The Monero (XMR) blockchain experienced its deepest chain reorganization (reorg) in history, involving 18 consecutive blocks.
This event replaced the previously accepted chain with an alternative longer one, effectively rewriting about 36 minutes of transaction history based on Monero’s average 2-minute block time and invalidating approximately 118 confirmed transactions.
The reorg occurred between block heights 3,499,659 and 3,499,676, and lasted roughly 43 minutes before the network converged on the new chain. The incident was linked to Qubic, an AI-focused Layer-1 blockchain project that operates a mining pool.
Qubic had amassed over 51% of Monero’s hashrate in recent weeks, using the rewards to buy and burn its own $QUBIC tokens. This concentration enabled a “selfish mining” strategy, where Qubic reportedly withheld blocks to build a private chain in secret, then broadcast it to outpace the public chain.
This isn’t a full 51% attack (e.g., no confirmed double-spends), but it exploits proof-of-work vulnerabilities, including propagation delays and Monero’s low overall hashrate. This follows a smaller 6-block reorg in August 2025, also attributed to Qubic, raising ongoing concerns about mining centralization.
Monero’s RandomX algorithm is designed to resist ASIC dominance and promote decentralization via CPU/GPU mining, but it hasn’t prevented hashrate concentration in pools like Qubic. 118 transactions were orphaned and became unconfirmed, potentially exposing users to double-spend risks.
Monero traditionally recommends waiting for 10 confirmations for finality, but this 18-block depth exceeded that threshold, highlighting a gap in security assumptions. While Monero nodes quickly adopted the new chain, the event underscores risks for a privacy-focused coin where transaction integrity is crucial.
No funds were directly lost, but it erodes confidence in short-term finality. Counterintuitively, XMR rallied 5-7% in the hours following, climbing above $300 from ~$287 to a near two-month high of around $333. Trading volume and open interest surged, suggesting market resilience or speculation rather than panic.
Community reactions on X (formerly Twitter) were mixed: some expressed alarm over the “Sword of Damocles” hanging over Monero due to reorg risks, while others noted the network’s quick recovery. Security experts like SlowMist founder Yu Xian warned that ignoring such issues could invite more severe attacks.
Monero’s proof-of-work model, while secure against ASICs, remains vulnerable to hashrate centralization—especially with its relatively low total hashrate compared to Bitcoin. Qubic’s actions, led by former IOTA co-founder Sergey Ivancheglo, are seen by some as “useful proof-of-work” (recycling mining rewards), but critics view it as exploitative experimentation that prioritizes $QUBIC over Monero’s stability.
The community is debating mitigations without compromising decentralization:
Temporary “signed” snapshots of chain state via community-managed DNS servers to limit deep reorgs. This could be adopted short-term by node operators but introduces trust elements.
Adjusting difficulty, increasing confirmation requirements (e.g., beyond 10 blocks), improving block propagation, or even exploring hybrid consensus. Multi-party checkpoints or reorg depth limits are also under discussion.
Encouraging more decentralized mining to reduce pool dominance.
As of September 16, 2025, no immediate hard fork or changes have been implemented, but developers are monitoring closely. Users handling XMR payments are advised to wait for more confirmations, 20+ during volatile periods.
This event highlights ongoing challenges for privacy coins under regulatory and technical pressure, but Monero’s history of resilience—surviving delistings, attacks, and upgrades—suggests it could adapt.
