Monero (XMR) suffers 18-block blockchain reorganization, raising fears of double-spend attacks

XMR has experienced an 18-block blockchain reorganization that has led to blockchain observers raising alarms about double-spending attacks on the network.

Monero users are now being advised that the usual practice of waiting for 10 confirmations before treating a transaction as final may be compromised following the 18-block reorganization on the network. 

⚠️The attack against Monero is back. Hours ago XMR experienced a 18 block reorg

If you accept XMR make sure to wait for more than the usual 10 confs pic.twitter.com/793j5WWXgZ

— OrangeFren.com (@OrangeFren) September 14, 2025

18-block reorganization sparks double-spending concerns 

Monero (XMR) users have been put on high alert after the network experienced a rare and unusually deep 18-block chain reorganization. The event, confirmed by monitoring screenshots shared on X, has raised widespread concern that the crypto’s network is now vulnerable to double-spending attacks.

A blockchain reorganization happens when competing versions of the chain are produced, and one overtakes the other. The “losing” chain’s blocks along with all the transactions inside them, will be discarded as a result. 

In Monero’s case, new blocks are created roughly every two minutes. An 18-block reorganization effectively rewrites more than half an hour of transaction history.

While minor reorganization of one or two blocks can occur naturally in proof-of-work systems, an 18-block reorganization is highly unusual and suggests deliberate manipulation or an abnormal concentration of mining power.

A security researcher and founder of SlowMist Cosine warned, “If the Monero community does not take block reorganizations seriously, this sword of Damocles will always hang over Monero’s head. It may not always result in a double-spend, but the ability to do so now exists — even without strictly controlling 51% of the network hash rate.” 

According to reports, the incident now means that the usual practice of waiting for 10 confirmations before treating a transaction as final may not be enough to prevent double-spending. 

Possible safeguards for Monero holders

This is not the first time Monero has faced concerns over mining dominance. In August 2025, the controversial mining project Qubic claimed to have gained the majority of hash power over the network, as reported by Cryptopolitan. At the time, major exchanges, including Kraken, temporarily suspended Monero deposits, citing the risk of a 51% attack that could lead to reorgs and double-spending. 

The new 18-block reorg appears to confirm that those earlier fears were valid. Blockchain data shows competing chains being built by large mining pools such as monero.hashvault.pro and supportxmr.com, with blocks from smaller or unidentified pools often being discarded. 

This environment makes it possible for malicious actors to replace transactions or execute double-spend attempts by selectively reorganizing the chain.

A customer could send XMR, wait for 10 confirmations, and then see that payment invalidated if a deeper chain replaces the one containing it. In such a case, the sender could effectively “double-spend” the same coins elsewhere.

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