Briefing
The Monero blockchain experienced its largest recorded reorganization on September 14, 2025, when the Qubic mining pool, controlling over 50% of the network’s hash rate, executed an 18-block rollback. This consensus manipulation invalidated approximately 117 confirmed transactions, forcing them back to the mempool. While no direct fund theft or double-spending was confirmed, the incident severely disrupted transaction finality and exposed a critical vulnerability in Monero’s Proof-of-Work security model.
Context
Prior to this event, Monero relied on a 10-block finality rule, presuming sufficient block confirmations would prevent such deep reorganizations. This incident follows a smaller 6-block reorg by the same Qubic pool in August 2025, signaling a growing concentration of mining power. Smaller Proof-of-Work networks, particularly those prioritizing ASIC resistance and decentralization, remain susceptible to hash rate accumulation and subsequent 51% attacks.
Analysis
The incident leveraged the fundamental “longest chain rule” inherent in Proof-of-Work consensus mechanisms. The Qubic mining pool secretly mined a longer private chain by withholding blocks, then released it to the public network. This longer chain superseded the honest chain, causing all nodes to switch to the attacker’s history and orphan the 18 blocks mined by other participants. The attack vector exploited a critical mass of mining power, allowing the malicious actor to rewrite recent transaction history and temporarily negate the network’s established finality assumptions.
Parameters
- Protocol Targeted → Monero (XMR)
- Vulnerability Type → 51% Attack / Blockchain Reorganization
- Attacker Entity → Qubic mining pool
- Financial Impact → No direct funds stolen; 117-118 transactions invalidated
- Affected Blockchain → Monero
- Reorganization Depth → 18 blocks
- Date of Incident → September 14, 2025
- Hash Rate Control → Over 50% by Qubic
Outlook
Immediate mitigation includes exchanges raising confirmation requirements for Monero deposits to counteract future short reorgs. The Monero community is actively discussing protocol-level solutions such as “Publish or Perish” to penalize block withholding and exploring merge mining or chain-locking mechanisms to enhance network security. This event underscores the necessity for continuous vigilance against mining centralization, serving as a critical reminder for all Proof-of-Work networks to assess their vulnerability to majority hash rate attacks.
Verdict
This Monero 51% attack profoundly challenges the perceived immutability and transaction finality of smaller Proof-of-Work chains, demanding immediate and systemic consensus mechanism enhancements across the digital asset ecosystem.
Signal Acquired from → ccn.com
