Monero (XMR) recently underwent an 18-block reorganization, prompting a security alert for transactions and leading to a significant price decline.

Executive Summary

Monero (XMR), a privacy-focused cryptocurrency, recently experienced an 18-block reorganization, signaling a potential attack and heightened double-spend risk. This event has led to increased caution for users accepting XMR payments and contributed to price volatility. Security experts, including SlowMist Yu Xin, have warned that without concerted community attention, the network faces long-term vulnerabilities to double-spending, even without a sustained 51% attack.

The Event in Detail

The Monero network detected an 18-block reorganization, a process where a longer chain of blocks replaces a previously accepted shorter chain, effectively rewriting a portion of the transaction history. This incident follows prior reports of network instability, including a claimed 6-block reorganization on August 12, 2025, which some experts initially linked to a potential 51% attack. During earlier events, Qubic founder Sergey Ivancheglo (CFB) suggested Qubic had gained significant computational power over Monero. Previous analyses indicated that the Qubic mining pool had amassed a dominant share of hashrate, which enabled the private mining of a longer chain to force node synchronization. Such reorganizations carry risks including double-spending, transaction censorship, and the reversal of confirmed blocks. While SeraiDEX developer Luke Parker contended that a 6-block reorg did not definitively confirm a successful 51% attack, an 18-block reorg represents a more substantial rewrite of the blockchain.

Market Implications

The 18-block reorganization has introduced uncertainty into the Monero market. Following previous security claims, XMR experienced a price drop, with reports indicating a decline of over 5% against the U.S. dollar within 24 hours and over 14% for the week in earlier instances. More recent unverified claims of a 51% attack caused XMR to drop more than 10% in price. The security concerns have led to exchanges, such as Kraken, suspending XMR deposits and later requiring significantly more confirmations—up to 720 confirmations—compared to the usual 10, to mitigate risks. This reflects a broader erosion of confidence in the transaction finality and security of the network, potentially impacting XMR liquidity and trading activity.

Broader Context

The recurring reorganizations highlight a fundamental vulnerability in Proof-of-Work (PoW) networks where hashrate can be consolidated. Ledger CTO Charles Guillemet previously estimated the cost of sustaining a 51% attack on Monero at $75 million per day. The situation has intensified the debate within the Monero community regarding its consensus mechanism. Proposals for enhancing network resilience include merge mining with Bitcoin, implementing geographically distributed hardware to prevent large mining pool dominance, and adopting mechanisms similar to Dash's Chainlocks, which use masternodes to finalize blocks and prevent reorganizations. Industry initiatives aimed at preventing future critical episodes include continuous monitoring of hashrate, collaborations among platforms to identify anomalies, and promoting decentralization among miners through awareness and incentives. If not effectively addressed, these vulnerabilities could diminish trust in Monero's privacy features and broader adoption, while also serving as a cautionary precedent for other PoW cryptocurrencies.