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The Monero 51% attack by Qubic reshaped debate on PoW security. Here’s how it happened, its impact, and what it means for privacy coins.
Author: Tanishq Bodh
In early August 2025, the Monero 51% attack made headlines after Qubic, an AI-focused blockchain founded by IOTA co-founder Sergey Ivancheglo, claimed control over more than half of Monero’s hashrate. This event triggered the deepest chain reorganization in Monero’s history, raising new questions about proof-of-work security for mid-cap cryptocurrencies.
Qubic’s mining pool allegedly reached over 51% of Monero’s network power between August 11–12, 2025, with some reports estimating as high as 65%. This allowed a six-block reorganization, orphaning around 60 blocks. While the takeover was framed by Qubic as a “demonstration” of its Useful Proof-of-Work (UPoW) model, the move sparked fears about double-spending, transaction censorship, and blockchain rewriting.
Monero developers and supporters called it more of a publicity stunt than a sustained threat. They emphasized that no double-spends or theft occurred, and that the network remained operational.
Qubic used economic incentives to draw miners to its pool. Participants earned higher-than-normal rewards, and half of the mined XMR was sold to buy and burn QUBIC tokens—reducing supply and boosting price.
At its peak, this strategy burned over 65.9 billion QUBIC tokens from mining nearly 3,500 Monero blocks. However, operating costs were massive around $75M daily, making long-term dominance unlikely.
Monero’s RandomX algorithm is designed to resist ASIC dominance by favoring CPU and GPU mining. But Qubic’s pooling approach showed how hashrate centralization can still occur, especially in smaller networks.
With majority hashrate, an attacker can:
In this case, Qubic mined empty blocks during the reorg, temporarily reducing transaction throughput.
The Monero 51% attack caused XMR’s price to drop between 15–25% over the week, with up to 17% in a single day. Several exchanges paused withdrawals to reduce risk during the reorg period.
Despite this, Monero’s network stayed online. Supporters argue this proves PoW’s resilience, while critics highlight the exposure of privacy coins to such attacks.
Qubic maintains it was “protecting” Monero from worse actors and showcasing UPoW’s strength. The Monero community remains skeptical, seeing it as unsustainable and possibly self-serving.
The event has reignited debate about:
The Monero 51% attack by Qubic was short-lived but significant. While it didn’t result in confirmed theft or double-spending, it exposed potential weaknesses in mid-sized PoW blockchains and stirred discussions about decentralization, miner incentives, and long-term network security.
As Qubic plans to move on, the episode serves as a reminder: even established networks like Monero must constantly adapt to protect against concentrated mining power.
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