You look at the mining pool hashrate rankings and see that the top three together could influence half of the Bitcoin network. That makes you nervous. This is indeed Bitcoin's most real security concern right now, but the risk is different from what most people imagine.
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The core risk is not that someone will launch a 51% attack. The core risk is that no one can stop transaction censorship.
First, look at the data: How concentrated has it become?
As of June 2026, Foundry USA, AntPool, ViaBTC and F2Pool together control more than 70% of Bitcoin's total hashrate. Their shares are roughly 31%, 18%, 13% and 10% respectively. According to D-Central, the Nakamoto coefficient is 3. That means only three mining pools need to work together to exceed half of the network's block production capacity.
Foundry USA alone controls nearly one third of the hashrate and mainly serves large institutions and publicly listed mining companies. The top mining pools together can control more than two thirds of the hashrate. This means transaction selection power is highly concentrated in the hands of a small number of entities.
The risk is not a 51% double-spend attack, but transaction censorship
In 2014, GHash.IO briefly reached 55% of the hashrate, but it did not attack the network. That is because if it had double-spent coins, the Bitcoin price would have crashed, and the value of its own mining equipment would also have fallen to zero. This is an economic calculation. Attacking Bitcoin is not in any mining pool's long-term interest, even if it really holds more than 51% of the hashrate.
The real risk lies in another direction. Mining pools control the creation of candidate blocks and the selection of transactions. In theory, they have the ability to exclude specific transactions or carry out double-spending. If a mining pool is required by regulators to block transactions from certain addresses, it is fully capable of doing so. And there is no other force in the market large enough to stop it. That is the most realistic threat from hashrate concentration today.
Why nothing major has happened yet
There are three layers of buffer mechanisms at work:
- Miners can switch away at any time. Miners are not permanently locked to a mining pool. If a pool behaves in a way miners dislike, they can immediately move their hashrate to another pool. This creates a constraint: the pool knows its users can leave at any time.
- The Stratum V2 protocol is being promoted. This upgrade allows miners to decide their own transaction templates instead of letting the pool fully control transaction packaging. It can fundamentally weaken a pool's control over transaction selection.
- The Bitcoin network has proven its resilience. When China cracked down on mining in 2021, the hashrate dropped sharply for a short time and then quickly migrated to places like the United States and Canada. Network security was not damaged in the long term.
What kind of attack is the real threat?
There is one scenario more worth watching than a 51% double-spend attack: slow infiltration disguised as AI business.
If an entity uses AI training as a cover and secretly controls a large amount of mining pool hashrate, its motive may not be to earn Bitcoin at all, but to disrupt the network. Traditional mining pools lose money if they attack the network. But if that entity's core business is AI computing demand, and Bitcoin mining is just a side business when it has excess computing power, then the constraint of losing money no longer applies. Once the mining pools it controls grow large enough to create chaos, the normal operation of the Bitcoin network would face a real threat.
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How to check the situation yourself
Open mempool.space or BTC.com and find the mining pool hashrate distribution section. Look at two numbers: the combined share of Foundry and AntPool, currently about 49%, and the combined share of the top four mining pools, currently over 70%. If these two numbers keep rising and no new mining pool enters the top five, it means centralization is deepening. That is the time to seriously consider countermeasures, not right now.
