Qubic, a Layer-1 blockchain designed to implement Proof of Work (PoW) to complete useful tasks, applied its mining pool to mining Monero in an attempted 51% attack against the protocol. In August 2025, the attack allegedly succeeded, with the mining pool performing a six-block deep reorganization of the Monero ledger and orphaning around 60 blocks.
Inside the Attack
The 51% attack is the original threat to blockchain security, made possible by the fact that blockchain consensus algorithms essentially implement majority rule. If an attacker controls over 50% of the hashrate of a PoW blockchain, they can create new blocks faster than the rest of the network combined.
Blockchains operate under the longest chain rule, which says that, if two competing versions of the blockchain are presented, the “longer” one wins (with some caveats regarding the meaning of “longer” in some cases). As a result, a 51% attacker who can build a new version of the blockchain faster than the rest of the network can rewrite history, performing double spends, censoring transactions, and reorganizing approved transactions. The one thing that they can’t do is create fake transactions for other users, since this would require access to the private key used to generate digital signatures for their account.
Qubic is a blockchain designed to reduce the waste associated with PoW by making miners perform useful calculations. In June 2025, the network elected to make these calculations the PoW hashes needed to mine Monero blocks. In addition to setting up a 51% attack, this allowed the network to earn block rewards on Monero that could be used to benefit the network itself.
In August 2025, Qubic successfully performed a six-block reorganization of the Monero blockchain, rewriting its history. While impressive — and expensive — this doesn’t necessarily mean that a 51% attack succeeded. Simply controlling a substantial percentage of the Monero hashrate might allow Qubic to build six blocks faster than the rest of the network by getting lucky. A true 51% attack would allow them to rewrite as much of the blockchain history as they wanted.
In response to Qubic’s efforts, a distributed denial-of-service (DDoS) attack was launched against the protocol’s systems. This caused a dramatic reduction in its hashrate by disrupting its coordination and ability to apply miners’ efforts to performing Monero PoW.
Lessons Learned from the Attack
Historically, most successful 51% attacks targeted smaller blockchains, where achieving 51% of the network’s hashrate was more difficult and expensive. This attack on Monero demonstrates that a sufficiently determined and well-resourced organization may be able to threaten larger protocols as well, moving this type of attack from theoretical to practical.
Managing the security of Layer-1 protocols requires considering all potential security threats and implementing security controls to manage them. For help with implementing best practice security controls for blockchain, reach out to Halborn.