Monero (XMR) experienced its largest chain reorganization in history on Sunday, September 14, 2025, when the Qubic mining pool executed an 18-block reorg that reversed approximately 117-118 transactions. The attack began at block 3,499,659 at 5:12 am UTC and completed at block 3,499,676 roughly 43 minutes later, marking the most significant security breach in the privacy-focused network's history.
This represents the second major attack by Qubic, a layer-1 AI-focused blockchain and mining pool that previously executed a six-block reorg last month after amassing 51% of Monero's hashrate. The recent reorg exceeded Monero's 10-block lock mechanism designed to protect transactions, raising serious concerns about network reliability.
Surprisingly, XMR traded relatively flat during the attack and then rallied 7.4% from $287.54 to $308.55 approximately eight hours later, outperforming the broader market which dropped around 1% on Sunday. The token has shown remarkable resilience, falling only 5.85% since Qubic's initial takeover around July 28.
The incident has triggered widespread community concern, with crypto pundit Vini Barbosa announcing he would stop accepting XMR payments until the situation is resolved. Blockchain security expert Yu Xian of SlowMist warned that this 'Sword of Damocles' will continue to threaten Monero unless the community takes reorganization risks seriously.
Researchers note that Monero's orphan block rate has reached approximately 30% - far above the historical average of 1-3% and even exceeding the 8.3% rate observed during previous Qubic attacks. Protocol researcher Rucknium suggested that Monero node operators might temporarily adopt DNS checkpoints as a solution, though this would come at the cost of increased centralization.