What is a chain reorganization and how deep was Qubic's Monero reorg?

A chain reorganization, or reorg, happens when Monero replaces some recently accepted blocks with a competing version of that part of its transaction history. The major Qubic-linked reorg reported in September 2025 was 18 blocks deep—roughly 36 minutes of blocks at Monero’s usual pace—although describing it as definitively “Qubic’s attack” goes beyond what the reorg alone proves.

What actually changes during a reorg?

Monero records transactions in blocks, with each block pointing back to the one before it. Sometimes, two competing versions of the newest blocks appear.

Your Monero software follows the valid chain with the most cumulative proof of work: the greatest total mining effort represented by its blocks. If a competing branch overtakes the branch your software was following, your software switches to it.

That switch is a reorg. Transactions in the replaced blocks lose those particular confirmations.

This does not automatically mean every affected payment disappears permanently:

• Some transactions may already appear in the replacement blocks.
• Others can return to the waiting area for unconfirmed transactions and be mined again.
• A transaction that conflicts with a spend accepted on the replacement chain cannot simply be included again.

How deep was the Qubic-linked event?

The September 2025 incident replaced 18 previously accepted Monero blocks. At an average block interval of about two minutes, that represents roughly 36 minutes of transaction history.

That is an estimate of the history displaced, not a precise measurement of how long an attack took. Actual block intervals vary.

You may also encounter reports of a six-block reorg associated with Qubic in August 2025. That was an earlier event, not another measurement of the September 18-block reorg.

The attribution matters. Qubic’s efforts to concentrate Monero mining power made it a focus of concern, but observing a deep reorg does not, by itself, establish who produced it or prove that payments were successfully double-spent.

Why was 18 blocks concerning?

A one-block reorg can happen naturally when miners find competing blocks close together. An 18-block reorg is a much more serious disruption.

Monero normally requires 10 confirmations before received funds become spendable. A confirmation means that your transaction has been included in the accepted chain, with further blocks adding confidence that it will stay there.

An 18-block reorg can reach past that normal waiting period. That matters if you accept a payment and hand over something valuable before its history changes.

A double spend means using competing transactions to spend the same funds differently. A reorg can enable one, but a reorg and a proven double spend are not the same thing.

The bottom line

The headline figure is 18 blocks, or about 36 minutes of Monero history, for the September 2025 Qubic-linked incident. Keep it separate from the earlier six-block report, and distinguish confirmed chain changes from claims about responsibility. If you receive XMR, remember that confirmations reduce risk; they do not make a payment mathematically irreversible.