrency

Occurrences of a 51% Attack

When a miner has over 50% market share in a transaction, they can double spend bitcoin by replacing an initial transaction with an alternative one through the miner's greater processing power. In many cases, a 51% attack relies on replacing the public history of the transaction with a version favored by the attacker.

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 - 51% Attack
 - Selfish mining
 - Effects of transaction costs on large miner behavior

Competitive issues in cryptocurrency markets

 Deuflhard, Florian and Heller, C.-Philipp, Antitrust Economics of Cryptocurrency Mining (September 3, 2021). Available at SSRN: https://ssrn.com/abstract=3917012 or http://dx.doi.org/10.2139/ssrn.3917012 

Antitrust Economics of Cryptocurrency Mining

51% Attack

After completing a cryptographic puzzle, most cryptocurrencies implement block reporting. Miners can delay this reporting to quickly work on another block while leaving the other miners to the older one and exhausting their computing speed in a process called selfish mining. Concerns have been raised about how selfish mining could be exploited to create a dominant position in the mining market.

Selfish mining

- A miner could interact directly with a block to prevent selfish miner interference.
- A cryptocurrency could use a new hash puzzle that would protect against other miners until it gains traction.
- Regulation could involve tracking of specific mining pools and their interaction with real-world markets.
- The protocol of Bitcoin could be changed to cause selfish miners to require more of the market in order to be profitable.
- It can be possible to approximate widely used methods of market concentration, which could then be compared to baseline values to see any discrepancies.

Proposed mitigations of monopolizing activities in mining

- Transactions with lower fees in the Bitcoin network have been displayed to be widely taken by large miners due to the need for a set of small miners in their model to increase revenue. 
- Large miners have been hypothesized to have a reason to overlook transactions with low transaction fees when proposing a new block, potentially threatening users without a fee and reducing the mempool size.

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