Moneroโs privacy chain endured its deepest-ever chain reorganization in the last 24 hours, when 18 consecutive blocks were replaced, briefly โrewritingโ roughly 36 minutes of ledger history and invalidating about 118 already-confirmed transactions. Multiple independent monitors flagged the event late Sunday into Monday, describing a rollback spanning block heights 3,499,659 through 3,499,676 before nodes converged on a new best chain. With Monero targeting two-minute blocks, an 18-block reorg translates to ~36 minutes of historyโan extraordinary depth for a mature proof-of-work network.
Monero Hit By Record 18 Block Reorg
โThe attack against Monero is back. Hours ago XMR experienced an 18-block reorg. If you accept XMR make sure to wait for more than the usual 10 confs,โ warned independent monitor OrangeFren as alerts circulated across crypto social channels. The post, amplified by several industry accounts, captured the immediate operational takeaway for merchants and services: raise confirmation thresholds until conditions normalize.
The incident comes a month after the controversial AI-oriented project Qubic claimed to have marshaled a majority of Moneroโs hashrate, a campaign that coincided with a six-block reorg in August and led to temporary deposit suspensions at some exchanges. Security researchers at the time cautioned that the August event did not, by itself, prove sustained 51% dominanceโstreaks of mining โluckโ can produce short, probabilistic reorganizationsโbut they also warned that persistent hashrate concentration raises the ceiling on how deep such reorgs can go. Mondayโs 18-block episode materially raises that ceiling.
Community voices reacted with a mix of alarm and realism. SlowMist founder Yu Xian commented via X: โIf no one in the Monero community takes the issue of block reorganization seriously, then this Sword of Damocles will always hang over Moneroโs headโฆ It may not necessarily carry out a double-spend attack, but having this capabilityโฆ It doesnโt even have to strictly exceed 51% of the hash powerโฆโ
Qubic founder Sergey Ivancheglo, who is known by the alias โCome-from-Beyondโ, posted via X: โMonero will stay because Qubic wanted it to stay.โ
#Monero will stay because #Qubic wanted it to stay.https://t.co/49MZzF5WaI pic.twitter.com/qd7wPVoDOu
โ Come-from-Beyond (@c___f___b) September 14, 2025
At a technical level, a chain reorg occurs when two valid histories compete and the network ultimately converges on the one with the most accumulated proof-of-work, discarding the other and any transactions exclusive to it. In Moneroโs design, two-minute target block intervals and dynamic block sizing aim to keep throughput smooth while preserving privacy primitives like ring signatures and stealth addresses.
In practice, however, a miner or pool with outsized hashpower canโat least in burstsโprivately extend a side chain and then release it, โout-workingโ the public tip and forcing nodes to reorganize. That is what appears to have happened here, with a depth that exceeded the de facto 10-confirmation comfort zone commonly used by wallets and exchanges.
The AugustโSeptember sequence has re-opened a contentious debate inside Monero about defenses. Proposals span the spectrum: โrolling 10-block checkpointsโ that finalize recent blocks to cap reorg depth; adopting elements akin to Dashโs ChainLocks to bolt on a finality layer; โdetective miningโ incentives to counter selfish-mining strategies at the pool level; and even merge-mining concepts that would piggyback Moneroโs security on a larger PoW base.
Each trade-off is sharp: checkpoints and ChainLocks introduce degrees of centralization or new trust assumptions, while pool-level changes may be unevenly adopted. The community has not yet coalesced around a specific fix, but the urgency has unmistakably increased.
Markets treated the event with surprising resilience. Within hours, XMR rallied between roughly 5% and 7%, trading above $300 on some venues.
Featured image created with DALL.E, chart from TradingView.com
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