Was the Qubic attack on Monero really a 51% attack?
Qubic’s campaign against Monero was a real attempt to gain majority mining power, but calling it a proven, sustained “51% attack” needs qualification. Qubic claimed majority control, and chain disruptions were reported during the campaign. Those claims and disruptions do not, by themselves, prove that Qubic continuously controlled most of Monero’s mining power or successfully carried out a double-spend.
What would make it a 51% attack?
A 51% attack happens when one operator controls enough mining power to override the honest miners’ version of the transaction history. Mining power, also called hashrate, is the computing work used to compete for new blocks of transactions.
The name is shorthand. An attacker does not need to show exactly 51.00% on a dashboard. What matters is whether they can reliably produce a competing chain with more accumulated mining work.
With that advantage, an attacker could:
- Reverse some recent transactions.
- Delay or exclude transactions.
- Replace blocks produced by other miners.
- Attempt a double-spend: pay someone, then replace the transaction history so that payment disappears.
They could not simply steal your wallet’s private keys, spend arbitrary coins belonging to you, or change Monero’s supply rules.
What Qubic’s campaign actually tells you
During its 2025 campaign, Qubic directed substantial computing resources toward Monero mining and publicly pursued majority control. This was not just a theoretical discussion about whether Monero could be attacked.
However, three different statements often get bundled together:
- Qubic claimed a majority share.
- Qubic demonstrated influence over block production.
- Qubic proved a successful majority attack with a specific outcome.
These are not interchangeable.
A mining pool’s reported share is an estimate, not a direct measurement of every computer mining worldwide. A pool combines miners’ work, but its participants may also leave. Even a majority share observed during one period does not establish permanent control.
You should therefore separate Qubic’s announcements from independently checked block data and from any demonstrated financial harm.
Why reorganizations are not enough to settle it
A chain reorganization, or reorg, happens when Monero replaces recent blocks with a competing branch that has more accumulated work.
Reorgs can happen naturally when miners find competing blocks close together. Deeper or repeated reorgs are more concerning, especially during a publicly announced attack campaign.
But a reorg alone does not tell you:
- Exactly how much mining power the attacker controlled.
- How long that control lasted.
- Whether a payment was deliberately reversed.
- Whether an exchange or another recipient lost money.
An attacker with less than half the mining power can sometimes cause a reorg through luck and strategic block withholding. Conversely, majority control can threaten the network without producing a publicly documented double-spend.
The bottom line
The most careful description is a majority-control campaign with claims of success and reported chain disruption, rather than treating every stronger claim as settled.
If you see “Qubic successfully 51%-attacked Monero,” ask what was independently demonstrated: majority mining power, a deliberate reorg, censorship, or a double-spend. Those are different findings. Missing proof of one does not make the campaign harmless—but neither should a headline substitute for that proof.
