What to know:

• Qubic, led by IOTA co-founder Sergey Ivancheglo, says it now controls over 51% of Monero's hashrate, enabling potential chain reorganizations, double spends and transaction censorship.

• The takeover, described by Qubic as a strategic experiment, leveraged economic incentives and a "useful proof-of-work" model to draw miners away from other pools.

• The price of Monero's XMR token fell 6% in the past 24 hours as traders questioned whether the network can prevent sustained control by an outside entity.

Monero, the leading privacy-focused cryptocurrency, is facing one of the most serious security challenges in its history.

Qubic, a project led by IOTA co-founder Sergey Ivancheglo, says it now controls more than 51% of the network's hashrate. In blockchains secured by proof-of-work algorithms, that's the same method used by Bitcoin, that level of control can allow an attacker to rewrite transaction history, block transactions or carry out double-spend attacks.