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Over the past few weeks, 51% attacks on Proof-of-Work (PoW) blockchains have once again made headlines. Following recent campaigns led by Qubic, concerns about blockchain security are resurfacing, with a central question: could Bitcoin itself ever fall victim to such an attack?
The timing is notable. Two major mining pools are edging close to controlling half of Bitcoin’s hashrate, sparking debate over whether the world’s largest blockchain could ever be compromised.
What Is a 51% Attack?
Proof-of-Work blockchains rely on miners who provide computing power to secure the network. The total computing power is measured as the hashrate. A blockchain is considered immutable because altering its distributed ledger is nearly impossible under normal circumstances.
However, if a single entity or coalition controls 51% or more of the hashrate, it could manipulate the blockchain. This might allow double spending, reversing transactions, or halting block processing ,effectively undermining the very integrity of the network.
1/ Two Bitcoin pools now control more than 51% of the network hashrate!🚨
That crosses a decentralization red line. A 51% attack becomes technically possible
Why this matters and why Ethereum these days looks more decentralized and sustainable than BTC. 🧵 pic.twitter.com/sAERij4Rnz
— Leon Waidmann 🔥 (@LeonWaidmann) August 21, 2025
From Theory to Reality
Originally, 51% attacks were largely theoretical, since the cost of amassing such computing power outweighed potential gains. But with the rise of hashrate rental services, attackers can now temporarily acquire enough power to launch targeted assaults.
Qubic’s recent campaigns, in which one network attacked another, highlight this evolving landscape. While these attacks typically target smaller blockchains, history shows they are not uncommon:
- Expanse (EXP) was attacked in July 2019.
- Litecoin Cash (LCC) suffered six attacks in the same month.
- Vertcoin (VTC) was attacked in December 2019.
- Bitcoin Gold (BTG) endured two attacks in January 2020.
Could Bitcoin Be Next?
Technically, Bitcoin has never suffered a 51% attack. The scale of its hashrate makes such a feat astronomically difficult and prohibitively expensive. However, history offers warnings: in 2014, the mining pool GHash reached 51% control, before voluntarily reducing its share to ease concerns about centralization.
Today, two mining giants dominate: Foundry USA (32.3%) and AntPool (17.2%), together holding 49.5% of Bitcoin’s hashrate. While this doesn’t automatically mean danger, some observers worry about the risk if they were ever to collude.
Experts also note that while private groups lack the resources to sustain such an attack, state-level actors might, in theory, be able to. Still, the economic incentive remains weak, as controlling Bitcoin temporarily would be enormously costly with little financial upside.
JUST IN: 🚨 #Bitcoin is now at risk of a 51% attack because two mining pools (Foundry USA and Antpool) control over 51% of the hash power.
Bitcoin today: 2 pools (Foundry + Antpool) >51% → de facto centralization.
Kaspa today: 85% unknown miners (individual)→ de facto… pic.twitter.com/BoptfOkGUy
— 𝐂𝐫𝐲𝐩𝐭𝐨 𝐏𝐫𝐨𝐬𝐞𝐥𝐲𝐭𝐞 (@Crypt0Proselyte) August 20, 2025
Conclusion: Economics Over Fear
For now, Bitcoin remains resilient. While the theoretical threat of a 51% attack cannot be fully dismissed, the economic reality aligns with Satoshi Nakamoto’s original conclusions: launching such an attack against Bitcoin is simply not profitable or sustainable, thus could further incentivise for crypto trading.
