Monero’s ‘Largest’ Reorg Yet Erases 36 Minutes of Transaction History

Monero has shrugged off its biggest chain rollback in its 12-year history.

The network reorganized 18 blocks, erasing 36 minutes of transaction history and invalidating 118 transfers, according to independent monitors disclosing the incident Saturday.

The native token belonging to the Monero blockchain, XMR, has been little impacted, with it rising more than 5% on the day. It is up nearly 12% on the week to $302.54, CoinGecko data shows.

“This is the largest reorg Monero has ever seen,” an XMR community podcaster known as Xenu wrote on X, adding that the event shattered the old rule of thumb that ten confirmations were enough to consider a transaction final.

The rollback occurred between September 14 and 15 at block height 3,499,659, when mining pool Qubic released a hidden chain that overtook the main network. Xenu later claimed that Qubic had mined in isolation “because of selfish mining, even after all their reorgs.”

Representatives for Monero’s open-sourcing project and Qubic did not immediately return Decrypt’s request for comment.

Selfish mining is when a miner withholds blocks they find and later publishes a longer chain that rewrites recent transactions. The tactic takes advantage of proof-of-work rules that reward cumulative work without distinguishing between steady contributions and privately hoarded blocks.

Under Monero’s proof-of-work rules, the longest valid chain is considered the “real” history, so nodes immediately switch over.

In the latest case, Qubic’s withheld chain grew longer than the public one. When it was released, nodes automatically accepted it as the valid history, forcing the network to discard the previous 18 blocks and the 118 transactions they contained.

The move effectively erased more than half an hour of Monero’s recent activity, even though other miners had already confirmed those payments. The reorg also demonstrated that ten confirmations did not provide adequate assurance, leading exchanges to extend their requirements.

Last month, Qubic attempteda 51% attack on Monero as it developed ways to “experiment” and “help” guard the network against future attacks. 

A 51% attack happens when someone controls more than half of a blockchain’s computer power and can change how new transactions are recorded, block or reverse them, and potentially spend coins twice (double-spend).

While that attempt was questioned, some community estimates at the time suggested it may have briefly reached majority control.

Payments once considered final were erased, pointing to how concentrated mining power threatens the network. Some users said they would stop accepting Monero until stability returns, while others urged miners to move to smaller pools.