Monero Faces 51% Attack: AI Protocol Qubic Claims Assistance for the Privacy Coin's Resistance
In August 2025, Qubic, led by former IOTA co-founder Sergey Ivancheglo, claimed to have gained majority control (over 51%) of Monero's hashrate. This move, framed by Qubic as a stress test and demonstration of its "Useful Proof of Work" model and "Outsourced Computations," sparked significant debate and concern throughout the crypto community.
The immediate impact of the alleged attack included the ability to reorganize blocks, censor transactions, and attempt double-spends on Monero. A six-block reorganization occurred, representing roughly twelve minutes of chain reversion due to Monero's two-minute block time. The market reaction included a significant price drop in Monero (XMR), falling about 6-8% within 24 hours after the events surfaced. Exchanges like Kraken temporarily paused Monero deposits to mitigate risks, later resuming with heightened confirmation requirements (up to 720 confirmations).
However, some community experts and the RIAT Institute contended that no full 51% attack truly occurred, suggesting Qubic's share of the network was less than a real majority. The event was more accurately described as a selfish mining attack, which can be effective with 33-40% hashrate rather than a full 51%. The Monero community reacted by proposing defensive countermeasures such as "detective mining," which targets selfish mining without needing a hard fork. Longer-term discussions are underway to possibly overhaul Monero’s proof-of-work consensus to better resist such attacks in the future.
The broader implication for proof-of-work blockchains highlights the growing feasibility of so-called "economic attacks" where mining power is strategically acquired and used to undermine competing chains as part of broader project economic models. The importance of network decentralization to resist majority hashrate control is emphasized, since consolidated mining pools pose systemic risks. The necessity for rapid software-level defenses and possible consensus upgrades to maintain blockchain security in the face of increasingly sophisticated hash power campaigns is also highlighted.
Qubic's end goal is for Monero's security to be provided by its miners, potentially increasing profitability for Qubic's miners at the expense of non-Qubic miners. The cost of maintaining control of a decentralized protocol, as demonstrated by Qubic, can be extremely high, estimated at $75 million per day by Ledger CTO Charles Guillemet.
The success of Qubic's claimed 51% attack on Monero is disputed by some experts. AMLBot and Horizen Labs have expressed doubts about the independence of the verification of the attack. Nikita Zhavoronkov, CEO of Blockchair, defined the attack as low-impact and not long-lasting, but still a successful 51% attack. Retrodrive from Qubic claimed it mined 63 out of 122 blocks, surpassing the 51% threshold.
Monero, being a privacy-centric blockchain, has a reputation for being used by malicious actors in crypto and may be targeted by governments. The attack reintroduces concerns about the vulnerability of blockchain networks to 51% attacks. The QUBIC token saw a 25% increase in value over the past 24 hours, while Monero's XMR experienced a 6% decrease in value from Monday. The XMR price saw a decrease in value from Monday, potentially influenced by the Qubic attack. Retrodrive stated that Qubic does not want to harm Monero.
Eugene Ivanchenko, leader of Qubic, stated that the attack was aimed at helping Monero prepare for future fights against three-letter government agencies. Other blockchain security experts also confirmed data indicating the attack was successful. A report on the event will be made available by Retrodrive. The incident serves as a reminder of the ongoing need for robust blockchain security measures and the importance of maintaining network decentralization.
- Qubic, led by former IOTA co-founder Sergey Ivancheglo, reported gaining majority control (over 51%) of Monero's hashrate in August 2025.
- The move, deemed a stress test, showcased Qubic's "Useful Proof of Work" model and "Outsourced Computations," inciting debate within the crypto community.
- The alleged attack enabled Qubic to reorganize blocks, censor transactions, and attempt double-spends on Monero.
- A six-block reorganization occurred, representing roughly twelve minutes of chain reversion due to Monero's two-minute block time.
- As a result, Monero (XMR)'s price dropped about 6-8% within 24 hours after the events surfaced.
- Exchanges like Kraken temporarily paused Monero deposits to mitigate risks, later resuming with increased confirmation requirements (up to 720 confirmations).
- The RIAT Institute and some community experts contended that no full 51% attack truly occurred, suggesting Qubic's share was less than a real majority.
- The event was more accurately described as a "selfish mining attack" that can be effective with 33-40% hashrate.
- Monero's community proposed defensive countermeasures such as "detector mining" to resist selfish mining, and discussions are ongoing to possibly overhaul Monero’s proof-of-work consensus to better guard against such attacks in the future.
