Is Monero Safe? The Qubic 51% Scare Explained
Short version: no confirmed double-spend ever happened, and nobody has been shown to have lost money. But in 2025 a mining pool called Qubic did briefly concentrate a lot of Monero's mining power and cause a real, deep chain reorganization. Here's exactly what happened, what it did and didn't do to the network, and where things stand now.
Qubic pointed a large amount of rented CPU power at Monero and, on 14–15 September 2025, caused an 18-block reorganization — the deepest ever reported. Around 115–118 transactions were briefly un-confirmed, but they were not stolen (most were simply re-mined into the new chain), and no double-spend or exchange theft was ever confirmed. The "52.7% takeover" headline was disputed and never independently verified as sustained. By 2026 Qubic's share had fallen back to a minority. For normal use, nothing changes; for large amounts, the standard advice — wait for enough confirmations — is exactly why this caused no lasting harm.
What is Qubic, and why did it point hashrate at Monero?
Monero is mined with an algorithm called RandomX, deliberately built to run on ordinary CPUs. That keeps mining fair and blocks the specialized ASIC hardware that centralizes other coins — but it has a flip side: CPU power is a rentable commodity. Anyone with a lot of general-purpose compute can point it at Monero for a while.
Qubic, a crypto project with a large pool of CPU/AI compute, started doing exactly that around 18 May 2025, and its share of Monero's hashrate climbed from under 2% to more than a quarter over the following weeks. Its founder announced a plan to try to hold a majority during August 2025 as an "economic demonstration," with mined XMR funding buy-and-burns of the project's own token.
Did Qubic really get to 51%?
This is where most of the coverage overreaches. Qubic claimed a brief peak of 52.72% around 12 August 2025, but that figure was pool-reported and disputed almost immediately. Independent analysts put the real, sustained share far lower — an academic measurement study later found its sustained share averaged roughly a quarter to a third, and concluded Qubic "does not maintain a stable majority at any point" and "gained no reward advantage over honest mining." Qubic got close enough to frighten everyone and to cause real disruption, but a clean, sustained 51% takeover was never demonstrated.
What actually happened: the September 2025 reorg
The one concrete piece of damage came on 14–15 September 2025 (a timezone quirk means you'll see both dates), when the chain underwent an 18-block reorganization, reported as the deepest in Monero's history. Around 70 minutes of chain history (18 blocks) were replaced by a longer chain Qubic had mined in secret.
Crucially, "reorganized" does not mean "stolen." Of the roughly 115–118 transactions in the discarded blocks, most were simply re-mined into the new chain — the spends still went through. Careful analysis afterward (by Monero Research Lab contributor Rucknium) found no evidence any of it was used to actually double-spend against a merchant or exchange. It was a warning shot that proved the network could be disrupted, not a heist.
Did anyone actually lose money?
No. As Rucknium put it after studying the reorg: no merchant or exchange has publicly claimed to have been the victim of a double-spend from it. Through 2026, no confirmed theft or successful double-spend from the Qubic episode has surfaced. Your Monero was never at risk of vanishing.
Why is this even possible?
It comes straight from the design trade-off above. RandomX makes mining egalitarian and ASIC-proof, which is good — but it also means hashrate can be rented from cloud CPUs or borrowed from other CPU-mined networks. That's a property of every CPU-mineable coin, not a bug unique to Monero. The upside is that renting enough power to attack is expensive and, as the academic study found, yields no financial reward — so there's little incentive to sustain it.
How Monero pushed back
The response was social, not a change to the rules:
- Miners boycotted Qubic's pool, moving their hashrate to smaller pools and to P2Pool. Qubic fell from the largest pool to around seventh.
- P2Pool — a decentralized mining pool where each miner runs their own node and builds their own blocks — can't be centrally pointed at an attack chain. Growing it is the direct way the community keeps mining decentralized. If you mine, mining on P2Pool is how you help; see run a node.
- Exchanges added safeguards — Kraken briefly paused XMR deposits and re-enabled them with more required confirmations.
Monero did not change its consensus rules to defeat Qubic, and there was no emergency hard fork. If you read that "Monero forked to stop the attack" or that "FCMP++ activated in early 2026," that's wrong — the network is still on protocol v16, which you can confirm from the live box above and on our hard-fork status page.
Where things stand in 2026
By 2026 Qubic's share of Monero mining had receded to a minority — roughly the low-double-digits — and it never re-established a sustained majority. The network has been producing blocks normally, and the live box at the top of this page shows the current hashrate from our node. The structural fact remains — rentable CPU hashrate means a well-funded actor could flare up again — but the community's answer is working, and the incentive is weak.
Is Monero safe to use and hold?
Yes. For everyday payments, nothing about how you use Monero needs to change. For large transfers or exchange deposits, the sensible habit is the same one that made the September reorg harmless: wait for more confirmations on big amounts. Received funds already unlock only after 10 confirmations (~20 minutes); for a very large sum, waiting for a few dozen is cheap insurance against a deep reorg. You can watch any transaction's confirmations on the transaction checker.
MoneroHelp runs its own node and keeps pages like this current. A little XMR helps.
Frequently asked
Is Monero under a 51% attack?
No. A mining pool called Qubic claimed a majority of Monero's hashrate in August 2025, but the claim was disputed and an independent study found it never sustained a majority (roughly a quarter to a third on average). By 2026 its share had fallen back to a minority and there is no active attack.
Did Qubic double-spend Monero?
No confirmed double-spend or exchange theft ever occurred. The one real event was an 18-block chain reorganization on 14-15 September 2025 that briefly un-confirmed roughly 115-118 transactions; most were re-mined into the new chain, not stolen.
Were any coins stolen during the Qubic attack?
No. No user funds were stolen and no successful double-spend was confirmed at any point during the Qubic episode, according to Monero Research Lab analysis.
Is Monero still safe to use in 2026?
Yes. For everyday payments nothing changed. For large transfers, waiting for extra confirmations is prudent — which is exactly why the September reorg caused no lasting harm. Monero did not change its consensus rules and is still on protocol v16.