TL;DR
- Qubic founder Sergey Ivancheglo claims its mining pool now controls over 51% of Monero's hashrate. (source)
- A six-block chain reorganization was detected, raising concerns about possible network manipulation.
- Experts like Ledger's Charles Guillemet warn such dominance could enable censorship or double-spending attacks.
- Some developers, including Luke Parker, dispute the claim, pointing to data gaps and questionable calculations.
- Monero's XMR price dropped over 8% as the debate intensified, with the community split on the threat's severity.
Monero has long been considered one of the most secure and private cryptocurrencies in the world. Built on the RandomX proof-of-work algorithm, it is designed to favor general-purpose CPUs over specialized mining hardware, making it accessible and decentralized.
But that decentralization is being tested. This week, Sergey Ivancheglo, founder of Qubic and known in the crypto community as "Come-from-Beyond," claimed his project's mining pool had achieved 51% control over Monero's total hashrate. If true, it marks one of the most serious threats to the network's security in years.
Ivancheglo framed the move as a wake-up call:
The Six-Block Shake-Up
Concerns escalated when blockchain data suggested a six-block deep reorganization had occurred. In plain terms, this means that six blocks of confirmed transactions were replaced by an alternative chain - a sign that someone with significant hashing power was able to overwrite recent network history.
Charles Guillemet, CTO at Ledger, warned that Monero might be in the midst of a real-time majority control incident:
This raised alarms because with majority control, an attacker could:
- Delay or block transactions from being confirmed.
- Reverse their own payments to double-spend coins.
- Potentially undermine trust in Monero's privacy guarantees.
How Qubic's Mining Model Works
Qubic is not a typical Monero mining pool. It uses a "useful proof-of-work" (uPoW) approach, which incentivizes CPU mining of Monero's RandomX algorithm. The mined XMR is then sold for USDT, and the proceeds are used to buy and burn QUBIC tokens, creating a deflationary loop for its own ecosystem.
This model allows Qubic to funnel external computing power into Monero mining - rapidly increasing its share of the hashrate. From mid-May to late July 2025, Qubic's mining power reportedly jumped from under 2% to over 25%, at times topping pool rankings before pulling back amid criticism.
What a 51% Attack Really Means
For proof-of-work blockchains like Monero, controlling over half of the network's hashing power theoretically enables:
- Block Reorganizations - Replacing recent transaction history with an alternative chain.
- Transaction Censorship - Preventing certain addresses or transactions from being included in new blocks.
- Double-Spending - Reversing transactions to spend the same coins twice.
While Monero's privacy features make tracing individual transactions extremely difficult, they do not prevent the network from being manipulated at the block-production level if majority control is achieved.
Community Pushback and Skepticism
Not everyone is convinced Qubic has crossed the 51% threshold. Luke Parker, lead developer at SeraiDEX, argued that what happened was a targeted six-block reorganization attack, not a sustained 51% takeover.
He pointed out that Qubic's hashrate data is no longer public, making it hard to verify the claims.
Some also noted that Qubic could be making the claim to attract attention to its own project and token economy, especially given its unique token burn mechanism tied to Monero mining profits.
Market Reaction and Confidence Concerns
The controversy quickly spilled into markets. Monero's XMR fell more than 8% in 24 hours, trading near $248 with a market cap around $4.6 billion. The drop reflects both traders' uncertainty and broader fears about how a prolonged hashrate imbalance could affect the network's credibility.
For privacy-focused coins, trust is everything. Even the perception of a possible network compromise can weaken confidence, potentially pushing users toward alternatives.
The Bigger Picture for Privacy Coins
If Qubic or any single entity - can maintain majority control, it could prompt a deeper conversation within the Monero community about mining incentives, network decentralization, and potential protocol changes.
Some have suggested emergency measures, such as:
- Adjusting the RandomX algorithm to make large-scale takeovers more costly.
- Encouraging more miners to join smaller pools to spread out hashing power.
- Introducing temporary block validation rules to limit the effects of reorgs.
Whether these measures are necessary will depend on whether Qubic sustains its mining dominance or if this was a short-lived experiment.
Final Thought
Whether Qubic's claim represents a genuine threat or a calculated publicity move, the episode has exposed how vulnerable even the most privacy-oriented blockchains can be when mining power becomes too concentrated. For Monero, it's a reminder that privacy and decentralization go hand in hand - and losing one can quickly put the other at risk.
