Monero faces chain reorganization fears after Qubic says it controls 51% of hashrate

A long-running campaign to amass more than half of Monero’s hashrate appears to have succeeded, prompting concern among crypto observers about the privacy-focused blockchain.

Qubic founder Sergey Ivancheglo claimed his project’s mining pool has “achieved 51% over Monero”, a level that would theoretically let it reorganize blocks, censor transactions, or attempt double-spends on the privacy coin’s network. He added on X that Qubic’s push would ultimately help the community prepare for future threats.

The Block previously reported on Qubic’s months-long bid to concentrate Monero hashrate. Qubic uses a “useful proof-of-work” model that incentivizes CPU mining of Monero’s RandomX algorithm, then converts mined XMR to USDT to buy and burn QUBIC tokens, creating a deflationary loop for its own token economy.

From mid-May to late July, Qubic’s share of Monero’s global hashrate surged from under 2% to above 25%, at times topping the pool rankings before dropping after community backlash, according to data cited in The Block’s earlier coverage.

Ivancheglo has since discussed a plan to demonstrate majority control between Aug. 2 and Aug. 31 and has said Qubic would temporarily stop reporting its pool hashrate to highlight takeover risks.

What a 51% attack means

Charles Guillemet, chief technology officer at hardware-wallet maker Ledger, wrote on X that “Monero appears to be in the midst of a successful 51% attack,” citing a “major chain reorganization” and asserting Qubic now controls a majority of hashrate. Guillemet estimated maintaining dominance could cost about $75 million per day, and warned confidence could erode quickly if competing miners see their blocks orphaned.

A 51% attack occurs when an entity or group controls most of a proof-of-work network’s hashing power, enabling it to reorder blocks, censor new transactions, and reverse its own payments, AKA double-spend.

On Monero, whose RandomX favors general-purpose CPUs, Qubic’s uPoW strategy aims to pull external compute and capital into Monero mining, then recycle the proceeds by selling XMR for stablecoins to buy and burn QUBIC.

If sustained, the majority hashrate would give Qubic controlling influence over Monero’s near-term block production. The economics hinge on whether token appreciation and burn dynamics outweigh the high cost of renting or mobilizing compute.

Qubic scrutiny

Not everyone is convinced Ivancheglo’s game plan is actually profitable. In an X post on Aug. 12, Yu Xian, founder of on-chain security firm SlowMist, argued that the economic benefits are unclear.

There are also doubts about whether Qubic has indeed amassed more than half of Monero’s hashrate. SeraiDEX’s Lead Developer, Luke Parker, argued that Ivancheglo’s project executed a six-block reorganization attack, rather than a full 51% attack.

Parker also scrutinized Qubic’s majority hashrate claims, noting difficulties verifying the numbers as the project has excluded its data from public analytics. He also said that the info available on miningpoolstats suggests otherwise. “They may have 51% of the estimated hash rate/mined blocks but less than 51% of overall hash rate,” Parker wrote. “No matter what, the numbers don't add up.”

Amid the debates, XMR fell over 8% and traded around $248 at a $4.6 billion market capitalization, The Block’s price page shows. QUBIC changed hands near $0.0000023, with a market value around $280 million, according to CoinGecko.

QUBIC market cap data. Image: CoinGecko