Update (Aug. 12, 2025, at 1:30 pm UTC): This article has been updated to add a Qubic announcement.
Layer-1 blockchain Qubic said it had “completed its attempt to dominate the Monero network,” claiming a month-long push culminated Monday with 51% control of Monero’s hashrate.
According to a Monday blog post, the “month-long, high-stakes technical confrontation” concluded with Qubic reaching 51% of Monero’s hashrate. The effort coincided with a six-block-deep chain reorganization that discarded 60 previously valid blocks, according to the Monero Consensus Status dashboard.
A six-block-deep reorganization is when the blockchain replaces the last six confirmed blocks with an alternate chain that is longer or has higher cumulative work. While Qubic said this shows that it carried out a succesful 51% attack on Monero, others are unconvinced by the claims.
Developers dispute successful attack claim
The claim drew immediate pushback from developers who argued that the reorganization alone does not prove a successful 51% attack. SeraiDEX’s lead developer, Luke Parker, said in an X post that a six-block-deep network reorganization with block orphaning “does not mean a ‘51% attack’ was successful.”
“It does mean an adversary with a high amount of hash got lucky,” he added.
A 51% attack is when a single entity controls over half of a blockchain’s mining power or stake, allowing it to rewrite transactions or block them entirely.
Still, Zhong Chenming, the co-founder of crypto cybersecurity firm SlowMist, said in a Tuesday X post that “this time the 51% attack on Monero seems to have succeeded.” He added:
“The cost was also high, and it’s unclear what the economic benefits of doing this are in the end… In theory, the Qubic mining pool can now rewrite the blockchain, achieve double-spending, and censor any transactions.”
Related: Coin Metrics research shows BTC and ETH are immune to 51% attacks
How the chain reorg unfolded
Qubic is a layer-1 blockchain that employs a “useful proof-of-work” model to route mining toward artificial intelligence tasks, which recently rerouted its computing power to attacking Monero (XMR).
In a June 30 blog post, Qubic revealed that it had begun incentivizing Monero CPU mining via its own network. The mined XMR would then be used to fund buybacks and token burns for the Qubic ecosystem. “QUBIC miners now perform real-world tasks (Monero mining) that generate real market value, which in turn strengthens the QUBIC economy,” the post stated.
Sergey Ivancheglo, founder of crypto projects Qubic, NXT and Iota, admitted at the time that his Qubic network was staging a takeover of the Monero network. In an X post, he said that after getting control of most of the network’s hashrate, Qubic would reject the blocks mined by other pools.
Related: 51% attack on Ethereum more difficult than on Bitcoin — Justin Drake
The Monero community responded to Qubic’s economic attack against Monero in late July. The community responded to the ongoing attack with its own countermeasures, including an alleged distributed denial-of-service attack against Qubic’s mining pool. At the time of the alleged six-hour-long DDoS attack, the mining pool’s hashrate fell from 2.6 gigahashes per second down to 0.8 GH/s.
Amid the turmoil, Monero’s price fell by around 8.6% to $248, according to CoinMarketCap.
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