Security Experts Flag Possible 51% Attack on Monero, Citing 6-Block Reorganization

Several reports indicate that the Qubic project has captured 51% of the network’s hashrate, with some observers warning that Qubic mining pools have the ability to reorganize the blockchain and block certain transactions.

Alleged 6-Block Chain Reorg Spurs Fears in the Monero Community

On Aug. 12, 2025, at 6 a.m. Eastern time, the Slowmist security team claimed that the Monero (XMR) network was hit by a 51% attack, resulting in an alleged six-block chain reorganization. Slowmist noted that, according to a now-deleted X post and “provided blockchain reorganization charts, Monero recently experienced a six-block chain reorganization mined by an unknown pool.”

The firm added that data from the miningpoolstats web portal shows no single pool controlling more than 50% of the network’s hashpower. Qubic founder Sergey Ivancheglo, also known as “Come-from-Beyond” (CFB), likewise alleged that Qubic had seized control of Monero’s computational power.

“Looks like Qubic has achieved 51% over Monero, we are waiting for independent confirmations,” CFB detailed. “In the meanwhile Monero team is polishing details of their 51% attack protection.” Over the past 24 hours, XMR has dropped more than 5% against the U.S. dollar and has fallen over 14% for the week. Several others weighed in on the matter, with Slowmist’s founder offering his take:

Ledger CTO Charles Guillemet stated, “Monero appears to be in the midst of a successful 51% attack,” adding that “sustaining this attack is estimated to cost $75 million per day. While potentially lucrative, it threatens to destroy confidence in the network almost overnight.”

If it continues, the event could mark one of the most notorious assaults ever against a privacy-focused blockchain, testing the resilience of both Monero’s infrastructure and community. Long-term trust may hinge on whether countermeasures are swiftly implemented and effectively communicated.

Additionally, SeraiDEX developer Luke Parker contended that the reorganization does not necessarily mean a 51% attack was fully successful. “A 6 re-org does not mean a ‘51% attack’ was successful,” Parker wrote on X. “In that case, we’d see unbounded-depth re-orgs/no blocks mined by any other mining pool (assuming the adversary censors other mining pools, as this one does).”

Parker added:

The incident may also serve as a broader warning to other proof-of-work (PoW) networks, highlighting how concentrated mining power can upend assumptions about decentralization. How Monero responds in the coming days could influence its reputation for years ahead.