In August, Sergey “CFB” Ivancheglo announced a test: mobilizing Qubic to attempt a 51% attack against Monero. Since then, the debate has been raging. Did the attack take place, and under what conditions? Did it work? What are the implications for network security and for XMR?
Verdict: did the Qubic blockchain succeed?
In July, Sergey Ivancheglo, alias CFB, founder of the Qubic blockchain and former co-founder of IOTA, announced, using Qubic’s “useful Proof of Work (uPoW),” a 51% attack against Monero during the month of August for experimental purposes. And as is often the case with strong communities, including Monero’s, articles and comments about these events have attracted a lot of criticism.
For example, our latest article entitled “51% attack on Monero (XMR): Qubic pool controls the majority of miners” was criticized on social media for its overly assertive title, even though the content presented the facts in a pragmatic manner, with the necessary nuances for understanding. Looking back, what really happened?
At the beginning of the period announced by CFB, there was nothing particularly noteworthy to report. It was only from August 12 that the threat really became apparent: for short periods, the Qubic mining pool maintained enough hashrate to reorganize eight blocks in a row, then nine blocks on August 17.
This incident therefore had a relatively significant impact, but it does not really correspond to a fully successful 51% attack. CFB itself recently acknowledged that the 51% threshold had not been reached.
Here is the statement:
Despite mining more than 51% of all the blocks which got accepted into the longest #Monero blockchain, independent analysis has shown that the #Qubic pool did not have a 51%+ share of the total hashrate during that event. This supports CFB’s claim that “51%…
— Come-from-Beyond (@c___f___b) August 17, 2025
Here is the statement: Despite the fact that the Qubic pool mined more than 51% of all accepted blocks in the longest Monero blockchain, independent analyses have shown that this pool did not reach 51% (or more) of the total hash rate during this event. This supports CFB’s assertion that the term “51% attack” is inaccurate and should be renamed “34% attack.”
As the majority of blocks mined by the Qubic pool were empty, it is highly unlikely that any double spending occurred. These reorganizations only invalidated 6 to 7 blocks, representing all transactions made during approximately 16 to 18 minutes.
What should we take away from this attack on the Monero blockchain?
Although the Monero blockchain is back to normal and the Qubic project now seems to be targeting the Dogecoin blockchain, the community is divided into two camps: Monero optimists, who are proud of having resisted, and the more pragmatic, who note that Qubic, a fledgling project valued at only $350 million, still managed to reorganize a dozen blocks.
One can imagine that an attacker with greater resources, a better organized and more fortunate state or group, could launch a more severe offensive against Monero. However, game theory also applies to Monero: few players have a real interest in spending so much capital to carry out such an attack. The Qubic case is one of the few scenarios where the operation could have remained profitable and been sustained long enough to compromise the chain, but it was relatively unsuccessful.
