The Monero (XMR) cryptocurrency network experienced a 51% attack led by the Qubic mining pool starting on August 12, 2025. Qubic gained control of over 51%, and later 52%, of Monero's total hashrate, enabling it to mine approximately 80% of blocks within a two-hour period. This dominance allowed Qubic to trigger multiple blockchain reorganizations, raising concerns about the network's security and stability. The attack involved a six-block deep chain reorganization, which theoretically could enable double-spend attacks, censorship of transactions, and rewriting of blockchain history. Monero's price fell by up to 15% amid the incident, resulting in an estimated market capitalization loss exceeding $300 million. Qubic described the takeover as a planned experiment and has yet to disclose full details. Despite the attack being broadcasted in advance, the event has sparked debate over Monero's resilience and the broader implications for proof-of-work blockchain security. The episode highlights potential vulnerabilities in mining concentration and has drawn attention as a possible warning for other cryptocurrencies reliant on proof-of-work consensus mechanisms.